The ingress of body fluids or their constituents is one of the main causes of failure of active implantable medical devices (AIMDs). Progressive delamination takes its origin at the junctions where exposed electrodes and conductive pathways enter the implant interior. The description of this interface is considered challenging because electrochemically-diffusively coupled processes are involved. Furthermore, standard tests and specimens, with clearly defined 3-phase boundaries , are lacking. We focus on polymers as substrate and encapsulation and present a simple method to fabricate reliable test specimens with defined boundaries. By using silicone rubber as standard material in active implant encapsulation in combination with a metal surface, a corrosion-triggered delamination process was observed that can be universalised towards typical AIMD electrode materials. Copper was used instead of medical grade platinum since surface energies are comparable but corrosion occurs faster. The finding is that two processes are superimposed there: First, diffusion-limited chemical reactions at interfaces that undermine the layer adhesion. The second process is the influx of ions and body fluid components that leave the aqueous phase and migrate through the rubber to internal interfaces. The latter observation is new for active implants. Our mathematical description with a Stefan-model coupled to volume diffusion reproduces the experimental data in good agreement and lends itself to further generalisation. Neural prostheses emit electrical signals directly into nerve paths restoring lost neural functions. In this way, neural faults and dysfunctions can be treated. According to the EU Medical Device Regulation, neural prostheses are active implantable medical devices (AIMDs) which belong to the highest risk Class III of medical devices. This study addresses neural prostheses, which are implanted permanently into the human body and exposed to its fluids ,2,3. TheOur study focuses on the corrosion-triggered delamination of the silicone rubber from the encapsulated metal of the electrode with its lead or an encapsulated implantable pulse generator (IPG), leading to leakage and electrical dysfunctions due to the change of impedance and isolation properties. The driving force of this delamination process is assumed to be the diffusion of molecules of the body fluids into the interface leading to weakening of the adhesion between electrode and encapsulation and eventually leading to corrosion.Polydimethylsiloxane (PDMS) is one of the standard silicon rubbers, which are often used in a wide range of medical applications. PDMS is known to be nontoxic, easy to process and shows quite favourable mechanical and electrical properties. Together with its hydrophobic nature and long-term stability, PDMS is considered an ideal encapsulation of sensitive electronic circuits of AIMDs such as the cochlear implant (CI) . In spitOn that account, a collection of studies dealing with the modelling of sorption and transport processes of water vapour exists. High individuality of different PDMS blends and its filler components make each study quite specific depending on manufacturer and product and becoming more unique when the PDMS is used in a metal-polymer compound like implantable neural electrodes, with varying geometry, composition depending on manufacturer and fabrication method ,10,11,12Furthermore, all existing studies strictly focus on diffusion processes in bulk, not giving credit to transport and aging processes in the interface of such polymer-metal joints . This miThis may be ironic due to the fact that a major failure mechanism is the delamination of the PDMS mantle of the platinum wire as observed in cochlear implants. This delamination mainly arises at the part of the electrode where it has its contact . At thesA mathematical interpretation based on the works of Crank and Goodman was developed and is introduced in Manufacturers of cochlear implants award a product lifetime up to 30 years. Implant failure is an inherent ubiquitous risk . A studyOur study focuses on corrosion-triggered delamination of the PDMS from the metal electrode, leading to leakage and electrical dysfunctions due to the change of impedance and isolation properties. The driving force of this delamination process is assumed to be the diffusion of molecules of the body fluids into the interface, leading to corrosion weakening the adhesion between electrode and encapsulation. The main cause of failure of thin-film components such as microelectrode-arrays is the delamination of layers due to a humid environment. Providing a stable compound between precious metals and polymers can be quite challenging as it is not possible to blend the atoms of each material ,17. The Even with adhesive optimization of those compounds, delamination is still present. This makes further investigations of possible factors influencing delamination essential. A recent study , alreadyOne major trend in medical engineering is miniaturization of AIMDs to become minimally invasive and thereby reduce both surgical trauma and scarring in the healing phase after implantation. This miniaturization leads to implant sizes of a few millimetres containing complex integrated circuits in which leakage phenomena are the predominant cause of failure of the mechanism.Lifetime estimations and valuation of protection against biologic liquids are a big concern in medical approval processes. Due to the small size of cochlear implants, helium leak testing is inapplicable making soak test necessary to examine the resilience of AIMDs against ions and fluids. These tests confirm delamination to be a major failure mechanism . Based oPDMS as encapsulation material has a long tradition in AIMDs ,15. The A copper sputter silica wafer was used as a metal layer. Copper was chosen as it can be corroded within seconds. The manner of this type of corrosion in the experiments showed a sharp visual contrast between corroded and non-corroded metal, allowing easy optical analysis via a camera and a MatLab  script. The sputter deposition of the copper layer ensures homogeneous surface properties. Due to the minimization of the observation time, corrosion at the interface due to diffusion becomes the only time-relevant process. Details of copper deposition are described in Recent studies have only shown ion selective corrosion for the vapour state. Up to this point, diffusion out of the aqueous phase was not considered in most studies. Only a single paper has described diffusion of aqueous bromine in PDMS , so far.A solution of potassium polysulfide (sulphur liver)  dissolved in deionized water is chosen as a corroding agent. To only corrode the copper but sparing the Sylgard-184, a 2% concentration of potassium polysulfide was used. Single droplets were placed with a syringe on the edge of the copper and silicone .Pictures were taken every 5 min to display the time dependent corrosion process . InitialTo investigate the geometry of the 3-phase-boundary on the interface diffusion, different approaches were followed to form the edge of the PDMS droplet as it marks the point where interface diffusion starts. All attempts of manufacturing a pre-set edge using a mould failed due to the fact that the low viscosity of Sylgard-184 leads to the undermining effects of the polymer. Moreover, there was no way to form an edge without creating mechanic and thermal stresses in the interface, weakening the adhesion of the polymer-metal joint. Investigations of diffusion require a nearly perfect interface without discontinuities or material errors. Therefore, the approach was changed to a more sparing manufacturing process. PDMS droplets were set directly on a copper layered wafer, leaving the drop to form naturally. As a continuous surface energy can be assumed, which leads to a contact angle according to the Young equation and the different viscosities in the fluid and solid state, a uniform edge along the outer droplet radius will set in. By eliminating the need to extract the specimen from a mould, it prevents the interface from any mechanical stresses ensuring an undisturbed diffusion. The drops allow a radial symmetric look on the interface diffusion, allowing a one-dimensional modulation. Although the impact of volume diffusion has to be accounted for by adding a second dimension, it is mainly assumed to take place perpendicular to the interface. Silicone rubber was prepared at a 10:1 volume ratio of resin to curing agent, mixed thoroughly and degassed several times. Silicon wafers (3 inch) were used as substrates. They were cleaned via inverse sputter etching of about 4 nm  at a power of 200 W and a working pressure of 8 µbar. Substrates were heated at 200 °C for 10 min. A Cr/Ni adhesion layer with a thickness of 4 nm was sputter deposited  at 400 W power, succeeded by 200 nm copper deposited at 500 W. Substrates were tempered at 200 °C for 10 min. Layer thickness was measured via white light interferometry and phase shift interferometry .In order to vary the surface energy leading to a different contact angle to investigate the impact of the edge geometry (starting point) on the interface diffusion process, some wafers were additionally vapour coated with silicon oil SF V50 at room temperature .Directly after removal from the sputter system or after the siliconization process, respectively, several drops of 40 µL volume of the prepared PDMS were set with a cannula directly on the surface of the wafer. The PDMS was then polymerized at 40 °C for 48 h. The low temperature was chosen due to the different temperature coefficients of expansion of copper and the polymer, which would lead to thermal stresses in the interface. Moreover, the process at 40 °C lowered the viscosity of Sylgard-184 leading to a further expansion of the edge of the drop. This process regime resulted in drops with a diameter of 3 cm by using only volumes of 40 Before further diffusion experiments, the drop geometry was investigated, with particular attention paid to the edge geometry, since it is considered the starting point of the diffusion process. For the edge-characterization, a thin layer of platinum was applied via sputtering to provide a better image in the light interferometer.Light microscopic images show the geometry of the edge of the droplets, indicating a homogeneous surface energy in every point. Therefore, a constant contact angle was assumed for the complete edge of the droplet. This assumption was also applied for the contact angle, which was measured for the siliconized specimen. For both surfaces, an average contact angle of 5° ± 1° was determined. Nevertheless, high resolution images of the edges of the droplets on different surfaces  revealedThe edges of the specimen appeared smooth, while at higher magnifications , right aFirst corrosion experiments in selective points on the edge show how corrosion leads to delamination after minutes . We obseThe volume diffusion through PDMS towards inner metal parts was studied with cuvettes imaged in a time lapse series . Small sInterface diffusion in the metal-PDMS junction was observed within 120 min after the start of the experiment. The diffusion front parallel to the surface advances along the gradient of concentration much faster than in bulk. Because of the delayed start of bulk diffusion, we observe an isolated view on interface diffusion. With the chosen set up, bulk diffusion could be neglected. The diffusion front progresses over time for the case of undisturbed interface diffusion as shown on In the first segment, we observe a saturation curve-like progression. The front of diffusion penetrates the interface in a radial direction and corrodes the copper. This way, with progressing time, the diffusion flattens the concentration gradient along its way as the bonded corroding ions are immobilized on the copper surface and do not participate in a further diffusion process but inhibit it. With a greater travelled distance by the front of diffusion, the gradient flattens leading to saturation-like behaviour. This observation was transferred into discontinuous diffusion coefficients in our mathematical model as they change discontinuously from a constant value to another at certain concentrations. So, we modelled a moving boundary between the two phases of corroded and not-yet corroded copper, separating the region where all diffusing molecules are immobilized from the area in which they are allowed to diffuse freely as the concentration is still zero. With further progression of the diffusion, the boundary moves alongside the diffusion front. Mathematically, these kinds of boundary value problems are often referred to as Stefan-like. Stefan  used theIn his work, “the mathematics of diffusion” Crank  transferThe experimental setup is able to visualize the position of the moving boundary at any time. This experimental result has been used later to validate the outcome of our theoretical solution. According to Crank and Hill, the boundary conditions suitable for our problem are: These conditions have to satisfy Fick’s Second Law of diffusion:In order to solve these problems, mostly approximated analytical approaches are made. Crank suggests a polynomial approach for the unknown concentration profile to satisfy all boundary conditions. With Goodman’s integral method  a polynot: time and This leads to the solution of the position of the boundary according to time with X again the moving boundary, X describes the boundary in the photographs in which copper corrodes and turns from reddish into black. The square root dependency is consistent with our experimental data.The position The experimental set up allows two values of concentrations to be observed. One actively , one pasAfter a certain time, the second value of concentration gains significance. When the copper surface is corroded to a certain amount, the metal-polymer bond at the interface is compromised and the PDMS begins to delaminate. Because of the already blackened copper it is not possible to actively observe this process. Due to capillary forces, the corrosion-triggered delamination allows the corroding agent to migrate under the delaminated edge of the droplet leading to an increased jump in concentration. In this phase, a steady-state-like diffusion process can be assumed because the not-yet delaminated PDMS between the edge of delamination and diffusion front acts like a membrane. This steady-state behaviour is reflected in the almost linear part of the curve, which the diagram begins to evolve into in phase two . We hypoThe evaluation of the time lapse series of the bulk diffusion experiment  led to tThis result for the diffusivity of gard-184 . Althouggard-184  .The developed model for the first phase shows good compliance with the isolated process of interface diffusion. This model, developed as a Stefan-like moving boundary problem, is able to theoretically describe the interface diffusion process. The model agrees with the experimental values for phase one gained in our experiments, making it the first model to predict the movement of the diffusion front in the metal-polymer-interface. There are quite a few studies for interface diffusion lacking a mathematical point of view. The model developed in this study is based on the already existing diffusion mathematics formulated by Crank, which has already been validated and proven over decades.The model in this study uses Fourier’s laws of heat conduction from which Fick’s laws of diffusion were derived. Boundary conditions were inspired by the Stefan-problem, which characterize the melting or solidification of an ice cube. The one-dimensional model by Theodore R. Goodman describes a distinct melting line between the solid and liquid phase. This heat balance integral method, published in 1957, was subsequently used to describe several technical systems. For several cases, this method delivers viable solutions with minor numerical effort. Examples for applications are: heating by forced convection, heat conduction by transpiration, security analysis for nuclear reactors and also concentration-based diffusion processes ,27,28,29For polymers, the Stefan-problem is used to describe pyrolysis. The polymer is exposed to thermal radiation, increasing the temperature up to a critical temperature leading to pyrolysis. A pyrolysis front (moving boundary) is formed, which can be characterized by the Stefan problem . The equation for the position of the front of diffusion is quite simple showing a square-root-character . We alsoWith the knowledge of the correlation  between Slower diffusion in bulk allows an isolated view on the interface diffusion/delamination process in the starting phase. The results of the bulk diffusion experiments show that for further modelling, a more accurate study for the bulk diffusion is needed to take its impact in phase III into account. At this time, this represents the limit of our current model and further investigation is required.The developed model is still based on and therefore still attributable to the preceded and validated maths of Crank , GoodmanAlthough copper is not a standard material in medical implant technology, the revealed separate interface and volume mechanisms found can be further generalised to understand delamination failure between polymers and AIMD electrode metals such as platinum and platinum-iridium alloys. The interfacial diffusion with mobile phase boundaries according to the Stefan model and the diffusion of constituents of the body fluids, which leave the aqueous phase and cause damage over large areas, applies to all PDMS-metal interfaces. Although the chemical reactions of platinum metals occur at different potentials, damage is certainly observed under the usual medical stimulation conditions, e.g., in CIs. For their analysis, however, waiting for visible damage could be replaced by faster and more informative methods. Such as confocal RAMAN microscopy, for example, which works in the transmissive window of PDMS, would be suitable for this. It allows the identification of reactants, opening the possibility of detailed material parameterization for a delamination model that could support the development of digital twins of failure modes and thus improve AIMDs in the future.


Based on finite time thermodynamics, an irreversible combined thermal Brownian heat engine model is established in this paper. The model consists of two thermal Brownian heat engines which are operating in tandem with thermal contact with three heat reservoirs. The rates of heat transfer are finite between the heat engine and the reservoir. Considering the heat leakage and the losses caused by kinetic energy change of particles, the formulas of steady current, power output and efficiency are derived. The power output and efficiency of combined heat engine are smaller than that of single heat engine operating between reservoirs with same temperatures. When the potential filed is free from external load, the effects of asymmetry of the potential, barrier height and heat leakage on the performance of the combined heat engine are analyzed. When the potential field is free from external load, the effects of basic design parameters on the performance of the combined heat engine are analyzed. The optimal power and efficiency are obtained by optimizing the barrier heights of two heat engines. The optimal working regions are obtained. There is optimal temperature ratio which maximize the overall power output or efficiency. When the potential filed is subjected to external load, effect of external load is analyzed. The steady current decreases versus external load; the power output and efficiency are monotonically increasing versus external load. Similar to macro motor, micro energy conversion devices can transfer energy in micro field, such as Brownian motor and energy selective electron engine. Brownian motor is a micro-nano device that can use micro-scale energy ,2. It reFinite time thermodynamics (FTT) theory ,5,6,7 caAlthough the operating principles of micro-nano motors are quite different from those of macro motors, FTT theory is also applicable to the study of micro motors . ThermalSome scholars noticed that some irreversible losses cannot be ignored. Zhang et al.  studied For the one-stage HE, the heat in the cold reservoir (CR) is released into the environment directly and results in heat losses. To improve energy utilization efficiency, some scholars try to utilize waste heat by connecting the HEs by using FTT theory. For macro energy systems, Rubin and Andresen  modeled The combined TBHE model without an intermediate HR is also feasible in theory. When the intermediate HR is not considered, the topping and bottom TBHEs transfer heat directly through viscous medium. As a result, the temperature of CR  in Ref. , for macThe method used in this paper can also be used to research the combined TBHE model without the intermediate HR. Similar research work has been performed for macroscopic combined Carnot HE models without the intermediate HR ,109. TheFor the topping TBHE, the heat absorption rate is The temperature of VM d Bekele  firstly d Bekele . In addid Bekele  studied d Bekele  and Asfad Bekele  studied d Bekele  studied When the system works stably, the particle will drift in a directional manner. The heat caused by the friction of Brownian particles through the VM can be expressed as to Refs. ,75, the There are three kinds of heat flows for the TBHE. The first is the heat transfer rate when a particle passes through the potential field. The particle absorbs energy to overcome external load The second is the energy due to kinetic energy change when a particle moves. The heat flows can be expressed as ,123(17)The third is the bypass heat leakage rate essed as ,125,126.According to Equations (13)–(19), the total heat absorption rate The power output The total heat absorption rate The power output The total In order to simplify calculation, some dimensionless parameters are defined: the asymmetry of the potential The self-defined functions When the combined TBHE is free from external load P3*=The irreversible combined TBHE is connected by intermediate reservoir, which leads to the increased overall heat resistance increase.The TBHE exchange energy through particle moves, which is different from macro heat engine. The energy releasing from upper heat engine can’t be absorbed completely by lower heat engine and as a result the efficiency decreases.Because the intermediate reservoir is considered, the temperature of the reservoirs of upper heat engine changes and the velocity of particle motion changes. The upper heat engine absorbs less energy from the hot reservoir The performance comparison is illustrated by numerical calculation. The parameters are set as of Refs. ,108,109 It is supposed that the single or combined HE is operating between two heat reservoirs with fixed temperatures. For the one-stage TBHE, there are two heat transfer processes between two reservoirs. For the two-stage combined TBHE, the bottom HE has two heat transfer processes and there are four heat transfer processes in total. As a result, the overall heat resistance increases and the power and efficiency decrease. For the three-stage combined TBHE, two heat transfer processes are added, there are six heat transfer processes. As a result, the power and efficiency decrease continually. Therefore, as the number of HEs operating in tandem increases, the overall heat resistance increases and the power and efficiency decrease. Some scholars have studied the macro combined Carnot HE model and systematically expounded the effect of heat resistance on the performance of multi-stage combined Carnot HE ,109.One can find from the present analyses that, when the bottom TBHE is placed below the topping TBHE, the entire power output and efficiency can be greatly improved due to the expended working temperature of the entire system. The results can be applied to the performance optimization of the combined energy conversion systems with multiple HRs.When the temperature ratio When the potential field is free from external load, considering heat leakage and kinetic energy loss, the curves of When the potential field is influenced by external load, the steady current decreases with the increase of external load. The maximum power and efficiency monotonically increase versus the asymmetry of the potential. The power and efficiency are monotonically increasing versus external load.The overall heat resistance of combined thermal Brownian heat engine is bigger than that of single thermal Brownian heat engine, the power or efficiency of combined thermal Brownian heat engine are lower than that of single thermal Brownian heat engine.In this paper, an irreversible combined thermal Brownian heat engine model is established by FTT theory. The effects of basic parameters on the performance of the combined thermal Brownian heat engine are analyzed with or without external load. The optimal thermodynamic performance of the combined thermal Brownian heat engine is further studied. The main conclusions are:


The present study examined the association between risk perception and travel satisfaction related to the use of public transport (PT) during COVID-19 pandemic in Turin, Italy. A total of 448 PT users took part in an online survey conducted from January to March 2021. It investigated safety and risk perception related to the use of PT, and the users’ subjective experience, measured through the Satisfaction with Travel Scale (STS). These perceptions were compared for three time scenarios: before the pandemic, during the pandemic, and in the future at the end of the pandemic emergency. Results showed that COVID-19 influenced respondents risk perception both during the pandemic and in their projections about the future, especially for females. The risk of contagion from COVID-19 is perceived as higher inside a PT vehicle than in the adjacent/waiting spaces. Regarding travel satisfaction, the overall scores of the STS indicated that the pandemic has impacted reported well-being while travelling, both now and in the future. The dimension of activation shifted towards the negative pole and did not indicate a return to risk perception before the pandemic levels at the end of the crisis . Respondents reported a significant decrease in their level of pleasure and satisfaction during the pandemic, but expect that in the future these levels will go back to the levels previously experienced. Regarding travel satisfaction, PT users aged 36 to 50 years reported the highest level of satisfaction, while younger users (18 to 35 years) reported the lowest degree of satisfaction in all three time scenarios. Overall, the results clearly pinpoint that health-related perceived risk is becoming a key determinant for PT use. Within this context, different dimensions of travel satisfaction proved to be impacted differently by the pandemic, for both current and future scenarios. On 11 March 2020, the World Health Organization (WHO) . The sigRosa  outlinesResearch on risk perception in relation to COVID-19 has highlighted that women present higher levels of risk perception than men , and oldNonetheless, the perceived risk of potential for infection and the attitude towards this risk have proved critical elements in determining the reduction of mobility during this pandemic emergency ,4.Travel satisfaction is defined here as a specific satisfaction measure connected to subjective well-being based on how travellers evaluate their daily travel experiences . From a As seen before, in existing studies, risk in relation to PT use is measured in terms of perceived possibility of being a victim of a crime (e.g. harassment or violence) or an accident  –43, and Before the pandemic, previous studies conducted in the European context showed that users were generally satisfied with PT services, though satisfaction with daily travel differed significantly between urban areas ,44. ThesA number of studies argue that several socio-demographic variables are associated with different degrees of travel satisfaction –49. One This study is part of a new line of research which aims to develop an understanding of the psychological mechanisms underlying mobility choices and behaviours during the COVID-19 pandemic and provide future projections in relation to this ,39. SpecThese topics were investigated by collecting and comparing measures of risk perception and travel satisfaction with reference to three different time scenarios  and considering age and gender differences. Hence, the data collected was: retrospective; related to the present ; and related to future expectations.In a period of pandemic and for post-pandemic projections, providing mobility that is safe from a health perspective while continuing to support sustainable modes of transport, is a central challenge for PT agencies and all stakeholders involved in health and transport. Within this framework, understanding users’ satisfaction with travel and users’ risk perception, are key factors for transport planning, service improvement and effective communication to address PT users’ needs in relation to information and reassurance.The study was conducted using a self-administered, cross-sectional online survey among a total of 517 unpaid volunteers who were all users of PT in Turin, a metropolitan area of northern Italy. Participants were recruited through local media advertisements with an invitation to participate in a COVID-19 and mobility choice online survey. Of the participants in the initial sample, 69 (13%) did not complete the questionnaire or were excluded according to predefined exclusion criteria. Subjects who were not regular or occasional users of PT in Turin metropolitan area were excluded. PT users under 18 were also excluded because they needed a special consent from their parents to take part in the survey and we did not have the possibility to check for such permission appropriately. Moreover, we chose to not include participants over 70 years old, since they were strongly recommended not to use PT unless necessary during the considered period. Hence, the sample ranges from 18 to 70.The final sample included 448 participants divided into three age groups: 39% aged 18 to 35 years; 32% aged 36 to 50 years; and 29% aged 51 to 70 years. Across these age groups, 57% were women, 42% men and 1% chose to not specify. All demographic characteristics for this study are presented in The responses were collected over a period of seven weeks from January to March, 2021, during a period where mass restrictions for PT passengers were imposed to increase safety .Survey response time was monitored to flag any respondents who did not spend an adequate amount of time answering the questions. Prior to completing the survey respondents were asked to read a written introduction explaining the study, and to provide their informed consent. The collected data was analysed anonymously. The study was approved by the local ethics committee and conducted according to the Declaration of Helsinki, and informed consent was obtained from every participant.The survey took approximately 15 minutes to complete, comprised of four sections with most responses expressed using a Likert-scale. The first section of the survey aimed to collect data about: socio-demographics; compliance with security rules aimed to protect PT users from COVID-19 transmission ; attitudes toward the use of face masks; and trust in the authorities.The second section of the survey aimed to capture transport mode choice and moving habits. Seven options for mode of transport were provided: (1) Car, (2) PT, (3) Bike, (4) Bike sharing, (5) Car sharing and (6) Walking.The third section of the survey investigated perceptions of risk and safety in the use of PT during the COVID-19 pandemic emergency in relation to the possibility of infection. Finally, the fourth section collected data about travel experiences as measured through the STS. Perceptions of risk, safety and travel experience were investigated for each of the time scenarios: before, during and after the pandemic. This paper focuses on the results of the third and fourth sections of the survey.To investigate risk perception in the use of PT, the survey asked participants: ‘How do you perceive the risk of being infected with COVID-19 while traveling on public transport?’ A version of this question was asked for each of the three temporal scenarios . Respondents answered on a 7-point Likert scale ranging from 1  to 7 (extremely risky).To the best of our knowledge, there are no scales designed to measure health risk perception in relation to the use of PT. As such, a new risk perception measure was developed for this study and used to capture user perceptions during the COVID-19 outbreak. The scale employed was developed from an initial pool of 12 items identified within a literature review on the topic of risk perception investigated from a psychological point of view –28,52–54Traveling in a subway was chosen as prototypic example of in-vehicle experience because of it being underground, possibly crowded and needing ventilation efficiency. Responses were provided on a 7-point Likert scale ranging from 1  to 7 . The second subscale (composed of 6 items) investigated risk perception related to the experience of staying or moving in the adjacent/waiting spaces. Responses were provided on a 7-point Likert scale ranging from 1  to 7 (extremely risky).Travel satisfaction in the three different time periods  was measured using the STS, an instrument that measures customer service experience relating to travel without focusing on any particular travel mode.STS consists of two affective dimensions and one cognitive dimension see  35]. Th. Th35]. p < .05). Gender also proved to have a significant effect on risk perception , with female PT users indicating higher levels of risk perception. A significant interaction effect of gender and temporal scenario was present only for the future scenario , with female PT users declaring higher levels of risk perception. The demographic variable age showed no influence on participant perception of risk. The mean value of the items was calculated for each of the two risk perception subscales. The distribution of answers to the question: ‘How much do you perceive the risk of being infected with COVID-19 while traveling with the public transport?’, related to the three different time scenarios summarised in A repeated measure ANOVA was performed to compare the global scores of travel satisfaction and each of the three dimensions of travel satisfaction in the three temporal scenarios. Regarding the global scores, a significant difference emerged . Tukey HSD showed significant differences among all three temporal scenarios, with the highest levels of global travel satisfaction before the pandemic (p ≤ .05) and the lowest during the pandemic.Considering each one of the STS dimensions separately, repeated measure ANOVA revealed that the dimension of activation was significantly different in the three time scenarios  and Tukey HSD showed significant differences among all three scenarios with the highest levels of activation during the pandemic and the lowest before the pandemic (p ≤ .01). With regard to the dimensions of pleasure and satisfaction, ANOVA showed a significant effect of the time-scenario  = 354, p ≤.05; F  = 1.83, p ≤ .001, respectively). For both pleasure and satisfaction, Tukey HSD revealed no significant differences between the conditions ‘before the pandemic’ and ‘after the pandemic’. But a significant difference was found between ‘before the pandemic’ and ‘during the pandemic’ and for ‘during the pandemic’ and ‘after the pandemic (p ≤ .05), with the highest levels of pleasure and satisfaction recorded for ‘before the pandemic’  = 553, p ≤ .01]. When considering the three different dimensions of the STS separately, an interaction effect of gender and scenario emerged for activation, with males scoring higher rates of activation than females in the post-pandemic scenario . Regarding the pre-pandemic and pandemic scenarios, no significant differences emerged between males and females for activation and no effect of gender emerged for pleasure and satisfaction.The demographic variable of age did not have any significant effect on the global score of the STS, except for the period before the pandemic  with younger users recording lower scores for global satisfaction. Regarding activation, scenario showed a significant main effect , since activation level increased significantly during the pandemic. In addition, an interaction effect of age and time scenario emerged regarding activation , with younger users significantly more activated than older users in the post-pandemic scenario. Regarding the satisfaction dimension, both scenario and age showed significant main effects . Finally, age showed no significant effect on the pleasure dimension. A summary of the main and interaction effects of all variables reviewed in this study are shown in Tables To further investigate the association between risk perception and the three dimensions of the STS in each of the three temporal scenarios, a correlation analysis using Pearson’s correlation coefficient (r) was performed. Risk perception and activation were found to be negatively correlated, despite their relationship being weak. The strength of the correlation increased from the risk perception before the pandemic (r (448) = -0.22, p < .001) to the risk perception during the pandemic (r (448) = -0.30, p < .001) and the risk perception after the pandemic (r (448) = -0.33, p < .001) see . The corA Sobel test was also used to verify the mediating effect of the risk perception during the pandemic between the global STS global score before the pandemic and the global STS global score after the pandemic. First, results of simple linear regression showed that the global STS before the pandemic was a statistically significant predictor of the global STS after the pandemic . Next, when the mediator, risk perception during the pandemic was entered in the regression analysis, the STS global score before the pandemic proved smaller . On the other hand, the mediator emerged as a significant predictor of the dependent variable . To further investigate the mediator, the Sobel test was utilized to examine if risk perception during the pandemic significantly mediated the relationship between the two variables. The results confirmed that the mediator significantly mediates the relationship between the STS global score before the pandemic and the STS global score after the pandemic  were compared and the effect of gender and age was considered. Satisfaction with travel and risk perception in relation to the potential for the spread of COVID-19 infection, were considered two of the most important variables to understand potential impacts on mobility choices currently and into the future.Regarding risk perception over time, the results of this study suggested that the COVID-19 pandemic emergency significantly changed the perception of possibly contracting infectious diseases when traveling on PT, both now and for the future. As such, risk perception related to COVID-19 contagion should be considered as an important predictor of travel choices in the future . For thiThe results here presented highlighted a gender effect in risk perception, with women perceiving higher levels of risk, independent from the current pandemic emergency. Such results are consistent with literature on risk perception which shows gender-related effects ,63, for Regarding travel satisfaction, the STS global scores collected in this study indicated that the subjective experience of wellbeing during PT travel has changed across the three temporal scenarios .As far as age is concerned, the PT user group aged 36 to 50 years were identified as being the most satisfied with travel experiences across all three scenarios. This was followed by the group of older users aged 51 to 70 years and the younger group aged 18 to 35 years. This result is consistent with the literature that has found that users aged between 40 and 49 years are more likely to be satisfied with their journey  despite When considering the scenario ‘during the pandemic’, no significant difference in terms of age or gender emerged in this study. This suggests that the disruptive experience of the COVID-19 pandemic is far more impactful than other socio-demographic variables.Distinguishing the three dimensions measured by the STS, activation was the most negatively affected by the pandemic. The pandemic has increased respondents’ feelings of being worried or stressed about the prospect of using PT, and has increased a feeling of being hurried, especially for women.The youngest age group in the study experienced the highest level of negative activation in the use of PT in the period before the pandemic. This result is consistent with other studies that have identified that traveling by PT is stressful for this age group . In the While activation remained high in users’ projections of future PT experience, satisfaction and pleasure levels were currently impacted by the pandemic but should return to previous levels in the future. Feelings of worry and stress related to the risk of contagion remain high in PT users’ future projections, perhaps influenced by the subjective well-being of urban PT users and their choice to use PT.Interestingly, the data about travel satisfaction appears consistent with data about risk perception. In general, worry (measured by the STS in the activation dimension) can be understood as an emotional response to stress, potential risks or actual risks.Indeed, the correlation between risk perception related to the COVID-19 pandemic and the activation dimension of the STS strengthens over time, highlighting that this pandemic has changed the parameters by which PT users evaluate the travel experience.This result suggests that users’ perception of safety in relation to health is a critical factor influencing their overall satisfaction with PT travel. Such interpretation is strengthened by the results of the proposed mediation model: the global satisfaction with travel before the pandemic affects the global satisfaction with travel in the future through the partial mediating effects of risk perception during the pandemic, confirming that COVID-19 risk perception is nowadays a central issue when traveling by PT. Based on these findings, it can be concluded that risk perception plays an important role in the process of satisfaction evaluation that in turn may influence intentions and decisions about transport mode use.In sum, the understanding of the travel satisfaction will increasingly pass through the perception of risk. In this sense, risk perception has become one of the determinants of the overall level of the travelers’ satisfaction. It also means that travelers are aware and take risks  into account when evaluating the satisfaction with PT. These findings provide stakeholders with evidence about the importance to act for making PT safe and communicating it as such.Despite this investigation regarded a relatively limited urban area in north Italy (Turin), the present study may be mostly representative of many urban/metropolitan areas of regions and countries featured by similar socioeconomic conditions and public transport density, that were affected by the pandemic contagion and the related measures to contain the infection during the first period of the COVID-19 outbreak. However, caution should be given when generalizing the results of this study worldwide. In fact, for example, the sample is not representative of all regions in Italy but only of those areas where the experience of the COVID-19 pandemic has been dramatic due to a far high number of infections. However, we may assume that risk perception and concern for contagion are similar throughout Italy, in part because when the Italian government extended the lockdown millions of citizens living in the Northern regions  fled South on the last departing trains and buses. Two more limits need to be noted: first, this study did not adopt standardised measures to assess psychological distress or attitudes to risk-taking, instead we developed a questionnaire ad hoc that aimed to assess the constructs of interest. Second, some retrospective questions were used in order to collect travel satisfaction data. As such, the responses to these questions could be affected by psychological factors (e.g.: emotions and prejudices) involved in the recall of past experiences. The limitations of retrospective measurement of travel satisfaction and risk perception requires comparative data to be collected in the near future to enable more accurate longitudinal analyses.However, despite these limitations, this study offers insights to better understand the complexity of psychological determinants in the use of PT in Italy, one of the countries hardest hit by the COVID-19 pandemic. It will be vital that stakeholders and decision makers responsible for PT policies consider the psychological processes investigated and analysed in this study.There are several possible future lines of research arising from this work. One key area is examining changes in mobility patterns in advance and after the COVID-19 pandemic. Focussing on other socio-demographic variables  and comparing different parts of Italy, would also provide further insights into the factors influencing risk perception and travel satisfaction during this pandemic. It would also be interesting to include other psychological variables  that may account for individual differences and/or cross-cultural differences in risk perception and travel satisfaction.The present study results highlighted that COVID-19 influenced PT users’ risk perception and their travel satisfaction as measured by the STS, in particular the dimension of activation, both during the pandemic and in their projections about the future. Interestingly, especially women’s perceptions proved to be influenced by the pandemic outbreak. Moreover, according to our findings, it can be concluded that risk perception plays an important role in travel satisfaction evaluation that in turn may influence PT users’ decisions about travel choices.Our results provide a possible interpretation of some of the determinants underlying the mobility shift recorded since the outbreak of the COVID-19 pandemic, highlighting the role of health-safety concerns and travel satisfaction over time. The increase in levels of activation and risk perception during and after the pandemic may indicate a permanent change in how people think about and the views they hold in relation to PT use.Within this framework, this study explained that the COVID-19 outbreak—a period of uncertainty and risk—makes it relevant to assess factors- as risk perception- that influence travel satisfaction.Notwithstanding its limitations, this research provides PT stakeholders with insights into certain psychological reactions to PT use in the pandemic (particularly in relation to age and gender variables) for consideration in the development of risk communication campaigns now and in the future.These results highlight that the future of PT in cities, the satisfaction with travel by PT and the willingness of passengers to use it once the COVID-19 pandemic is over, are deeply influenced by the perception of risk of being infected by this contagious disease . This needs to be a key consideration in the management of PT services and for communication with PT users in relation to safety measures adopted by the PT organisations and the behaviour required of users. The open challenge for the urban PT system in response to the public health security crisis is to meet the needs that have changed during these emergency situations. Communication will have a central role for such purpose, by enhancing passengers’ risk awareness and limiting unnecessary or disruptive beliefs and incorrect risk perception. Namely, effective and targeted communication, designed on the basis of scientific knowledge about specific risk perception and subjective levels of travel satisfaction, may contribute in empowering passengers’ knowledge about safety measures adopted by PT companies, and in increasing their vigilance and correct attitudes regarding healthy and safe behaviours.On the other hand, PT stakeholders are requested to re-designing various aspects of PT services in order to avoid the spread of COVID-19 virus and other prospective diseases, so as to maintain a satisfactory level of safety thanks to preventive scientific-based measures . Furthermore, strategic and operational re-planning regarding travel patterns is needed in a medium-long term perspective to provide robust responses in case of future outbreaks.In conclusion, the impact of COVID-19 pandemic has represented an enormous challenge for the PT system and as a consequence for the issue of sustainable transportation. Nevertheless, it also presents an opportunity to deploy new safety measures and travel conditions that may in addition improve PT users’ safety perception and travel satisfaction more comprehensively.


Oil and gas (O&G) explorations moving into deeper zones for enhanced oil and gas recovery are causing serious safety concerns across the world. The sensing of critical multiple parameters like high pressure, high temperature (HPHT), chemicals, etc., are required at longer distances in real-time. Traditional electrical sensors operate less effectively under these extreme environmental conditions and are susceptible to electromagnetic interference (EMI). Hence, there is a growing demand for improved sensors with enhanced measurement capabilities and also sensors that generates reliable data for enhanced oil and gas production. In addition to enhanced oil and gas recovery, the sensing technology should also be capable of monitoring the well bore integrity and safety. The sensing requirements of the O&G industry for improved sensing in deeper zones include increased transmission length, improved spatial coverage and integration of multiple sensors with multimodal sensing capability. This imposes problems like signal attenuation, crosstalks and cross sensitivities. Optical fibre-based sensors are expected to provide superior sensing capabilities compared to electrical sensors. This review paper covers a detailed review of different fibre-optic sensing technologies to identify a feasible sensing solution for the O&G industry. In recent years, the recovery of hydrocarbons has become more and more difficult and challenging as exploration and production operations tend to seek new fronts into deep and ultra-deep harsh environments. As the energy demand continues to rise, there is a need for efficient management and optimization for production operations and systems to make this growing energy demand sustainable. This requires real-time monitoring of long and deep oil wells.Sensors and sensing data are vital elements in the oil and gas (O&G) industry. O&G exploration and production have been moving into unconventional depths (more than 3 km) in order to meet the growing demand for energy . This reExploration and production process monitoring helps to prevent or detect health and safety issues and to significantly enhance O&G production . DetectiCurrent O&G sensing techniques are mainly based on electrical sensors, which have many constraints when used in adverse environmental conditions . These eFibre-optic sensing technology can overcome the aforementioned limitations of their electrical counterparts, mainly due to their small size, electrical isolation, corrosion resistance, immunity to EMI and capability to operate in extreme environmental conditions ,15. The The goal of this review is to explore optical fibre-based sensing technologies that can significantly boost the performance and withstand extreme conditions prevalent in offshore O&G environments. Sensing in the O&G sector involves the measurement of different parameters like pressure, temperature, vibration, flow and acoustics. Sensing should be carried out throughout all stages of O&G production, which means that the sensor should be active on the surface, along the pipeline and even in the down-hole. The measurements are to be taken throughout the well, from the surface to the total depth of the oil well reservoir . Figure The elemental part of optical sensor design is the identification of the key technology which best suits the needs of the O&G industry. There are different optical sensing methodologies, but the selection needs to be carried out considering the environmental conditions, application , level of sensitivity and accuracy required in physical parameter measurements.With the worldwide decrease in oil reserves, exploitation of challenging reservoirs has rapidly started. In comparison to standard wells, the overall production of such reservoirs entails high and complex performance instrumentation, for example, fibre-optic distributed pressure, distributed strain monitoring, distributed temperature monitoring, etc. The development of fibre-optic sensory technology with regards to the oil and gas industry is currently on the rise and represents the future of well monitoring. Monitoring and data transmission via the utilization of fibre-optic sensors and standard optical fibre in cabling is currently common in the refining of natural gas and standard crude oil throughout the world. With the trend of hydrocarbon consumption outpacing its given discovery, techniques in Enhanced Oil Recovery (EOR) are being deployed worldwide so as to increase the apparent recoverable assets in the known reservoirs. Fibre-optic Monitoring represents an opportunity for the current oil and gas industry to manage and subsequently optimize its resources in a more effective manner and provide real-time data in a continuous way without interrupting production and reducing well intervention. Visiongain had forecasted that the expected expenditure on Fibre-Optic Monitoring by the gas and oil industry will be increasing globally . The risThis review paper mainly aims to cover sensors for oil and gas downhole applications. The following An optical fibre sensor is composed of an optical fibre, transducer or a sensing element, detector and a light source . OpticalFibre-optic sensing technology has several inherent advantages which makes them very attractive for a wide range of industrial sensing applications. The optical fibre is typically made up of a cylindrical waveguide that consists of a thin core with a refractive index covered by a cladding layer with a refractive index usually lower than that of the core for a single-mode fibre (SMF).An SMF is made up of the core and cladding layers which are usually from fused silica. Light is propagated through the core of the fibre by total internal reflection (Snell’s law)  and to aStandard telecom fibres are protected by coatings/buffer which protects the optical fibres from mechanical and environmental stresses. The primary coatings are applied on single or dual layers. The buffer material provides an additional layer of protection. For industrial applications like oil and gas, with harsh environments/HPHT (High-Pressure High Temperature) field conditions, special polymer coatings like polyamide ,26,27 anPCFs are a special class of optical fibres with a complex refractive index profile that employs a microstructured arrangement of low-index material in a background material of higher refractive index . NormallOptical fibres can be used as sensors for sensing various physical parameters like temperature, pressure, strain, etc., wherein the parameter to be sensed modulates various properties of light. From the basic principle of light propagation through an optical fibre, the light propagation changes when subjected to varying environmental conditions such as temperature and strain. By analysing the changes of the light properties through the fibre, the environmental conditions themselves can be determined. Fibre-optic sensors have been designed and developed to measure a wide range of physical parameters such as pressure , temperaFibre-optic sensors are further divided into two subcategories: intrinsic and extrinsic sensors. For intrinsic sensors, the light is confined within the optical fibre in which the physical quantity acts on. The performance of intrinsic sensors is largely dependent on fibre materials. While for extrinsic sensors, the light exits the fibre, get modulated by the external perturbation and is relaunched back into the fibre. The performance, in this case, is largely independent of the fibre material but on the sensing element.Propagation of light through the fibre is usually used to determine measurement information by spectral characterisation (wavelength/frequency and intensity), phase characterisation (interferometry) and polarisation. In the following sections, operation principles based on various techniques stated above are reviewed in detail.Wavelength modulation-based optical sensing is one of the spectral techniques which measures physical parameters by detecting a change in wavelength when the optical fibre interacts with the measurand. The most common type of wavelength modulated optical sensing is fibre grating-based sensing. A grating can be produced when the core of the fibre is exposed to an intense UV laser light and was first discovered in 1978 by Hill . The fibIntensity modulation-based sensing is another spectral technique that measures physical quantities based on the principle of direct detection of the change in optical power in either reflection or transmission. They are inherently simple devices where light from an optical source is propagated through the fibre and the intensity is altered at the transducer which is then returned to an optical detector. The light intensity detected by the detector is a function of the physical quantity measured. There are three different classes of intensity modulation-based sensing as shown in The transmission coupling-based sensor consists of two fibres with a small gap between them, wherein the amount of light coupled to the second fibre depends on the fibre acceptance angle and the distance between the fibres. One of the fibres can move in response to vibration or pressure thereby changing the distance between the fibres and hence the coupling loss . The refIntensity modulation-based sensors made with multimode fibre-optic micro bend have successfully been commercialized. It is based on the operating principle in which the mechanical periodic micro bend coupled the energy of both the radiation and guided modes thereby resulting in the attenuation of the transmitted light. Sensor configuration can be designed and constructed in such a way that the mechanical mirobending device transfers the applied perturbation to the optical intensity change. The micro bend modulated sensors have been reported to have good performance characteristics like good resolution , howeverPhase modulation-based sensing uses interferometric mechanisms such as the Mach–Zehnder, Michelson, Sagnac and Fabry–Perot to measure physical parameters see Fabry–Perot interferometer is generally designed to measure physical parameters through the formation of a cavity with two parallel reflective surfaces. When light is propagated through the Fabry–Perot cavity, multiple interferences of light are formed caused by the multiple reflections between the two reflective surfaces. The Fabry–Perot interferometer is subdivided into two categories, intrinsic Fabry–Perot interferometer (IFPI) and extrinsic Fabry–Perot interferometer (EFPI). In IFPI, the light is confined and modulated within the fibre. The cavity length and modulation are from within the confine of the fibre. In EFPI, the light exits the fibre and is modulated before being relaunched back into the fibre. The cavity length is formed outside of the fibre and the fibre serves as the medium for transmitting light into and out of the Fabry–Perot cavity. Different configurations for FP have been proposed and developed. One configuration is to place a different fibre aligned to the first one which forms a cavity between them and is then packaged using a glass or silica, see The IFPI is usually fabricated by splicing a special fibre and coating the two end-faces with a reflective film. The output signal is generated by the superposition of the multiple reflections at the end-faces of the special fibre. These reflections are a result of the reflectance of the coating, the cavity and the refractive index of the fibre. Whenever there is a change in the cavity length or the refractive index, the interference output can be tracked thereby being able to measure any physical quantity that caused the changes of the optical properties .In EFPI, the performance of the sensor depends only on the sensing element which gives it the flexibility to adapt to various sensing applications. An EFPI cavity is formed by placing an input fibre side by side to a reflecting fibre. As light is propagated through the input fibre, a fraction of it is reflected back as R1, approximately 4% and all others are transmitted into the cavity space to the reflected fibre end-face. Part of the transmitted light is also reflected back as R2 at the end-face of the reflected fibre which is then recoupled into the input fibre. Fibre-optic interferometric acoustic sensor array has established itself as a potential alternative to the conventional sonar array based on electro-ceramic transducers for underwater or subsea applications . A metalEFPI sensors have been developed and commercially available. The EFPI sensors have many advantages over the Mach–Zehnder and Michelson sensors such as their high sensitivity, small size, good flexibility, and their simple structures. These make them very attractive for various sensing applications. However, the EFPI have the potential of having low coupling efficiency due to the usual misalignment of the reflecting fibre.Liao et al. reported a high temperature (Up to 950 ºC) sensor based on micro taper in-line fibre Mach–Zehnder interferometer, which has got wide application prospect in the fields of high-temperature hot gas flow, as well as oil and gas field development . Zhao etPolarisation-based fibre-optic sensors typically involve an extrinsic birefringent component to perform the actual polarisation modulation . These sOptical sensors based on the photo-elastic effect was first introduced in 1982 by Spillman . After tFibre Bragg grating can be produced from a standard single-mode optical fibre. The core of the fibre is exposed to two ultra-violet light beams originating from the same laser source, the UV light beams constructively and destructively interfere . The resectively .FBG sensor is primarily composed of three main segments; the sensing part, which is composed of the bare FBG that does the actual sensing, the packaging and the FBG arrays; the instrumentation which is composed of the interrogating instruments and related components such as the switches, multiplexors, data acquisition system, data processing units, software and the graphical user interface; and the system integration unit which compose of project management and engineering aspects .Hill et al. observed photosensitivity of the optical fibre when exposing the germanium doped fibre core to the two coherent argon-ion laser counter-propagating radiation with elengths . The reselengths  in 1989 e theory  have beee theory . Lemairee theory  proposede theory  was devee theory  demonstre theory . Femtosee theory . Qiao ete theory . The lone theory .Recently, many fibre-optic sensors are made with FBG through the modulation of the refractive index profiles. Different types of FBG have been reported based on their coupling characteristics. First, there is short-period grating FBG . Here, t grating ,80. In t grating ,82. The hift FBG ,84,85.Distributed sensing uses scattered light to detect, measure and analyse certain backscattered characteristics and changes that occur in the optical fibre. Rayleigh, Raman and Brillouin type scattering are used in order to measure those changes. Distributed systems are based on two main techniques: optical time-domain reflectometry (OTDR) and optical frequency domain reflectometry (OFDR) with the purpose of measuring the optical loss distribution along a fibre in order to evaluate along the fibre its condition and to detect, locate and quantify the losses and defects all along. In this principle, a pulse of light is launched through the fibre and the optical properties of the backscattered light are measured .There are three main principles for distributed sensing, the backscattered light contains three spectral parts: Rayleigh, Raman and Brillouin as shown in Raman scattering contains information on the temperature to be able to calculate the temperature along the fibre. It is composed of the Anti-Stokes band (strong dependent on temperature) and the Stokes, which can give temperature with high precision at the location that the scattering is taking place. It is only temperature-dependent.Rayleigh scattering effect is elastic and is used for both temperature and strain monitoring. There are no gains or loss of energy meaning that occurs at the same wavelength as the light signal.The main characteristics of this system are a high resolution of measured parameters and short spatial resolution, but the maximal length of the sensor is limited to 70m. Therefore, this system is suitable for monitoring localized strain changes over relatively short distances . Most diBrillouin scattering is also temperature- and strain-dependent. The wavelengths of the Brillouin, Stokes and Anti-Stokes peaks occur close to the input signal, so sophisticated filtering methods are needed to filter the Rayleigh signal and improve SNR. Since Brillouin scattering is both temperature- and strain-dependent, most DTSS (Distributed Temperature and/or Strain Sensing) systems are based on Brillouin scattering . VariousThe DTS use the OTDR operating model based on Raman scattering in the optical fibre , which tTypical well monitoring applications of the DTS include flow metering, flow assurances, leak detection, gas-lift surveillance, and permanent temperature logging ,89. DTS The DAS uses optical fibre for sensing and telemetry. The DAS is an OTDR-based system that uses Rayleigh backscatters in a single-mode optical fibre to measure sound frequencies over large distances and in harsh environments. It is mostly used in the sensing of strains in well systems. Using an acoustic-based system allows the operators to reconstruct the source event and subject it to real-time analysis. The optical pulse that is propagating along the fibre experiences attenuation along the optical fibre. The acoustic signals obtained from each position along the fibre from the processed backscattered light are then analysed and sent to a display .The DAS system is ideal for the detection of relative strains wherein the operators use the optical fibre’s index of refraction and speed of light to determine the distance along the fibre that is experiencing the strain. However, the accuracy of the measurement is dependent on the distance that the light probe travels. For this reason, the DAS system pulses the light strobe a thousand times per second to obtain a high signal to noise ratio . The carDAS is preferable in wellbore diagnostics to monitor various factors. It can be used to determine the location of a leak flow caused by pressure bleeding. It can differentiate a single-phase flow from a two-phase flow. The peak noise can estimate the rate of leaking. The system can also distinguish between the matrix flow, borehole flow, fracture flow, and channelling . The appThe DTSS is based on the Brillouin scattering mechanism that employs the OTDR system. The system analyses both the thermal and strain effects of the target object by combining both the DTS and the DSS system. The DTS measures temperature properties while the DSS provides measurements that are used to the location and severity of deformation in well casing. The DSS also provides insightful data that estimate the stresses inherent at perforations during oil output stimulations. Most DTSS systems are based on the technology that combines the Brillouin Optical Time Domain Reflectometer (BOTDR) with the Brillouin optical time-domain analysis (BOTDA). The BOTDA preceded the BOTDR in sensor detection. It used two lasers that counter-propagate by exploiting the benefits of Brillouin amplification. However, the system was limited to a temperature accuracy of 3 degrees Celsius and a spatial resolution of 100 m over a sensing length of 1.2 km. The BOTDR was introduced because it could monitor the system from the opposite end . As a reThe DTSS system is ideal because it can use loop or single end measurements . For thaThe core principle of DPS is the conversion of hydrostatic pressure acting along a coated optical fibre, into a distributed mechanical strain . MeasureThe DPS is preferable because it does not require expensive optical devices. A light source and a light detector are the main elements that are needed in this technology. However, the DPS has some inefficiencies based on the intrinsic nature of the measurements. For example, the intensity of light often depends on quite a number of factors that cannot be controlled easily or cannot be controlled at all. The light source’s intensity of light is often subject to fluctuations because of certain factors such as temperature changes and ageing . OxidatiChemical sensors in fibre-optics operate in a manner that allows the transportation of the light by intensity or wavelength in order to provide vital information concerning the analytes in the immediate environment that surrounds the sensor .A distributed chemical sensor is normally developed by coating multiple Bragg gratings in a single glass fibre with chemical responsive coatings. In this configuration, FBGs are quasi-distributed or distributed point sensors.2S and the CO2 in carbon capture reservoirs and approximating the identity of specific molecules in diverse media and under extreme conditions. A DCS modified by the coated fibre Bragg grating will produce an axial strain in the fibre in the presence of a chemical compound [A chemically selective layer is used to replace a part of the fibre’s cladding. The layer detects the environment in terms of light polarisation, reflectance, and absorbance changes. The light or propagation characteristics of the optical fibre measure the changes in the target object. Typically, the DCS is vital in the detection of formation water, enhanced oil recovery water breakthrough, Hcompound . The meaHowever, this shift is only useful in determining the quantitative properties of the chemical compound. The accuracy of the results is also prone to external factors such as coating sensitivity, methods of processing the coating to the FBG, and the approach used to optimize the chemical selectivity of the polymeric coatings. For that reason, the selective measurement of a particular chemical compound will require the operator to tune the polymer coating on the FBG to suit the analytes .In recent times extracting oil and gas from reservoirs comes with unique challenges, increase in demand for energy with a shortage of oil produced has driven the petroleum industry to search for oil in more complex reservoirs. Due to the complexity of these reservoirs, the strategy for oil production has become more difficult and often economically impossible utilizing available technology. Reliable and efficient data are required for making decisions to combat the issues with uncertainty. Fibre-optic DTS based on the principle of Raman backscattering is more efficient for oil and gas well monitoring than conventional electronic sensors. However, one critical issue associated with the DTS system is the complexity in the intensity of the backscattering profile; this is as a result of local attenuation affected by physical perturbation as well as pure temperature effect . DiffereFibre cable produced by manufacturers has different attenuation rates; attenuation can be enhanced during installation or thereafter by bending or twisting of the fibre cable, tension and chemical ingression of hydrogen gas which is very common in the oil and gas industry ,105. TheFurthermore, in the event of hydrogen darkening optical transmission capability of the fibre is limited. Space limitation can also pose a huge problem. Hence this method cannot be applied in all wells. Increased noise in temperature trace is prominent in the double-ended system due to the long length of fibre cable and consequently requires more time for data acquisition during measurement to reduce the noise level.Another typical way in which this problem could be resolved is by sectioning out areas in the optical fibre cable known to have nonlinear attenuation and applying the Differential Attenuation Factor (DAF) for each section . The issThis method may not be practical in cases where the attenuation rate changes continuously in wells. In instances where the fibre is installed in a well, acquiring the correct value for DAF will be difficult or impossible in most cases because the temperature at the end point of the fibre is required to obtain the DAF using this method .The possible alternative to resolving these issues is the deployment of a single-ended optical cable configuration which can achieve the same accuracy as the double-ended configuration. DTS system works with the principle of Raman scattering for temperature measurement. The Raman signal has two bands, namely Stokes and anti-Stokes bands. To carry out this correction using a single-ended system, an extra light source is needed with a wavelength chosen in which the Stokes wavelength of the secondary sources (correction light source) coincides with the backscattered anti-Stokes wavelength of the primary source (measurement light source). The Stokes and anti-Stokes signals filtered from the two light sources produce the same attenuation rate during backscattering. This technique cancels out the DA automatically ensuring temperature measurements are accurate.Fibre cable can be set up in oil and gas wells using three methods of configurations: single-ended, duplexed single-ended and double-ended configuration . The metAll the configuration method applies the same principle of acquiring temperature data. The respective power level of the anti-Stokes and Stokes signals Stokes ratio) at each point of measurement along the length of the fibre in the well are compiled by the DTS system to generate the distributed temperature measurement. Temperature change across the fibre length at any given point will affect the Stokes ratio at the location of that point. The Stokes ratios of the entire length of the fibre are collected and interpreted by the DTS system . MisleadThe single-ended installation is a straightforward system configuration of the fibre-optic cable in the well with one end connected to the DTS instrument and the other end ran into the well for measurement over the area of consideration. This system assumes uniform differential attenuation along the entire length of the cable for calibration purposes. However, damage in the fibre cable or connector points and fusion splices can affect the attenuation rate; this phenomenon is known as step losses . This tyIn a well that requires DTS run over a long period of time, there is a great chance that different fibre or instrument boxes may be used over time, it is then essential for accurate calibration of the fibre cable to compare data. The temperature must be known down-hole to obtain accurate calibration for this configuration, a separate wire  can be run in the same capillary line to take measurement of the temperature at the end of the fibre line .If the temperature is known at the end of the fibre cable, the non-linear differential attenuation rate can be resolved by sectioning areas of the fibre with different attenuation factors and applying correction on the DTS system . HoweverPartially returned fibre is also another installation method. It is pumped down to the required depth and returned partially back up the hole through turnaround subs. To terminate fibre movement at the predestined retuned depth, a valve is set at that point. It is expected in such a method that symmetry is exhibited about the turnaround sub for both sides of the fibre cable during the measurement for linear differential attenuation.Double-ended fibre installation is similar to the duplex single-ended configuration other than the fact that the returning fibre cable from the turnaround sub terminates at the surface and is connected to the DTS instrument box. Here the instrument box makes observation from both ends of the fibre cable. This method offers the possibility of calibration for accurate survey in the presence of nonlinear differential attenuation along the length of the fibre cable hence unlike the single-ended, in a duplexed configuration, the entire length can be considered for correction . The douIn the event of a non-linear differential attenuation condition, the double-ended system can effectively handle this issue. From To fully understand how DTS works, it is imperative to recognise the various components that make up this system. The DTS instrument box has a laser source that is capable of launching multiple laser light pulses into the fibre-optic line through the directional coupler .x axis representing the distance (km); it is a graphical representation of the scattered intensity over distance along the fibre cable. As shown in the diagram, linear attenuation can be seen from point A to B indicating that the attenuation rate along the fibre at that section is constant. Point C is an indication of the reflection and loss on a connector while D is an indication of the splice. It is important to note that the attenuation rate can be affected by splice or connectors along the fibre cable. Section E is an indication of a damaged part of the fibre cable where attenuation is large and fluctuating. The method of optical fibre installation in the well is briefly discussed below.A typical OTDR trace is shown in In the retrievable installation method , typicalIn semi-permanent installation , coil tuAnother Semi-Permanent method practised in the industry involves the installation of a stainless-steel capillary tube strapped along the outer diameter of the production tubing as shown in Fibre cable can be installed permanently in the well by running it on the surface of the casing, liner or production tubing in the well as shown in Different kinds of losses like the leakage or mode confinement losses experienced by traditional optical fibres is a major limitation, especially when looking for long-distance fibre-optic remote sensing applications ,110. FurMultiparameter sensors provide versatile sensing solutions, enabling miniaturization of the sensors and also bringing new sensor functionalities with improved measurement performances for sensing different critical parameters . This paAmong the different optical fibre-based sensors, FBG sensors have several distinct advantages and are competent to sense almost all physical parameters such as temperature, strain, pressure, vibration, etc., and also offer multi-point sensing with appreciably good range. Specific advantages of FBG sensors over other types of fibre-optic sensors are: They give wavelength encoded measurements and hence information is not susceptible to light power fluctuations . MultiplMany techniques have been already explored for the discrimination of physical parameters in FBG sensors. A common approach is to engage two FBGs in close proximity, wherein one is the sensing FBG and the other is the reference FBG which is kept isolated from one of the parameters . FurtherHowever, utilizing multiple FBGs for distinguishing various physical parameters restrict the number of sensors that can be deployed down-hole. The major factor deciding the number of FBG sensors that can be designed on the optical fibre is the wavelength range, which is also termed as the “wavelength window”. Individual sensors need room to vary up and down in wavelength corresponding to changes in their environmental parameters . One metThe intrinsic pressure sensitivity of FBGs is very low, therefore polymer coatings are required to improve its sensitivity . These pMajor challenges of multi-point multimodal FBG sensing are listed below: (i) Cross-sensitivity—it is a key problem in FBG sensors, where one physical parameter (temperature or strain) influences the value of the other. (ii) Cross-coupling—nonlinear coupling of sensor parameters makes it hard to decouple measurands of different physical variables. (iii)Transmission and sensing range—the range of the sensor is very important because physical parameters are to be measured from ultra-deep oil wells. (iv) SNR–SNR can be improved by increasing the reflected signal amplitude and by reducing the signal attenuation and crosstalks. (v) Number of sensors—the finite optical spectrum limits the number of sensors that can be designed. More sensors are required to achieve multi-point or distributed sensing down-hole. (vi) Sensitivity—enhanced sensitivity to physical parameters like pressure and temperature can be achieved with suitable polymer coatings.Although FBGs are capable of sensing almost all physical parameters like temperature, strain, pressure, vibration, etc.; however, they respond to multiple sensing parameters in a coupled fashion. Therefore, to measure these physical parameters separately or both simultaneously, the effects of temperature and strain need to be decoupled from each other. Therefore, multi-parameter measurements become important, as it allows to reduce the size, cost and complexity of the sensing system, and also provides parameter discrimination in situations where cross-sensitivity is a critical issue . AdditioEven though intensive research studies have been carried out on specialised fibres like PCFs ,131 and The main advantage of PCF is their high light confinement characteristics which are otherwise difficult to achieve in ordinary conventional fibres. LCPCF  open new perspectives in sensing applications . With thThe new multimodal sensor configuration is expected to be a multi-wavelength time-multiplexed fibre-optic sensing system capable of multi-point strain sensing, distributed temperature sensing and key point chemical or temperature measurements. Quasi-distributed FBGs with multi-wavelength-based detection enhances the capabilities of the sensor combination, enabling sensing of critical parameter like strain at multiple points. Microstructuring and liquid crystal infiltrations of cladding holes enhance the signal intensity from the sensor configuration and also enables wavelength tunability required for the rare-earth-based sensors. Rare earth doping of PCF core further enhances the signal intensity and generates Raman signatures useful for Raman-based DTS systems. Furthermore, localised rare-earth doping enables multi-point fluorescence sensing of critical parameters like temperature or chemicals. The multimodal sensor can be interrogated using different optical pulses with different frequencies or wavelengths. A multi-wavelength light source system can be used consisting of different laser wavelengths needed for probing the quasi-distributed FBGs, multi-point rare earth fluorescence sensors and Raman DTS.The various distributed sensing technologies , FBG sensors, interferometry-based sensors are all efficient for single parameter measurement—either temperature, pressure, vibration or strain. For the condition monitoring of multiple critical parameters like high pressure, high temperature, vibration, etc., the oil and gas industry is intently looking for multi-point or distributed multimodal sensors. Multimodal sensors will provide a flexible sensing solution, enable miniaturization of the sensors, and add new functionalities with improved measurement capabilities for sensing various important parameters. Furthermore, multimodal fibre-optic sensor configuration will be capable of sensing multiple parameters from different locations. An integrated fibre-optic sensor configuration, combining features of LCPCF, FBG and rare earth doping; extracting their molecular, atomic and vibro-rotational characteristics would be a good option for sensing multiple modalities in the oil industry.The main limitation of existing fibre-optic sensors is they use a single wavelength region (mostly communication window around 1550 nm) for probing the parameters. The highlight of the proposed multimodal sensor is that they operate in multiple wavelength regimes, adding scope for new functionalities and sensing capabilities. Microstructured fibre is also expected to improve the light confinement characteristics which in turn enhances the signal intensity from the multimodal sensor configuration. Infusing liquid crystals into the PCF sensor will aid in the tuning of spectral bands from the communication window to other wavelength regimes (NIR and visible).Electrochemical change induced in the PCF by infusion of liquid crystals will result in a change in the effective refractive index of cladding and also tunes the optical bandgaps. Application of electrical voltage for aligning the LC directors and subsequent refractive index changes both plays a key role in the shift of different spectral bands. Bandgap tuning enables the LCPCF sensor to operate within the photonic bands that have peak sensitivities. Liquid crystals can be infiltrated into the PCF holes via capillary effects, a technique known as vacuum-assisted LC infiltration is commonly used.Doping the core of microstructured fibres with rare earth elements can further enhance the light confinement characteristics and also invoke fluorescence and Raman signatures when probed with suitable laser excitations. These specialised features of LCPCF can be effectively utilized in the new sensor configuration for tuning the light confinement wavelengths to suit the fluorescence and Raman spectral regions of the rare earth doped sensors.Furthermore, integrating FBGs into the sensor configuration will enable multi-point multimodal sensing capabilities. The integrated sensor combination is expected to overcome the limitations of existing fibre-optic sensors with regards to cross-sensitivity, SNR, sensing range and multimodal sensing capability. The new sensor configuration operating in multiple wavelength regimes in a multiplexed fashion will have the potential to carry out multi-parameter sensing.A detailed review of different fibre-optic sensing techniques was carried out, to identify a feasible sensing solution for the oil industry. The different classifications of the fibre-optic sensors and different types of fibre-optic sensors used for oil and gas applications were discussed. The challenges posed by the oil and gas harsh environments were discussed. The technical challenges associated with the different fibre-optic sensing technologies, especially the most commonly used FBG sensors and distributed fibre-optic sensors were described in detail. Moreover, a thorough review leading to a new idea for multimodal sensing using a novel fibre-optic configuration for oil and gas applications was discussed. Lastly, to improve the performance of the multimodal sensor in terms of its SNR, different approaches and techniques were identified. The proposed configuration of the novel multimodal fibre-optic sensor consisting of rare-earth-doped microstructured–FBG sensor is expected to overcome the technical difficulties and challenges of the existing sensors in the O&G industry. The integrated sensor combination is likely to overcome the limitations of FBG sensors with regards to cross-sensitivity, SNR, sensing range and multimodal sensing capability. The new sensor configuration operating in multiple wavelength regimes in a multiplexed fashion is competent to perform multi-parameter sensing.


When a traditional visual SLAM system works in a dynamic environment, it will be disturbed by dynamic objects and perform poorly. In order to overcome the interference of dynamic objects, we propose a semantic SLAM system for catadioptric panoramic cameras in dynamic environments. A real-time instance segmentation network is used to detect potential moving targets in the panoramic image. In order to find the real dynamic targets, potential moving targets are verified according to the sphere’s epipolar constraints. Then, when extracting feature points, the dynamic objects in the panoramic image are masked. Only static feature points are used to estimate the pose of the panoramic camera, so as to improve the accuracy of pose estimation. In order to verify the performance of our system, experiments were conducted on public data sets. The experiments showed that in a highly dynamic environment, the accuracy of our system is significantly better than traditional algorithms. By calculating the RMSE of the absolute trajectory error, we found that our system performed up to 96.3% better than traditional SLAM. Our catadioptric panoramic camera semantic SLAM system has higher accuracy and robustness in complex dynamic environments. With the development of the mobile robot industry, robots increasingly require their own positioning when performing a variety of complex tasks. At present, visual simultaneous localization and mapping (SLAM) technology is one of the most common methods used to realize robot localization . HoweverThe distribution of features in the scene observed by the camera is one important factor that affects the performance of visual SLAM. As the camera field of view (FOV) increases, the number of features observed will also grow. When large field of view cameras are used in SLAM, it has more precise and robust performance than when ordinary pinhole cameras are used. A catadioptric panoramic camera can provide a 360-degree view of the scene, at a lower cost than a fisheye camera; thus, we chose to apply a catadioptric panoramic camera to the SLAM task.In the past few decades, a variety of SLAM and visual odometry systems have been proposed. Some representative examples are ORB-SLAM2 , and dirThere are also many studies on panoramic camera SLAM and visual odometry. Caruso et al. proposed a real-time, direct monocular SLAM method based on LSD-SLAM by incorporating the unified camera model. Heng et al. presented a semi-direct visual odometry for a fisheye-stereo camera . In 6],,6], a diThese methods are all based on the assumption that the objects in the scene are static. When the extracted feature points are from dynamic objects in the scene, it will affect the accuracy of the camera pose estimation. However, in reality there are not many completely static scenes; most scenes contain dynamic objects which will occlude the features being tracked, which may result in reduced accuracy of pose solution or loss of system tracking. Therefore, improving the accuracy and stability of SLAM in dynamic scenarios is an important research direction .The SLAM system in the classic dynamic environment does not consider using machine learning to detect dynamic targets. The dynamic objects in the environment are considered as outliers. The typical algorithms for removing outliers are RANSAC  and robuAlthough these geometric-based visual SLAM systems can eliminate the influence of dynamic objects in the scene to a certain extent, they also have certain limitations. These methods lack semantic information and cannot use prior knowledge of the scene to judge the motion state of the object. Therefore, many SLAM systems combined with semantic segmentation networks have been proposed to deal with dynamic objects in complex environments.Xiao et al. , based oIn order to solve the above problems, we propose a catadioptric panoramic camera semantic SLAM system for dynamic environments.The rest of the paper is structured as follows: In Knowing the types of objects in the scene can help us deal with complex tasks in a dynamic environment. However, traditional methods cannot provide this prior information. With the rapid development of deep learning, semantic segmentation networks can output pixel-level semantic information. Therefore, combining semantic segmentation networks with traditional SLAM systems can complete complex tasks in dynamic environments. Because the semantic information is used to mark the potential dynamic points in the environment, the image is input into the SLAM system with prior information.MASK-RCNN is a representative instance segmentation network , but it YOLACT divides instance segmentation into two parallel tasks: Generating a non-local prototype mask over the entire image and predicting a set of linear combination coefficients per instance. It then generates a full-image instance segmentation from these two components: For each instance, the prototype is linearly combined with the corresponding prediction coefficient, then the predicted bounding box is used for cropping .n represents the number of instances of cars and people in the image. The size of the original RGB image input to the network structure is Through the results of instance segmentation output by the YOLACT instance segmentation network, we obtain pixel-level semantic information of the panoramic image. Under normal circumstances, according to the semantic label corresponding to the pixel in the panoramic image, the type information in each instance can be distinguished, the corresponding dynamic instance can be eliminated, and the pose estimation of the catadioptric panoramic camera is not involved. However, vehicles in reality are not all dynamic. When a static vehicle occupies most of the view of the panoramic image, if the vehicle is removed as a dynamic point at this time, a large amount of useful information will be lost, or feature points will be insufficient and the camera pose calculation will fail.The result of YOLACT instance segmentation has nothing to do with the motion state of the objects in the scene. Semantic tags cannot be used to distinguish the motion state of objects in the scene. We need a method to distinguish the motion state of objects and reflect the real movement of objects in the scene.After analyzing the projection model of the catadioptric panoramic camera , the sphP is on the corresponding ray P is static, there should be a correct feature match, and we can know that point In R is the rotation matrix, t is the translation vector and P is a static point, We need to request the essential matrix before we can use the sphere epipolar constraint to determine the state of the scene point. Usually, The Formula (4) can be written as a linear formula related to There are nine unknowns in Formula (6), but due to the equivalence of scale of With the essential matrix, In order to verify the performance of the SLAM system based on the catadioptric panoramic camera in a dynamic environment, experiments were conducted on the Omnidirectional public dataset . This daA number of image sequences containing dynamic objects were selected in the dataset, and we divided them into low dynamic sequences, high dynamic sequences and sequences containing high dynamic and low dynamic objects. To verify the SLAM algorithm proposed in this paper, we evaluated the absolute trajectory error (ATE) and relative pose error (RPE) indicators for our SLAM system. ATE is used to calculate the error between the estimated trajectory and the ground truth, which can directly reflect the accuracy and global consistency of the pose estimation algorithm, and is usually used to evaluate the performance of the entire SLAM system. RPE is an indicator for evaluating system drift. It calculates the difference between two pose changes in a fixed period of time.N represents the total number of poses included in the trajectory.Formula (7) is used to calculate RMSE of ATE. We used RMSE, mean error, median error, and standard deviation (SD) to evaluate ATE indicators, and average relative translation to evaluate RPE indicators. The trajectory calculated using our method was compared with the monocular catadioptric panorama VO method and the ground truth. The monocular catadioptric panoramic VO  is basedAn indoor scene experiment on experimental platforms was conducted. As shown in We set a fixed route, and the trajectory calculated from the images collected by the RGB-D camera in a static environment was used as the ground truth. During the experiment, we added walking humans as dynamic objects. Image data were collected by the RGB-D camera and the catadioptric panoramic camera. The collected image data were used to compare the performance of our method with the state-of-the-art dynamic SLAM system. DS-SLAM and VDO-SLAM  were adoIn the experiments, DS-SLAM, VDO-SLAM and our method all ran stably in the environment with dynamic objects. Our method obtained the highest accuracy, with an improvement of 30.53% over DS-SLAM and 24.43% over VDO-SLAM. This is because we used a catadioptric panoramic camera, which has the advantage of a large field of view. After removing dynamic objects, there still are rich scene features that can be used to calculate the camera pose.In this paper, we have presented a semantic visual SLAM system for a catadioptric panoramic camera in a dynamic environment building on ORB-SLAM2. We added a separate thread running YOLACT to obtain pixel-wise semantic segmentation, and a tight coupling method of spherical epipolar geometry and semantic segmentation was proposed to detect and remove dynamic features effectively. We used an instance segmentation network to detect potential moving targets in the image, output the pixel-level segmentation results, and then use sphere epipolar geometry to verify the motion state and find the real dynamic features. When extracting ORB feature points, dynamic targets are eliminated, thereby improving the accuracy and robustness of camera pose estimation. In order to verify the performance of our system, experiments were conducted on a public omnidirectional dataset. The results show that in a dynamic environment, the positioning accuracy of our system is greatly improved compared to the traditional method. Comparing the RMSE of absolute trajectory error of different methods, the positioning accuracy of our system in a high dynamic environment was up to 96.3% higher than the traditional method, which proves the effectiveness of our system when faced with dynamic environments.We built an experimental platform and conducted real world experiments. We compared our method with the dynamic state-of-the-art SLAM system. As we use the semantic segmentation result and geometry to detect and remove dynamic objects in the image in a tightly coupled manner, and the catadioptric panoramic camera we use has the advantage of a large field of view, in the experimental results, our method obtained the highest accuracy, with improvements of 30.53% over DS-SLAM and 24.43% over VDO-SLAM.However, our current system only uses two frames of information to verify the dynamic points, and lacks the use of global information. The next step should be to improve this aspect.