Lipids are a complex group of chemical compounds that are a significant component of the human diet and are one of the main constituents of milk. In mammals, lipids are produced in the milk-secreting cells in the form of milk fat globules. The chemical properties of these compounds necessitate developing separate processes for effective management of non-polar substances in the polar environment of the cell, not only during their biosynthesis and accumulation in the cell interior and secretion of intracytoplasmic lipid droplets outside the cell, but also during digestion in the offspring. Phospholipids play an important role in these processes. Their characteristic properties make them indispensable for the secretion of milk fat as well as other milk components. This review investigates how these processes depend on the coordinated flux and availability of phospholipids and how the relationship between the surface area (phospholipids) and volume  of the cytoplasmic lipid droplets must be in biosynthetic balance. The structure formed as a result (i.e. a milk fat globule) is therefore a result of specified structural limitations inside the cell, whose overcoming enables the coordinated secretion of milk components. This structure and its composition also reflects the nutritional demands of the developing infant organism as a result of evolutionary adaptation. An increasing number of studies have demonstrated that breastfeeding is one of the key factors affecting the health of infants and has a positive impact on mothers by minimizing the likelihood of breast and ovarian cancer , 2. ThisOne of the constituents of milk are lipid compounds, which are an important component of the human diet throughout the whole life. Lipids are a complex group of chemical compounds, the significance and role of which are probably the least recognized among all groups of biomolecules. Due to their complex composition and incomplete knowledge in this regard, it is difficult to define the nutritional recommendations concerning this group of compounds, even though they constitute a significant part of the cell metabolome. The reason for this may be slower progression in available instrumentation to adequately characterize complex lipid fingerprints. Genetic and bioinformatic methods are not very useful in the analysis of lipids since they are not directly linked with the body genome. Therefore, progress in this research area is proceeding slightly slower than other classes of biomolecules . AlthougMilk lipids in the form of milk fat globules (MFG) are produced in the milk-secreting cells, a single layer of which lines the internal walls of mammary gland cavities, called alveoli. The diversity of synthesized lipids and complexity of biosynthetic mechanisms in the mammary gland point to multiple potential roles, to ensure the appropriate growth, health and survival of the progeny of mammals and, at the same time, reflect structural determinants stemming from the chemical properties of these compounds. This has required adapting separate processes for effective management of non-polar substances in the polar environment of the cell, not only during their biosynthesis and accumulation in the cell interior and secretion of intracytoplasmic lipid droplets outside the cell, but also during digestion in a progeny’s body. These processes have to be strictly coordinated and phospholipids play an important role in these processes.The digestion of lipid substances proceeds along with digestion of other dietary components. As a result of complex physicochemical mechanisms, this leads to the transformation of non-polar compounds into more polar components, which are capable of overcoming the unstirred layer of fluid on the surface of epithelial cells of the intestine and enables their absorption by villi . Hence, Our knowledge of the properties, composition and occurrence of particular groups of lipid compounds is expanding due to the application of such analytical methods as chromatography, mass spectrometry (MS) and nuclear magnetic resonance (NMR) spectroscopy. Complete databases of lipid compounds typical of particular tissues have also been established , 6. ThisThis review investigates the properties of phospholipid compounds occurring in milk and their effect on the structure of a milk fat globule and identifies their potential role during milk secretion in the mammary gland.The content of lipid compounds in milk is species-dependent and ranges from 3.8 to 3.9% in human milk and from 3 to 5% in cow’s milk. They occur in the form of the so-called milk fat globules with sizes ranging from 0.2–15 μm. Inside the globules, there are triacylglycerols which represent 97–98% of milk lipids , about 1Some of the membrane proteins are glycoproteins and can be covalently linked with phospholipids or neutral lipids . Apart fAlthough the lipid of the milk fat globule membrane comprises both neutral and polar lipids, however, the triacylglycerol fraction may originate from contamination by the core of fat globule during isolation of the membrane . The conThe main polar lipids of milk consist of phospho- and sphingolipids. They are basic components of the membranes due to their amphiphilic structure. The first are built of glycerol esterified at two sites with fatty acids (FA) and with phosphate with a different organic group  attached at the third site in the glycerol backbone. For sphingolipids, the structure varies and is based on a sphingosine (the sphingoid base), i.e. a long-chain aliphatic amine which additionally contains two or three hydroxyl groups. The attachment of a fatty acid to an amine group results in the formation of ceramide. After the addition of the organophosphate group, a sphingophospholipid is formed. Once phosphocholine is attached to the ceramide, sphingomyelin is formed, whereas a sphingoglycolipid is formed upon the attachment of a saccharide group , 17. PhoThe structure of a fat globule membrane is asymmetric – the internal and external surfaces differ from each other with their composition, which is due to the wide range of functions served by a given surface. The components of the MFGM are arranged so that the external part of the bilayer is constituted mainly by phospholipids containing choline (PC and SM) as well as glycolipids, cerebrosides, and gangliosides (molecules containing polar components as saccharide residues), directed towards the aqueous environment, whereas the interior surface of the bilayer contains phosphatidylethanolamine, phosphatidylinositol and phosphatidylserine , 20.Structurally, the MFGM is a so-called liquid lipid-protein mosaic. According to the widely-accepted current model of the membrane, the so-called “liquid lipid-protein mosaic”, proteins are immersed or flow in a liquid bilayer of spatially-oriented lipids . MembranConsidering the structure itself and structure-related physicochemical properties, the role of phospholipids includes stabilization of milk fat in the dispersed form, prevention of flocculation and coalescence of globules, as well as protection against adverse effects of lipases. Apart from their structural functions in the membrane, phospholipids are also significant in nutrition in the period of infant development. Several studies have demonstrated the high biological activity of these compounds and their metabolites, which allows classifying them as components with a positive effect upon human health . Their bIn terms of chemical structure, molecules present in the world around us may be divided into those which possess a charge and those which have no charge in their structure. The possession of charge results in specified properties that are referred to as hydrophilic and lipophilic, respectively. Phospholipids merge these two properties and constitute a structural hybrid composed of a part which possesses a charge and a part with no charge, the so-called “hydrophilic head and hydrophobic tail”. These are referred to as amphipathic compounds. This structure determines the specific properties of phospholipids. As each system tends to form the lowest energy state, the most beneficial would be if the hydrophilic part could contact the hydrophilic environment and avoid the hydrophobic environment. By analogy, contact of the hydrophobic part with the hydrophilic environment would be energetically unbeneficial. In an aqueous environment, phospholipid orientation enables the hydrophilic part to contact water and simultaneously the hydrophobic part to hide from contact with water. Such conditions are met when phospholipids are at the water surface and when their hydrophilic part is directed towards water. In this case, the surface tension of the water decreases. Another potential arrangement is a lipid bilayer in which the hydrophobic parts are hidden from contact with water inside the bilayer. If such a bilayer binds into a spherical structure, a liposome will form that separates the two spaces. This ability has resulted in the possibility of developing a structure separated from the external environment, i.e. a cell with a separating bilayer called a biological membrane. Considering that one tip of the phospholipids is polar and the other is non-polar, an ideal situation would be the hydrophilic part having contact with the hydrophilic environment and the hydrophobic part having contact with the hydrophobic environment. This is likely when the phospholipid is at the contact site of these two environments, i.e. at the so-called interface. Another property of phospholipids stemming from their structure is their emulsifying capability, which consists in the ability to disperse the non-polar molecules in the polar environment in a dispersed form .Although the general structure of phospholipids is similar, the significant variations in their chemical make-up in biological membranes entail some differences in their properties. These properties depend on both the length and saturation of attached fatty acids and the type and polarity of the head group. Different studies of these properties in artificial membrane systems indicate that the chain length, degree of unsaturation and properties of the polar head group and the type of pairing of the hydrocarbon chains may affect the rate of diffusion and transport through the membrane . PhosphoThese general basic examples show how the specified properties and characteristics of phospholipids determine their behaviour in various environments. They also point to some limitations a cell has to overcome when using non-polar substances in a polar environment as well as to the specific role of phospholipids in these processes.Processes related to lipid metabolism and leading to the biosynthesis of milk fat include a number of steps. One of the main constituents of milk triacylglycerols and phospholipids are fatty acids. They may come from ingested food or maternal fat stores. In addition, some fatty acids (saturated mainly of 10–14 carbon atoms) may be synthesized de novo in the liver, mammary gland and other tissues . Fatty aAfter crossing the membrane, fatty acids under the influence of acyl-CoA synthetase are activated into the form of an acyl CoA. The activated fatty acids connected to specific cytosolic FABP (fatty acid binding protein) are transported to intracellular organelles for further reactions. Depending on the needs of the cells, fatty acids can be desaturated under the influence of stearoyl-CoA desaturase (SCD) and/or be converted into triglycerides, phospholipids or cholesterol esters . GlyceroThe biosynthesis of milk fat is a complex process which requires coordinated control of many cellular processes and metabolic pathways occurring at different stages of development and functioning of the mammary gland. These processes are regulated by the so-called gene networks, which are interactions and relationships between transcription factors and other regulatory elements that affect the expression of specific genes , 46. DurProteins belonging to the family of SREBP proteins are transcription factors binding sterol regulatory element . They regulate the expression of genes involved in cholesterol and lipid homeostasis. Their regulatory domain interacts with the SCAP protein (SREBP cleavage activating protein) which acts as a sensor responsive to the level of cholesterol. When the cholesterol level is lowered, the protein SCAP escorts SREBP protein to the Golgi apparatus where, under the influence of two specific proteases, the domain that interacts with DNA is released. This fragment after dimerization translocates to the nucleus and, when attached to the promoter regions of DNA, activates the entire group of genes responsible for the synthesis of cholesterol, fatty acids, triglycerides, phospholipids and NADPH (as a cofactor necessary for the synthesis of these compounds) . RudolphAnother regulator of lipid metabolism in the mammary gland may be a PPAR γ protein (peroxisome proliferator-activated receptor γ). This protein is involved in the process of adipocyte differentiation and its expression can be stimulated by either insulin or SREBP-1. It belongs to the transcription factors activated by lipophilic agents and regulates the expression of genes related to the metabolism of fats, proteins and carbohydrates . It has While the relationship between transcription factors regulating the biosynthesis of milk fat has not been precisely defined, many important elements playing a primary role in this regulation have been identified. Most of research concentrated on the regulation of fatty acid or triacylglycerol synthesis. For example Cases et al.  showed tThis brief overview of some elements relating to the regulation of milk fat biosynthesis indicates the need for further research and greater attention to the role of phospholipid biosynthesis in the functioning of the mammary gland as well as in relation to the overall metabolism of lipids.The formation of lipid droplets in the cytoplasm is linked with endoplasmic reticulum. Also phospholipid synthesis occurs in the cytosol in close proximity to the ER membrane. After incorporation into the membrane, fragments of the membrane can eventually bud off and transport phospholipids across the cell. Triacylglycerols accumulate on the surface or inside the ER membrane. By forming a “bulge”, they may detach from the ER membrane and migrate to the cytoplasm and become surrounded by the outer cytosolic part of the ER membrane which contains polar lipids and proteins , 59. ThiAfterwards, by means of an unknown mechanism, the droplets are transported to the apical part of a secretory cell membrane. Studies have demonstrated high contents of microtubules, actin microfilaments and intermediate filaments in the apical regions of secretory cells , 69, as Fat globules are intracytoplasmic lipid droplets secreted outside the milk-producing cells. These are the only known cells capable of secreting whole lipid droplets outside the cell. It has required “developing” a separate mechanism enabling this process.The lipid droplets are secreted outside the cell, probably according to a unique mechanism different from classical exocytosis. This mechanism differs completely from the secretion of triacylglycerols by other cells (e.g. secretion of chylomicrons or other lipoproteins), although two potential mechanisms have been postulated.In the classical model, alveoli are subject to fusion, which leads to the formation of an intracytoplasmic vacuole containing both casein micelles and lipid droplets. The contents of these vacuoles are then secreted outside the cell via classical exocytosis . A potenThe second, currently accepted potential mechanism assumes that lipid droplets are coated with a cellular membrane originating from the external, apical part of the secretory cells and, in this way, gain an additional lipid bilayer . This biThis mechanism results in the formation of a specific structure and, although this unique structure results from the mechanism of lipid droplet secretion outside the cell, this mechanism is also expected to ensure specified benefits to the progeny. It is known that the lipid droplets secretion process probably follows the mechanisms presented in Fig. Milk production is a great challenge for mammary epithelial cells. It requires well-coordinated uptake of systemic building blocks for the production of nutrient-rich milk. The observations discussed above have shed some new light on the organization and coordination of the milk production process in the mammary gland. Considering the flux of phospholipids and their indispensability inside the epithelial cells of the mammary gland, they may be one of the key elements in regulating the secretion of milk components.The availability of polar lipids may also greatly affect the size of MFG. Greater availability of phospholipids may enable better dispersion of triacylglycerols in the form of smaller MFG which, in turn, will be reflected in the better availability of fat and better digestion in the organism of the progeny.By this means, both the flux and availability of phospholipids may determine the ratio of hydrophilic to lipophilic substances in milk, simultaneously enabling their coordination and uninterrupted secretion. The specified volume of transporting vesicles corresponds to the specified amount of the membrane material that will be attached to the apical membrane. This indicates that a specific amount of phospholipids may be used for the secretion of a specified amount (volume) of fat.Taking into account the changes in milk composition during transition from colostrum to the mature milk, a negative correlation may be observed between the size of fat globules and the content of phospholipids. Using formulas for sphere surface area and volume, the correlation between the number of formed globules with uniform size and the formed surface of milk fat may be expressed with the following formula:where: x denotes the number of globules formed and y denotes globule surface area.3, a surface of 48.34 cm2 will be achieved once 1000 globules of the same size are produced from 1 g of fat. An increase in the number of produced globules to 2000 will result in surface increase to 60.90 cm2. This formula approximately shows the increasing demand for phospholipids (as a monolayer) at an increasing degree of dispersion for a specified amount of triacylglycerols. It indicates that an appropriate amount of phospholipids may be an element controlling fat dispersion degree. Changes in globule size may be determined by the availability of phospholipids , but also have to meet the specific needs of a new-born for phospholipids and triacylglycerols, respectively. This demand may vary throughout the developmental process and, initially, during intensive development of the alimentary and nervous systems a relatively higher supply of phospholipids may be of greater significance. Furthermore, phospholipids contain significant amounts of polyunsaturated fatty acids (PUFA), which are an indispensable component of cellular membranes and are essential for proper brain and retina development in neonates [According to the formula, assuming a density equal to 1 g/cmneonates . Zou et neonates  investigConsidering the aforementioned limitations, the phospholipids supply to a new-born’s body may be enhanced through so-called “lactosomes”, i.e. small lipid globules free of triacylglycerols, the presence of which has recently been detected in milk . They maThe process during which a fat globule is coated with a triple phospholipid membrane may yield some benefits to the progeny ingesting milk. Fat globules coated with the triple phospholipid layer provide significantly higher (ca. threefold) amounts of phospholipids compared to the monolayer. For a developing young organism, the appropriate amount of phospholipids provided with food may be of key significance for the proper development of not only the nervous system, but the organism as a whole.During intensified production of milk fat in the mammary gland, lipid droplets are also subject to coalescence and fusing into larger droplets in a cell’s interior and, with their sizes increased, they are then secreted outside the cell. As mentioned above, a high degree of lipid droplet dispersion inside cells of the mammary gland, with a concomitant increased intensive production of MFG, requires a high supply of phospholipids. One of the means to reduce the demand for phospholipids with a the simultaneously enhanced production of milk fat involves a reduction of fat dispersion degree via coalescence and the formation of larger fat globules. For such globules, the surface area-to-weight ratio is lower and, resultantly, the demand for phospholipids is reduced. In other words, with a specified “pool” of phospholipids, the increase in fat globule size allows for a significantly greater secretion of milk fat. Thus, the availability of phospholipids may affect the size of intracytoplasmic lipid droplets and may be one of the factors regulating their size.The above considerations have been confirmed in conducted research. Lopez et al.  pointed It is also noteworthy that the process of fusion or breakdown of droplets into smaller ones results in an excess or deficit of phospholipids. It seems advisable, therefore, to consider whether the generated excess or deficit of phospholipids leads to a stable structure characterized by insignificant loosening  or whether it requires an immediate reorganization of the interface material. However, it seems likely that the process of globule fusion requires removing the excess of phospholipids from the interface in order to ensure stability and durability of a newly-formed system Fig. . By thisA similar mechanism, in which a key role is ascribed to phospholipids, may be applied to regulate the size of intracytoplasmic lipid droplets in other cells. Fat cells (adipocytes) are characterized by considerably larger sizes of intracytoplasmic lipid droplets. It is known that the key role in regulating fat globule size and in management of cell lipids reserves is played by PAT family proteins . HoweverThrough the presentation of characteristic structural traits of milk fat globules and mechanisms linked with their secretion, this work indicates the potential role of phospholipids in this process. To date much work has focused on total lipid composition and how the individual fatty acid composition of milk is altered by maternal diet or targeted gene deletion, but little is known about phospholipid synthesis in mammary gland and its role during lactation. The characteristic properties of phospholipids make them indispensable for the secretion of not only milk fat, but also other milk components. Since these processes are associated with the flux and availability of phospholipids, they have to be strictly coordinated to proceed smoothly. This indicates a strong correlation between secretion of lipid and water-soluble components. The structure formed as a result of these processes, i.e. a milk fat globule, is therefore a result of specified structural limitations inside the cell, the overcoming of which enables the secretion of milk components, although it also needs to reflect the nutritional demands of a developing infant’s body, resulting from evolutionary adaptation.


Many fault detection methods have been proposed for monitoring the health of various industrial systems. Characterizing the monitored signals is a prerequisite for selecting an appropriate detection method. However, fault detection methods tend to be decided with user’s subjective knowledge or their familiarity with the method, rather than following a predefined selection rule. This study investigates the performance sensitivity of two detection methods, with respect to status signal characteristics of given systems: abrupt variance, characteristic indicator, discernable frequency, and discernable index. Relation between key characteristics indicators from four different real-world systems and the performance of two fault detection methods using pattern recognition are evaluated. Fault detection is a key status monitoring function that identifies the presence of faults in a system, and the times at which they occurred. Fault detection methods use pattern recognition with large datasets have been applied to various engineering areas  since thOne issue with these methods is that the features that are extracted from the time domain are not always enough to distinguish between the no-fault and the fault states. For example, the amplitude of the time series shown in Such characteristics appear in the acoustic and vibrational data that are usually collected from mechanical systems, such as rotors and bearings ,13. If aSeveral model-based fault detection methods using frequency features have been proposed. The performance of model-based fault detection, by nature, is dependent on a proposed system model itself. However, the operational outputs of many industrial systems are often different from the intended behaviors due to unknown disturbances, system degradation, and measurement noises . Hence, Data-driven approaches have been develop to make fault status decisions by analyzing a large amount of historic data in the case of absence of any predefined system model . Many stTherefore, the first objective of this study is to develop a pattern extraction method that discretizes the frequency components of multivariate data. In The second objective of this study is to provide a guideline for selecting appropriate labels for fault pattern extraction via a comparison between the performance of different pattern extraction methods and the representative characteristics that they extract from the time series. In the process, several key characteristics indicators (KCIs) and an aggregated characteristic indicator is suggested. The guideline is also proposed regarding the type of labels that should be selected based on the key characteristics. The key characteristics of the time series and the experimental study are introduced in Conventional discretization methods downsize time series into a finite number of bins that preserve the temporal information of the original time series. Such methods have been applied to fault detection, owing to their efficiency in extracting patterns using discretized labels that are derived from statistical features of the discretized bins . HoweverThe discretization-based fault pattern extraction method aims to extract a set of fault patterns from the training data of a given system for fault detection. Therefore, the information of no-fault state and fault state, namely fault time markers, are assumed to be given a priori. The method comprises three steps: label definition, label specification, and fault pattern extraction.ith sensor and The label definition step involves estimating the distribution model of the time series, and dividing the given time series into a finite number of bins. Assume that a given time series is collected from a system containing Then, the probability density function (PDF) is estimated to determine the distribution model of the given data set. The probability density function, After that, a set of cut-points sth segment. Here, the label for each segment is determined by its relative location of the average value of the amplitude. Then, The label specification step divides the data into m labels for a given time index, i.e., a column vector of D(X), are converted into event codes. The event codes are combination of all the possible discrete state vectors, where a total number is To represent the state of the system in terms of the labels that are assigned to each sensor, X depending on whether there is an element of D(X).The set of fault patterns As performance of the fault detection is determined as follows: the number of the fault states where a specific fault pattern is found. Herein, the label definitions play a key role in this scheme. In the next section, we propose a new label definition using the frequency-domain features to expand the coverage of the discretization method.Frequency components have been popularly used to fault detection, Fast Fourier transforms is one of methods to extract frequency components from the original sensor signals of the system . Howeversth time segment of the ith sensor, Before defining the labels, the time series ormation . The slis = 19. ith sensor data, is divided into s segments, and FFT is applied to each segment for extracting the dominant frequency. Finally, a sequence of dominant frequency, The procedure is done with the window size of a time segment = 40 s, the sampling rate = 1 kHz, sth time segment signal of the ith sensor. We can obtain the Fourier coefficient We can now represent the overall sensor dataset iW for the ith sensor data is then defined as follows.Let sth segment of the ith sensor data is denoted as For the sake of simplicity, without a loss of generality, we employ the dominant frequency having the highest peak in the frequency spectrum as a key feature to characterize the inherent information in the frequency domain. The dominant frequency in the aVar) and discernibility index (DI) from the previous research, and discernable frequency (DF). Furthermore, we define CI as an aggregated characteristic index of DI and DF. We analyzed four datasets of multi-sensor signals and represented the main feature of each dataset by the three KCIs and the aggregated index CI. We discuss the relationships between fault detection performance and KCIs. The fault pattern extraction performance is highly affected by whether appropriate labels are selected to characterize the time series. To ensure that the labels are suitable, we need to determine the relation between the characteristic indicators for the collected signal and the fault pattern extraction performance. In this section, three KCIs are introduced to represent the characteristics of the time series of the given signal: abrupt variance aVarDI measures the degree of statistical overlap between no-fault and fault states, as follows [x in the no-fault and the fault states respectively.In contrast, the  follows :(7)DIi=DF to represent the frequency similarity between fault and no-fault states in terms of the composition of the time-frequency components. Existing features in the frequency domain, such as the dominant frequency and power spectral density, cannot represent time-frequency characteristics of the data because they utilize a classical FFT [DF is therefore defined as follows.We further define the ical FFT . The DF ith sensor signal in the no-fault and the fault states respectively. DF calculation, giving a frequency similarity of 0.331.CI is further defined to simultaneously highlight the performance of the DI and DF for the given data set. It is calculated by multiplying the complement of the DI with the DF, as follows.The In this section, we evaluate the performance of the discretization-based fault pattern extraction method by considering both frequency and amplitude variation, and discuss the relationship between the KCIs of system status signals and performance of fault detection. The detailed description of the experimental data is listed in FP1, and those generated by the frequency variation of sensor values as FP2. The two pattern generation methods were applied to real-world datasets collected from four mechanical systems. The following subsections describe each system and provide brief information regarding the performance of pattern extraction with respect to the KCIs. The results are summarized in Previous studies used a statistical representative sensor value and/or its linear trend in each time segment to construct a set of labels. We will call the fault patterns that are generated by the amplitude variation of sensor values as TM  as a data acquisition device. The system used a IPG YLS 2000 AC fiber laser source  with a maximum output discharge of 2 kW.Laser welding monitoring data, shown in We controlled the gap between the galvanized steel sheet parts by inserting a conventional metal thickness gauge having thickness of 0.1 mm to 1.0 mm. The travelling path of the laser was defined as ascending direction. In this study, we generated five defective weldments by controlling the artificial gap (=0.5 mm) between specimens, and forty-five normal weldments without gap.aVar and DI but high DF and CI. By using FP1, we could detect all the five weld defects, while only two defects were identified by FP2.The computed KCIs are listed in Automotive gasoline engine data, shown in Step 1. Turn on the engineStep 2. Control the amount of manifold air flowStep 3. An engine knocking occursIn this study, we artificially generated the fault that stops the engine by increasing an amount of air in the intake manifold. It was conducted with following steps:The data in no-fault state was collected during Step 1 and Step 2, while the data in the fault state was collected in Step 3.aVar, DI, DF, and CI. By FP1, we could detect 46 engine faults, while all of the 46 faults were identified by FP2.The sensor signals show high TM , and a noise detection software that we developed.The BSR noise monitoring data, as shown in 2. We then monitor the acoustic signals measured right above the door trim by a microphone array in order to identify BSR noises. We determined the state of a door trim whether it generated abnormal sounds when pressed by a pneumatic pusher.A car door trim was slowly pressed down by a pneumatic pusher with a pressure of 10 kgf/cmaVar and CI but high DI and DF. By FP1, we could detect 80% of the fault states, while all the fault states were identified by FP2.The sensor signals show low aVar, DI, DF, and CI. By FP1, we could detect 93% of the fault states, while all of the fault states were identified by FP2.Marine diesel engine data, shown in FP2 decreased as the DF and CI increased; in particular, the laser welding data showed the lowest performance (40%) together with the highest DF and CI values. When the DF is high, FP2 cannot generate enough fault patterns due to low event code variation between the no-fault and the fault states. The performance of FP2 also decreases as DI increases.The four different datasets were analyzed with two different discretization-based fault pattern extraction methods. The results demonstrated that the fault pattern extraction performances were closely related with the KCIs. The performance of CI value will be relatively low. We observed that the frequency variation based discretization method produced a better result when the CI is low.Based on these empirical studies, we can provide a guideline for selecting an appropriate label definition method in accordance with the KCIs of the given multivariate time series data. By definition, if the difference between no-fault and fault states is more distinct in the frequency domain, then the In summary, we have proposed a new fault pattern extraction method using frequency variation-based discretization. For fault pattern extraction, first, the time series of a sensor signal is discretized to create a set of labels representing the dominant frequency of each time segment. Second, the pattern is generated by converting series of labels into a set of event codes. Third, the fault patterns are determined as the patterns that only occur in a fault state of a system.CI, the aggregated index of the DF and the DI, can be used as a good reference for selecting an appropriate label definition method. For example, the fault pattern performance of FP2 decreased as DF and CI increased. In contrast, aVar was only weakly related to the FP2 performance. Furthermore, the FP1 performance decreased as the DI increased, whereas CI decreased. The experimental result confirmed that the frequency variation based discretization method produced better result when the CI is low. Since the fault pattern performance is also closely related to the discretization parameters, and therefore, further empirical study is necessary.In addition, we have investigated a relation between the KCIs and the performance of fault pattern extraction using both amplitude and frequency variation, providing a guideline for selecting appropriate label definitions. The results show that the


The host cell represents a hostile environment that viruses must counter in order to establish infection. Human cytomegalovirus (HCMV) is no different and encodes a multitude of functions aimed at disabling, re-directing or hijacking cellular functions to promulgate infection. However, during the very early stages of infection the virus relies on the outcome of interactions between virion components, cell surface receptors and host signalling pathways to promote an environment that supports infection. In the context of latent infection—where the virus establishes an infection in an absence of many gene products specific for lytic infection—these initial interactions are crucial events. In this review, we will discuss key host responses triggered by viral infection and how, in turn, the virus ameliorates the impact on the establishment of non-lytic infections of cells. We will focus on strategies to evade intrinsic antiviral and innate immune responses and consider their impact on viral infection. Finally, we will consider the hypothesis that the very early events upon viral infection are important for dictating the outcome of infection and consider the possibility that events that occur during entry into non-permissive cells are unique and thus contribute to the establishment of latency. The host response to infection is multi-faceted and is a concert of cell intrinsic, innate and adaptive immune responses. In higher-order eukaryotes, the adaptive response is orchestrated by specific immune cell types and plays an important role in the resolution of infection. However, all nucleated eukaryotic cells can respond to pathogen infection via the induction of cell intrinsic and innate immune responses, which represent the first line of defence upon infection .One of the first host cell ‘trip wires’ is the detection of the pathogen during the initial contact and entry of the pathogen into the cell. Cells express a number of pattern recognition receptors (PRRs)  that detIn this review, we will discuss the interaction of human cytomegalovirus (HCMV) with these functions and the multiple strategies encoded by HCMV to subvert them. We will particularly focus on the subversion of these responses in the context of the establishment of lifelong latent infections and explore the alternative mechanisms of evasion employed by HCMV under conditions where a number of virally encoded inhibitors of the antiviral response are not expressed and the downstream consequences of this.HCMV is a herpesvirus in the subfamily betaherpesvirinae. It is a ubiquitous infection, with seroprevalence approaching 100% in some populations , althougDespite the need for a vaccine, HCMV can be treated. Ganciclovir (GCV) and derivatives, along with foscarnet and cidofovir, are potent inhibitors of viral replication . HoweverA major site of HCMV latency is the CD34+ haematopoietic progenitor cell population resident in the bone marrow ,28. ThesWhat is becoming increasingly clear is that the events that occur during the very early stages of viral infection of non-permissive cells  have an important role during the establishment of latency. For example, a failure of the virus to antagonise key antiviral structures has been implicated as a mechanism to drive latency by default due to a repression of lytic gene expression . FurtherThe initial entry events during the infection of cells that can support lytic replication has been extensively studied for many years, often taking advantage of knowledge derived from studies of other herpesviruses such as herpes simplex virus.A wealth of studies has demonstrated that entry of HCMV into permissive cells is coordinated by a number of virally-encoded glycoproteins that interact with specific receptors on the cell surface—however, it is the identity of these receptors that has remained contentious . The claThe relative paucity of knowledge on the entry of HCMV into sites of latency is no doubt due in part to the relative difficulty of working with the primary cell types, in which HCMV establishes latent infections in vivo, which are CD34+ myeloid progenitor cells ,28, compHowever, work mainly from the Yurochko laboratory provides the first hint of increased complexity regarding the infection of a subset of non-permissive cells . CirculaOur own interests lie in the mechanisms HCMV utilises to establish latent infections of CD34+ progenitor cells. Successful infection of both CD14+ and CD34+ cells has been linked to interactions between the virion and EGFR. EGFR signalling leads to the activation of a vast array of signalling pathways including ERK/MAPK and PI3K signalling. However, when it comes to infection of sites of latency, great attention has been given to the possible role of EGFR during this process, particularly in monocytes.Firstly, it was demonstrated that primary monocytes do actually express EGFR , as it wIn lytic infection of fibroblasts and epithelial cells, HCMV DNA can be detected within the nucleus approximately 30 min post-infection . HoweverAlternatively, this extended entry pathway may be an important determinant of successful quiescent infection of monocytes. Pre-treatment of monocytes with LPS rendered them transiently permissive for IE expression 24 h following infection . HoweverWhat these studies of entry into latent and lytic infection may reveal is that the virus can utilise multiple entry pathways. This begs the question: could this be important? Does the presence of different entry pathways allow the virus to tailor entry to navigate different host cell responses the virus may encounter in a cell-type-specific manner?Nucleated eukaryotic cells encode a number of defence mechanisms to protect them from pathogens. Central to this is the activity of cell autonomous/innate immune responses. The innate immune response to HCMV is of critical importance to the intracellular environment encountered by the virus and the routes of entry determine some of the ways the virus is detected. Furthermore, given the variety of entry routes utilised by HCMV to infect different cell types, the innate immune response may be shaped differently in different cell types.One of the major challenges facing HCMV in its attempt to establish latency is a requirement to evade host immune responses following infection in the absence of expression of the vast array of cell modulators encoded within its genome. One such host response is the activation of apoptosis as a mechanism to remove a replication niche for the pathogen. For instance, during lytic infection, the expression of proteins such as UL36, UL37, UL38 and TRS1 as well as lncRNAs such as β2.7 restrict cell death pathways A, allowiA simple solution to the conundrum would be that viral infection of non-permissive cells does not activate cell death pathways at all. However, studies with multiple viruses imply that this is not the case. This is exemplified in the context of HCMV, CD14+ monocytes, CD34+ cells and the THP1 cell line B. It hasA similar model applies to the infection of CD14+ cells, whereby MCL-1 upregulation was also demonstrated to be required for the survival of CD14+ monocytes following HCMV infection. However, differences are evident. Firstly, the effect in monocytes was PI3K-dependent . SecondlIt is highly likely that the ability of HCMV to act as a poly-ligand through engagement of multiple receptors is critical for the survival response. Essentially, survival is the outcome of the activation of multiple signalling cascades working in concert to promote viability and only when these pathways are activated at the same time is protection observed.Underpinning the cell-intrinsic innate immune signalling apparatus that detects different components of virus particles is the activation of the production of interferon (IFN) and antiviral cytokines. This system consists of a series of pattern recognition receptors (PRRs) that detect various pathogen-associated molecular patterns (PAMPs). The role of the innate immune response to a viral infection is dual—to initiate a primitive antiviral state in the infected and neighbouring cells, and to help recruit cells of the adaptive immune system to commence a more specific response.It is likely that viral glycoproteins such as gB and gH are detected at the cell surface, whereas other components such as viral nucleic acid can be detected at both the cell surface and intracellularly . Toll-liBeyond cell-surface receptors for viral components, the other major route for detection of viral infection involves the detection of viral DNA after the viral nucleic acid has become exposed to the cell. Mechanisms of detection of viral DNA have come under intense scrutiny following initial reports that demonstrated activation of IRF3 in response to viral, bacterial and synthetic DNA .Multiple receptors, including cGAS, IFI16, DNA-PK, DAI, LRRFIP-1 and RNA Polymerase III have been shown to recognise DNA, activating innate signalling that leads to the transcription of interferon and other antiviral cytokines . FollowiA key player on which the DNA sensing pathways converge is the adapter molecule STING. As such, STING has been shown to be critical for the activation of IRF3 in response to DNA via TBK1 during HCMV infection . SimilarThe mechanism of detection of HCMV DNA upstream of STING is not yet fully established and could possibly involve multiple detectors acting in cell-type-specific ways. It has been reported that DAI plays a role in the detection of HCMV DNA in fibroblasts , despiteBeyond IRF-3 signalling, another PRR termed AIM2 has been shown to recognise DNA and initiate the formation of an inflammasome that leads to the elaboration of IL-1β and IL18-independent of IRF-3 signalling. It is not fully understood if inflammasomes play an important role in the control of HCMV infection. Inflammasomes are multimolecular bodies that convert pro-IL-1β and pro-IL-18 into their active forms, IL-1β and IL-18. AIM2 has been well characterised as a DNA sensor present in macrophages; however, the exact role it plays in HCMV has yet to be established. It has been noted that monocyte activation in response to HCMV is independent of the inflammasome adapter protein ASC , perhapsWhilst cytokines and chemokines undoubtedly primarily function to mount an immune response against HCMV, the cytokines involved have also been shown to have an effect on the latency of herpesviruses. HSV-1 latency in vitro has been shown to be enhanced by the production of type I IFNs , and sevWhat all these studies of HCMV may reveal is that the nature of the innate immune response to HCMV differs depending on which cell types are studied and the entry mechanism used. The influence of the differing responses of cell types and how this might impact on the progression of the infection of HCMV warrants further investigation. One potential conceptual challenge is the question of whether the host or the virus benefits the most from an IFN-based response against the virus. Whilst IFN may be important for keeping herpesviruses at bay, if it contributes to or even enhances the establishment of latency then, on an epidemiological level, this is potentially advantageous. An obvious benefit from lifelong cycles of latency and reactivation is the increased opportunity for transmission to new hosts.The lack of HCMV virus production during latency suggests that many of the pathways used to detect viruses may not be activated during latency. However, whether HCMV DNA is a potential long-term PAMP during latency is unclear. This question is linked to the broader issue of how the viral DNA is distinguished from cellular DNA by the DNA sensing machinery. In the case of vaccinia, a cytoplasmic DNA virus, the cellular location of the DNA may be the defining feature. However, HCMV DNA exists in a circularised form in the nucleus during latency , and, fuThe sensors playing a role in sensing of HCMV have yet to be fully tested. Whilst a role for HCMV was suggested by the existence of an inhibitor of IFI-16 in pp65, more recent studies have demonstrated that genetic knockout of AIM2-like molecules (including IFI-16) using CRISPR have shown that IFI-16 is dispensable for the induction of IFN in response to HCMV, conflicting with data generated using an siRNA approach in human fibroblasts . The preThe modulation of the cellular environment is initiated at the point of entry via the activation of signalling pathways. Furthermore, within the virion are several tegumental phosphoproteins such as pp65 and pp71 that are rapidly delivered to the nucleus upon infection of differentiated cells such as fibroblasts. Here they can counteract antiviral responses dictated by interferon-induced PML bodies and components of the DNA sensing pathway. For instance, pp71 targets Daxx ,118, leaThe hypothesis that the activity of PML bodies may be beneficial for the establishment of latency is an intriguing possibility. However, contrasting data have demonstrated that knockdown of individual PML body components  ,122 in TFinally, it may be worth considering the possibility that the differential localisation of tegument proteins during infection of non-permissive cells is beneficial during latency for other reasons. An abundance of pp71 in the cytoplasm may be important to prevent the activity of STING and consequential DNA sensing of the incoming latent genomes. Alternatively, the occlusion of pp65 and pp71 from the nucleus may promote a transient antiviral state that is interferon-dependent based on a failure to prevent sensing and IFN signalling. Studies in HSV have implicated that type I interferons may support the establishment of latency and thus a transient induction may benefit HCMV. In some ways, it is not unsurprising that HCMV utilises aspects of the antiviral IFN response. Multiple studies of lytic infection have suggested that IFN-stimulated genes such as tetherin  and IFITWhilst the precise impact of cellular restriction factors on the ability of HCMV to establish latency is still subject to investigation, perhaps in vitro models have the potential to generate a misleading picture of HCMV latency in vivo. For example, it is possible that interferons and pathogen detection have no role to play in the establishment of latency in early progenitor cells. A recent and compelling study of stem cells indicated that these cells, which are highly resistant to viral infection, encode high levels of endogenous interferon-stimulated genes (ISGs) . CrucialIn conclusion, the outcome of infection is a concert of effects triggered by the pathogen and the host. Understanding the triggers of the host responses and the subsequent mechanisms pathogens use to subvert them can give us insight not only into the host–pathogen interaction but can also shed new light on the complex function of the underlying components that regulate these multifaceted host responses.


It has frequently been reported that some users of conventional neurofeedback systems can experience only a small portion of the total feedback range due to the large interindividual variability of EEG features. In this study, we proposed a data-driven neurofeedback strategy considering the individual variability of electroencephalography (EEG) features to permit users of the neurofeedback system to experience a wider range of auditory or visual feedback without a customization process. The main idea of the proposed strategy is to adjust the ranges of each feedback level using the density in the offline EEG database acquired from a group of individuals. Twenty-two healthy subjects participated in offline experiments to construct an EEG database, and five subjects participated in online experiments to validate the performance of the proposed data-driven user feedback strategy. Using the optimized bin sizes, the number of feedback levels that each individual experienced was significantly increased to 139% and 144% of the original results with uniform bin sizes in the offline and online experiments, respectively. Our results demonstrated that the use of our data-driven neurofeedback strategy could effectively increase the overall range of feedback levels that each individual experienced during neurofeedback training. Neurofeedback is a type of biofeedback technology that has generally been used to train the ability of self-regulation based on the real-time analysis of neural signals such as electroencephalography (EEG), magnetoencephalography (MEG), and real-time functional magnetic resonance imaging (fMRI) . Over thAmong the various neural signal acquisition modalities including EEG, MEG, and fMRI –14, EEG Despite the recent development of EEG-based neurofeedback strategies, however, the EEG-based neurofeedback systems still suffer from some limitations. One of the most representative limitations is the large interindividual variability of the EEG features that have been used for neurofeedback –22. Due In the present study, we attempted to make the users of the neurofeedback systems experience a wider range of auditory or visual feedback. To this aim, we proposed a new neurofeedback strategy, named as the data-driven user feedback strategy, which uses nonuniform bin sizes to divide the entire range of an EEG feature into many bins, each of which is assigned to a corresponding feedback level, based on the offline EEG database acquired from a group of individuals.Two groups of healthy subjects were enrolled in our experiments. The first group, consisting of 22 healthy subjects , participated in offline experiments to construct an EEG database, and, the second group, consisting of five healthy subjects , participated in online experiments to validate the performance of the proposed neurofeedback strategy. Subjects who participated in the offline experiment were not enrolled in the online experiment again because we wanted to test whether the new subjects could show good performance using the proposed neurofeedback strategy based on the other participants' database. All participants had normal or corrected-to-normal vision and none had a previous history of neurological, psychiatric, or other severe diseases that might otherwise affect the experimental results. Before each experiment, comprehensive information on the experiments was given to each participant, and written informed consent was obtained from each subject. This study was reviewed and approved by the Institutional Review Board Committee of Hanyang University.To verify the feasibility of the proposed neurofeedback strategy, we conducted offline and online experiments. In the offline experiment, each participant in the first group performed a “meditation” paradigm, which helped the study participants to relax. The meditation paradigm consisted of resting, first meditation, and second meditation periods . During EEG signals were recorded using a multichannel EEG acquisition system . Two EEG electrodes (Fp1 and Fp2) were mounted on the prefrontal area of the subject's scalp according to the international 10–20 system, assuming a headband-type portable EEG neurofeedback system. The ground electrode was replaced with two electrodes, a common mode sense (CMS) active electrode and a driven right leg (DRL) passive electrode, both of which were located in the posterior region. The EEG data were sampled at 2,048 Hz, and then the spectral power of the alpha band (8–12 Hz) was calculated using a fast Fourier transform.As aforementioned, neurofeedback can be used to enhance cognitive performance of an individual or treat patients with neuropsychiatric diseases and neurological disorders. Although there is no formal procedure for the conventional neurofeedback training, the entire dynamic range of an EEG feature is usually evenly divided into multiple segments, each of which is then assigned to a corresponding level of auditory or visual feedbacks. In this case, some users who have narrow dynamic ranges of the EEG feature might accordingly experience relatively narrow ranges of feedback compared with other users who have broad dynamic ranges. The dynamic range of the EEG feature of an individual can be recorded before the neurofeedback training and set differently for each individual, which is referred to as the customization session; however, this customization session generally needs to be conducted every time a user wants to try the neurofeedback system because of the high intertrial variability of EEG data , 27. In f(x) = ax3 + bx2 + cx + d, where x is the number of EEG epochs included in an initial bin range and coefficients a, b, c, and d are unknowns that need to be determined. The order of the polynomial was selected empirically after confirming that the third-order polynomial could model the overall shape of the scale factor fairly well. We also limited the maximum of the scale factor to 50 to avoid an excessive increment of the number of subdivisions. The coefficients of the polynomial were determined using an optimization procedure with an objective function defined as the average increment in the number of feedback levels that each individual experiences. More specifically, the following equation was used as the objective function of the optimization:Nconv and Nprop represent the average numbers of feedback levels that each individual experiences when conventional and proposed neurofeedback strategies are used, respectively. In calculating the objective function, the total number of feedback levels of the conventional neurofeedback strategy, Nconv, was set to be always identical to that of the proposed neurofeedback strategy, Nprop. Contrary to the proposed strategy, the entire range of the EEG feature was evenly divided in the case of the conventional neurofeedback strategy. A Nelder-Mead simplex direct search algorithm implemented in a MATLAB optimization toolbox  was used for the optimization. The algorithm was designed to solve classical unconstrained optimization problems of minimizing a given nonlinear function without a need for the calculation of derivatives [In the first step, a database of an EEG feature was constructed, while a group of participants was performing a specific mental task (a meditation task in this study). Using the database, a grand histogram was obtained. The grand histogram could show overall distribution of the EEG feature values. In this study, we used frontal alpha band (8–12 Hz) power as the EEG feature –31. Alphivatives . We selet-test was applied to confirm a statistically significant difference in the alpha band powers in the resting and meditative states. The result of the statistical analysis showed that the alpha power was significantly increased during meditation periods (2.19 ± 0.69) compared to resting periods (1.88 ± 0.76) . This result is in line with the findings in many previous studies that consistently reported increased frontal alpha band activity during meditation tasks [To evaluate whether study participants performed the given meditation task well, a statistical analysis was performed. A paired on tasks –31.http://dx.doi.org/10.1155/2016/3939815).To demonstrate further the practicality of the proposed neurofeedback strategy, we asked participants to stare continuously at the real-time feedback  as well as the picture of a valley during the online experiment. The size of the circle and the length of the bar varied with respect to the changes in the EEG meditation feature, but the feedback levels were set differently according to the conditions . The supplementary movie file demonstrates that the user of the neurofeedback system could experience wider ranges of feedback without any training sessions or predata acquisition sessions  experienced reduced numbers of feedback levels after applying the proposed neurofeedback strategy (see Figures The supplementary movie file demonstrates that the user of the neurofeedback system could experience wider ranges of feedback without any trainning sessions or pre-data acquisition sessions.


Additive manufacturing (AM) is the fabrication of real three-dimensional objects from metals, ceramics, or plastics by adding material, usually as layers. There are several variants of AM; among them material extrusion (ME) is one of the most versatile and widely used. In MEAM, molten or viscous materials are pushed through an orifice and are selectively deposited as strands to form stacked layers and subsequently a three-dimensional object. The commonly used materials for MEAM are thermoplastic polymers and particulate composites; however, recently innovative formulations of highly-filled polymers (HP) with metals or ceramics have also been made available. MEAM with HP is an indirect process, which uses sacrificial polymeric binders to shape metallic and ceramic components. After removing the binder, the powder particles are fused together in a conventional sintering step. In this review the different types of MEAM techniques and relevant industrial approaches for the fabrication of metallic and ceramic components are described. The composition of certain HP binder systems and powders are presented; the methods of compounding and filament making HP are explained; the stages of shaping, debinding, and sintering are discussed; and finally a comparison of the parts produced via MEAM-HP with those produced via other manufacturing techniques is presented. Design concepts are generated from scratch or using 3D scanners, computed tomography (CT) scans, or magnetic resonance imaging (MRI) in the case of medical implants.A 3D computer aided design (CAD) model is prepared.The CAD model is analyzed and optimized with the aid of computer optimization techniques such as finite element analysis (FEA).The CAD model is commonly transformed into a Standard Triangulation or Tessellation Language (STL) file and imported into an AM setup. Nevertheless, the STL format lacks many features such as color or materials in the parts. For these reasons a new format was implemented by ASTM ISO, the Additive Manufacturing File Format (AMF) . BesidesThe geometric shape in the STL or other format files is sliced into thin layers and the movement of the depositing or fusing unit (“printing head”), and substrate (“printing platform”), as well as other parameters are programmed by specialized software that prepares the G-code, which is a numerical control programming language.The AM machine builds the tridimensional object layer by layer with the specified parameters.The built part is removed from the building platform and the removal of support structures used to build complex geometries is conducted. The excess unbound building material needs to be removed in a cleaning step depending on the type of AM technique used.After the object is removed and cleaned, it might require further post-processing such as polishing, coating, or thermal treatment to obtain a functional part.Additive manufacturing (AM) is a technology for fabricating real three-dimensional 3D) objects, using metals, ceramics, or plastics, which may be used in various applications . AM is dD objectsThe main advantage of AM over conventional manufacturing processes is dealing with geometric and material complexities that cannot be created, technically and/or economically, using subtractive and formative manufacturing processes . AM has Over the last three decades, many AM technologies have been developed. The standard EN ISO/ASTM 52921:2017  defines The focus of this review is on material extrusion additive manufacturing with highly-filled polymers (MEAM-HP) with a particular emphasis on its application for the fabrication of metallic and ceramic components. MEAM-HP, in this case, is a multi-step/indirect process, which makes use of a sacrificial polymeric binder material to shape metallic and ceramic powder particles. The polymeric binder is usually removed in subsequent  treatments and the powder particles are bonded together in a conventional sintering step.This review paper is organized in six subsequent sections following this introduction. Material extrusion additive manufacturing (MEAM) consists of softening a material and pushing it through an orifice in order to deposit that material in layers to build a 3D structure . ExtrusiUnlike other AM techniques, extrusion-based additive manufacturing techniques are well suited for multi-material deposition and can be used for a wide range of thermoplastic materials ,22,23,26The basic principle of material extrusion additive technology involves the loading and liquefaction of the material, moving the material through a nozzle or orifice by applying force or pressure, plotting liquefied material according to a pre-defined path in a controlled manner, and layer-by-layer bonding of the material to itself or a secondary build material to form a coherent solid structure . After aTM and Markforged calls theirs atomic diffusion additive manufacturing (ADAM). The profiles are fit into cartridges and are then fed into a plasticizing unit where the highly-filled thermoplastic composite is soft enough for extrusion. The soft material accumulates in a reservoir and finally a mechanical drive system  pushes the soft material and deposits it onto the building platform in a layer-by-layer manner [Two companies based in the USA, Desktop Metal Inc. and Markforged Inc. ,30, currr manner ,30,31. Ir manner ,33, withr manner . AlternaIn general, ram extrusion machines with cartridges are meant to be used for shaping parts that eventually will be made out of only metal or ceramic, thus the rods have a large amount of powder and the printed parts are sintered to obtain a dense part. The rods use similar materials as used in the well-established process of powder injection molding (PIM) . On theiTM. However, such a name could be applied to other AM techniques that melt materials and deposit them onto a platform or onto previously deposited layers of material, such as pneumatic extrusion, microinjection molding of droplets , screw eeformer . Fused feeformer ,40,41. Peeformer .FFF machines are ram extruders, with the filament being the ram that pushes the softened material out of the printing head. In conventional FFF machines the filament is first pulled by the driving wheels and then it is pushed by the same wheels into a liquefier and later into a nozzle. Therefore, sufficient mechanical strength is required for the filament to retain its shape after being forced through the drive wheels  to transThe most common non-filled thermoplastic materials used in FFF are acrylonitrile butadiene styrene (ABS) and polylactic acid (PLA). However, other examples of non-filled thermoplastics filaments commercially available include: acrylonitrile styrene acrylate (ASA), polyamide (PA), polycarbonate (PC), polyphenylsulfone , polyetherimide (PEI), thermoplastic polyurethane (TPU), polyethylene terephthalate (PET), thermoplastic elastomer (TPE), high impact polystyrene (HIPS), polyvinyl alcohol (PVA), polyether ether ketone (PEEK), polyvinylidene fluoride (PVDF), polyoxymethylene (POM), polyhydroxyalkanoate (PHA) blended with PLA, and some other blends of the previously mentioned polymers ,24,26,44The process of ram extrusion of filaments was pioneered by Stratasys and in 1991 they introduced the first AM system of this kind. Their FDM system had two extrusion heads and used two spools of material; one material was used to build the part and the second was used for the support material. Based on the FDM system, a novel system for the manufacturing of multi-material parts was presented by the Rutgers research group, the fused deposition of multiple ceramics (FDMC) . Four exThe production of rods or filaments represents an additional task that requires special extrusion lines and know-how to obtain filaments or rods with constant cross-sectional area and minimum ovality, which are prerequisites to deposit the adequate amount of material and therefore for a reliable process in fused filament fabrication (FFF) machines. However, not all materials can be made into filaments that can be spooled, but at the same time are rigid enough that they can be pushed by the feeding mechanisms of FFF machines see . TherefoA screw extruder is divided into several zones. In the solid conveying zone pellets are transported to the melting zone, where pellets are softened under heat and friction, and the metering zone in which the molten material is submitted to high pressure before its eviction through the nozzle. The rotating screw has a pumping effect and thus it moves the material from the feeding zone to the nozzle . ControlBellini et al. ,47 develCruz et al.  developeTwo companies have developed screw-based MEAM setups for making small parts and both are currently commercializing their machines. One of them is AIM3D GmbH  ,50. The 3 and 10.8 × 3.9 × 4.4 m3. The motion system is driven by linear motors and the absolute position accuracy is ±0.127 mm. Using BAAM, the manufacturers have been able to print sections of car bodies and sections of buildings. The materials that have been tested include pellets of acrylonitrile-butadiene-styrene (ABS), polyphenylenesulfide (PPS), polyetherketoneketone (PEKK), and polyetherimide (PEI), as well as composites materials containing carbon, glass fibers, and NdFeB particles [Cincinnati Inc.  and Oak Ridge National Laboratories  have developed a screw extrusion machine for large size additive manufacturing. The setup is called big area additive manufacturing or BAAM. It consists of a single screw extruder mounted vertically on a machine frame, similar to the frames used for laser-based AM machines. The extruder has a feed-rate of 36 kg/h and a unique automatic taping mechanism, which is used to flat the deposited material to increase the contact between deposited layers. The setup is available in two sizes: 7.8 × 3.7 × 3.3 marticles ,54.Highly-filled polymers are compounds of polymers with added particles at concentrations well above 20 vol % in which the interactions between fillers cannot be neglected . In thisThe idea of using highly-filled polymers for the additive manufacturing of metal and ceramic parts was first introduced in the 1990s; it was named fused deposition of metals (FDMet) ,59 and fIt is important to mention that the SDS process could use other additive manufacturing techniques such as indirect powder bed fusion, binder jetting, vat polymerization, and material jetting. For indirect powder bed fusion, sinterable particles can be coated with a thermoplastic and these coated particles are then fused together with low power lasers, since only the polymer needs to be sintered or melted ,64. For MEAM-HP has shown great promise as a cost-effective alternative for the fabrication of metal, ceramic, and metal-ceramic parts , particuAs mentioned before, PIM and MEAM-HP rely on the use of highly-filled polymers; the similarities and differences between their feedstocks will be described in the following section.Feedstocks for MEAM-HP and PIM are multicomponent systems consisting of a polymeric blend, sinterable powder, and additives. All of these components are needed to fulfil the requirements at different steps of the overall SDS process. Details about these different feedstock components will be described in the following sections.The main binder component is the component present in the largest amount and it is removed first during the debinding step. The main binder component represents between 50 and 90 vol % of the total binder systemThe backbone is the component used to hold together the shape of the part while the main binder component is removed during the first debinding stage. The backbone is thermally decomposed prior to sintering. The backbone represents 0 to 50 vol% of the total binder system.Additives like dispersant agents, compatibilizers, and stabilizers help to disperse the filler particles in the polymeric binder, preventing agglomeration and phase separation. Additives represent between 0 to 10 vol % of the binder system.The polymeric component of the feedstock is referred to as the binder system. The binder system greatly influences the production process and the quality of the sintered parts, even though it is completely removed during the debinding step. Usually a binder system consists of different types of polymers, waxes, and additives . GeneralIn fact, it is possible to use a binder system with only one polymeric material, but then the debinding step is limited only to thermal degradation, which is a very slow process compared to solvent or catalytic debinding ,76,77. FAs can be seen in On the other hand, binder systems for MEAM-HP, which are used as filaments, have components that lead to flexible feedstocks that can be spooled. One way to make feedstocks flexible is to add elastomers ,62,100 oThe effect of binder formulation on the properties of filaments have been investigated and linked to the ability of those filaments to be printed to build a 3D object. For example, Kukla et al.  studied 3N4) via filaments. The filaments produced with the feedstocks were too brittle to be spooled. However, the high material stiffness enabled the production of parts by hand-feeding the equipment. For this reason, Agarwala et al. [Agarwala et al.  initialla et al.  later imBhat et al.  studied These examples demonstrate that finding the right combination of polymers and their proportions in the binder is not a simple task and for this reason, most of the actual binder formulations that work are not described in detail in the literature. However, it is clear that the selection of the optimal binder systems can be directly based on the compatibility with the powder, resulting viscosity, and mechanical properties (modulus and flexibility) required for MEAM with filaments.Many additive manufacturing technologies rely on the use of powder as their building material; MEAM-HP is not an exception. The powder material used in MEAM-HP is in principle the same metal or ceramic powder as that used in PIM; this represents an advantage compared to AM techniques that rely on powder bed technology, which require a very specific particle size distribution for the process to work properly. In general, MIM utilizes particles with an average size between 5 to 15 μm . TypicalMany metals and ceramics are available for PIM  and in pChanging the characteristics of the powder can drastically influence the mechanical and flow properties of the feedstock materials, as has been reported in the PIM and the particulate composite literature ,128,129.12O19Sr), and yttria stabilized zirconia (YSZ), see 12O19Sr and YSZ) have the stiffest and less flexible filaments and they have to be fed into the printer head manually, i.e., one strand at a time. Among the metal-filled filaments, 316L was the stiffest, followed by the Cu, Ti6Al4V, NdFeB, and AlSi10Mg. All of the metallic filaments with the exception of NdFeB could be continuously printed on conventional MEAM machines having a feeding unit with counter-rotating wheels. The NdFeB-filled filaments had low stiffness and were fragile and they could only be printed on a MEAM machine with a feeding mechanism consisting of conveyor belts as opposed to rotating wheels. The conveyor belts provided more contact between the feeding mechanism and the filament and prevented the breakage of the filament during feeding into the liquefier unit [One main factor that affects the properties of highly-filled filaments is the change in the physical and chemical properties of the metal or ceramic fillers. This change can be attributed to the different size, morphology, and chemical composition of the different metals and ceramics, which lead to a distinct interaction with the binder system. Examples of how changing the particles affects the properties of filaments have been presented in the AM literature. For example, Kukla et al.  presenteier unit . It can Keeping the same powder type but with different characteristics has also been investigated. For example, Wu et al.  studied 12O19Sr . It was observed that increasing the content from 55 to 60 vol % made the filaments significantly less ductile ; however, the stiffness remained almost the same. The filaments with the highest powder content had to be fed manually and even the printed parts were easily broken if not handled properly during the removal from the building platform. 12O19Sr, a more brittle and fragile filament; but contrary to the filaments with Fe12O19Sr, both steel filaments could be fed continuously from the spool to the printer head. What can be concluded is that the maximum volume content that yields printable filaments is very much material-dependent since the mechanical properties are greatly dependent on the particle-matrix chemical interaction [The powder content in the filament has also been investigated. Gonzalez-Gutierrez et al.  characteeraction .The processability of filaments by MEAM is not only dependent on the mechanical properties of the filament, but also on the flow properties of the feedstock as well as on the processing conditions, the geometry of the filament, and the design of the printing head. This was discussed by Venkataraman et al.  for ceralated as :(1)σcr≈For a non-Newtonian fluid with apparent viscosity fined as :(2)ΔP=8Venkataraman et al.  assumed 5 to 5 × 105 s−1 in the range of shear rates commonly employed in filament MEAM (100 to 200 s−1) [As can be observed in Equation (3), the buckling of the filament will also depend on the radius (200 s−1) .3N4. In this case, the parameter A similar work was conducted by Rangajaran et al. , who invn et al. , the bucalidated , but furFeedstocks for MEAM-HP can be produced in a similar manner as PIM feedstocks: the metal or ceramic powder is mixed with the molten binder constituents and the filler is dispersed in the binder. One of the main requirements is that the resulting compound has a homogeneous distribution of powder particles and binder components. This helps to minimize the segregation of components during the shaping process and later on to obtain isotropic shrinkage after debinding and sintering. Avoiding the segregation of components is crucial to prevent visual defects, excessive porosity, warpage, and cracks in the sintered part . An exam3N4, zirconia, and other ceramics, the powder is first coated with surfactants  during a ball-milling step before blending with the rest of the binder components. After milling, the powder has to be sieved to further remove agglomerates and increase the homogeneity of the fillers in the feedstocks [Feedstocks can be prepared either in batch or continuous processes. Batch operations include roll mill kneaders and mixers, while continuous processing can be done in screw extruders and shear rolls . In geneedstocks ,133,137.edstocks .3N4 feedstocks.Filaments are generally produced via extrusion. For example, in capillary rheometers ,137 for It is very important to produce filaments with tight tolerances on the diameter, because the feeding mechanisms in a filament-based MEAM machine sets the feeding speed, and thus the flow rate, based on the assumption that the filament has a constant diameter. If the filament diameter is smaller than the specified value, the flow rate is lower than desirable and thus strands with smaller widths and thicknesses are deposited. This is referred to as underflow and it results in poor bonding between the deposited strands or creates voids between adjacent strands that will not be closed even after sintering. If the diameter is larger than the specified value, the filament cannot be fed into the liquefier or it may lead to overflow. Overflow leads to poor definition of the fine features of the part. Also, the filaments should be as round as possible  to be properly gripped by the feeding mechanism and so ensure constant feed without slippage . In ordeThe effect of different compounding strategies on the properties of filaments has been observed in zirconia feedstocks . It was The fabrication process is crucial to obtain good quality sintered parts. If there are large gaps between the deposited strands of material and other defects, these defects will remain in the sintered parts, affecting the mechanical performance and functionality of the final part. Therefore, optimizing the printing parameters to obtain a homogeneous part is very important.The building of parts via MEAM-HP has been reported in the literature. Several studies have been performed to investigate the effect of the printing parameters on the properties of the built parts via MEAM ,73,138. Other parameters that affect the quality of the final part are the improperly deposited strands  or the spacing between strands, which can lead to incomplete bonding between adjacent strands or between layers. In turn, all of these factors lead to a systematic variation in density and defect appearance in shaped parts . For unfAfter building the part via MEAM, the deposited strands are very noticeable and the lines do not disappear after sintering, as it can be seen in Another possibility for smoothing the surface of shaped parts could be the use of solvent vapor. Kuo and Mao  developeThe polymeric binder system must be removed without disrupting the MEAM-shaped part; this process is commonly referred to as debinding and it is also applicable for parts produced by PIM. Polymers have to be removed completely from the green part since carbon residues can influence the sintering process and affect the quality of the final product in a negative way. Moreover, binder removal is one of the most critical steps in the SDS process , since d3N4 particles. The first cycle was carried out in a nitrogen atmosphere, where the binder was wicked out of the material using a setter bed. The second cycle was performed in air, where the residual binder was burned off. The composition of the binder system can be modified to improve the thermal debinding rates, which tend to be very slow to prevent damage to the shaped part. This was shown in feedstocks containing 55 vol % piezoelectric powder, in which the proportions of base binder, tackifier, wax, and plasticizer were adjusted not only to improve the processability by MEAM, but also to exhibit good burnout properties to reduce the time required to thermally decompose the binder without any blistering or cracking in the finished samples [The most common way to remove the binder from the shaped/printed parts is to heat the binder so it melts and flows out of the part and/or the binder is thermally degraded and diffuses out of the shaped part. The temperatures used to carry out thermal debinding depend on the formulation of the binders used. For example, Onagoruwa et al.  had a bi samples .12O19) have more surface area to which the binder system can adhere and smaller pores between the particles; these two factors can slow down the diffusion process first of the solvent and then of the dissolved polymer [Another way to remove the binder system is to use first a solvent extraction step in a special solvent debinding unit, followed by a thermal debinding step in the same furnace where sintering will take place. This strategy has been investigated in feedstocks consisting of a cyclohexane-soluble thermoplastic elastomer (TPE) and a polyolefin as an insoluble component ,125. The polymer .When doing solvent debinding, it is important to remove a certain level of the soluble binder component. Gonzalez-Gutierrez et al.  showed wThe Austrian company EVO-tech GmbH in collaboration with BASF SE offers filaments for MEAM that can be catalytically debound . Such teIt is worth noting that in solvent and catalytic debinding methods, a skeleton of insoluble or non-degraded polymer remains to impart adequate strength and shape retention up to the onset of sintering. This remaining backbone is thermally removed usually between 200 °C and 600 °C in a pre-sintering step, depending on its chemical composition .The last step of the process to obtain dense metal, ceramic, or metal/ceramic parts is sintering. Sintering is a thermal treatment that transforms metallic or ceramic powders into bulk materials. Sintering is performed at temperatures below the melting temperature of the major constituent in the powder, generally within 70 to 90% of the powder’s melting point .Rearrangement, particle movement, and mass transport happen during sintering. Before the sintering occurs, the parts have a highly porous structure formed by particles with a large free surface and therefore a high surface energy. As heat is applied and the temperature increased, the system tends to reduce the surface energy by the formation of solid bonds between the particles. When temperatures increase beyond one half to two-thirds of the melting temperature of the powder material, atomic diffusion and chemical changes on the surface of the particles occur and lead to the formation of solid bonds. These bonds, known as necks , continuShrinkage of the shaped parts by MEAM is always observed after sintering due to the reduction of the porosity and the densification of the parts Examples of printed and sintered parts are shown in It is important to mention that the shrinkage does not occur in the same amount in all dimensions, as observed by Lous et al. , AgarwalOne advantage of using MEAM as a shaping technology is that multi-material parts can be easily fabricated compared to powder bed technologies or vat photopolymerization. The multi-material process merely requires a printing head that can work with two or more materials simultaneously. However, the main limitation for applying MEAM-HP for the fabrication of multi-material components is the co-sintering step. For successful co-sintering, the powders have to be sintered together at the same temperature and in the same atmosphere. In order to avoid excessive mechanical stress under cooling, the coefficient of thermal expansion and the shrinkage behavior of both materials should be similar .One of the first demonstrations of the possibility of producing multi-material components was described by Jafari et al. . FilamenMore recently, the possibility of producing parts made of 17-4PH steel and zirconia via MEAM-HP has been demonstrated ,150. ThePost-sintering operations of the MEAM-printed parts such as the improvement of surface texture, accuracy, aesthetics, and a matte surface finish are often desired depending on the application of the printed part. A simple bead blasting of the surface can help even the surface texture, remove sharp corners from stair-stepping, and give an overall matte appearance. If a smooth or polished finish is desired, then wet or dry sanding and hand-polishing are performed; this is particularly important if no surface treatment was performed before debinding and sintering. In many cases, it is desirable to paint the surface  prior to sanding or polishing. Painting the surface has the dual benefit of sealing porosity and, by viscous forces, smoothing the stair-step effect, thus making sanding and polishing easier and more effective .Depending on the material used, AM parts can be effectively colored by simply dipping the part into a dye of the appropriate color. If painting is required, the part may need to be sealed prior to painting. Common automotive paints are quite effective in these instances. Another aesthetic enhancement  is chrome plating. Ni, Cu, and other coatings can also be applied to the surface for aesthetic improvements. This is particularly important for materials that are sensitive to oxidation and for which the oxidation process causes a significant decay in the wanted properties, for example, the magnetic performance of NdFeB parts .Another post-sintering operation is the infusion process for the production of multi-material parts. The production of metal-ceramic composites was investigated by Onagoruwa et al.  and BandMEAM-HP is still in its early stages of development. Therefore, there is not a lot of literature comparing the properties of parts produced by this technique to parts produced by traditional manufacturing methods such as casting and PIM or to other AM techniques such as powder bed fusion, binder jetting, or vat photopolymerization. Just a few examples were found, and they are all presented in this section.Bandyopadhyay et al.  tested tAgarwala et al.  preparedGriffith and McMillin  preparedMaterial extrusion additive manufacturing (MEAM) is a very popular way to shape three-dimensional objects with thermoplastic materials. However, when using highly-filled polymers (HP) combined with the post-shaping operations of debinding and sintering, metallic, ceramic, and metallic-ceramic parts can be produced.MEAM can be performed in three main types of setups, which differ in the way the material is fed to the nozzle with either plungers, screws, or as a filament. The use of filaments is the most widespread. In order to carry out the shaping, debinding, and sintering successfully, appropriate feedstock materials have to be developed for each of the MEAM types. The feedstock materials are made by combining different ingredients in a binder system with powder of the desired material. The binder system has to provide enough flowability, when in the molten state, and enough mechanical strength for handling and processing the material before melting and after re-solidifying. The mechanical strength is particularly important when performing filament-based MEAM, since the filaments should not break or buckle during the feeding of the material into the nozzle. The binder system should also thermally decompose or dissolve without affecting the shape of the formed part during the debinding step, and the binder system should degrade and be fully eliminated before sintering of the particles takes place to prevent contamination of the produced part that could lead to poor properties. For this reason, it is clear that the development of such feedstocks is a complicated task and thus the full description of their recipe is not publicly available, since it represents a competitive advantage for their developers.Once the feedstock material has been developed, the building job has to be optimized by varying the processes parameters and the building strategy , since these factors and other factors will affect the properties and shape of the final part. The debinding of shaped parts by MEAM-HP can be performed in a similar manner as in powder injection molding (PIM), but the majority of results available in the literature are for thermal and solvent debinding. Finally, sintering is conducted the same way as in other powder technologies such as PIM. It is important to point out that there will be a significant anisotropic shrinkage similar to the one observed in powder injection molding; therefore, preliminary trials should be performed to optimize the final shape and performance of parts produced by MEAM-HP, debound, and sintered.When comparing the results of MEAM-HP with other shaping technologies, it was observed that MEAM-HP produces objects with acceptable properties when the overall process is optimized. After optimization, dense parts of metal, ceramic, or metal-ceramic can be produced for structural applications. However, it is possible that even if the parts retain some porosity after this manufacturing technique, this could be an advantage for certain applications where a large surface area is beneficial; examples of this include filtration, medical applications, or catalytic applications ,154. In The development of improved simulation tools that can predict the properties of built parts by knowing the material properties, processing parameters, and building strategies. This could allow the optimization of the printing process before it is actually done.The development of simulations tools for the debinding and sintering processes.The development of automatic monitoring systems that supervise the building process that stop and adjust the processing parameters to reduce the amount of unwanted defects on the printed part.The development of mechanisms to smooth the surface of the deposited strands as they are being deposited to reduce the surface roughness of the printed parts and to increase the mechanical properties of printed parts.The development of new binder systems for filaments and pellets, which are faster to remove and can use other solvents, such as water.The development of new feedstock materials with different filler particles that can be co-sintered for the fabrication of new multi-material components with new functionalities.The development of simulation tools that could speed up the development of new highly-filled polymers to be used in MEAM. Such simulation tools should take into account the compatibility and interaction of the different polymeric components, as well as the chemical and physical characteristics of powders.The improvement of the reliability of screw-based MEAM machines to replace the filaments by pellets, since filaments are hard to make and limit the amount of powder that can be added to the feedstock.The improvement of the properties of the interface of multi-material components so as to promote good adhesion between the different materials and obtain multi-material components with a long service life.Some of the work that could be done in the future to improve MEAM-HP includes:As it can be seen, there are areas in research and development in the field of MEAM-HP that should be improved in order to achieve a reliable process that can be used for the fabrication of unique products with multifunctional properties.