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Bioelectronics presents a promising future in the field of embedded and implantable electronics, providing a range of functional applications, from personal health monitoring to bioactuators. However, due to the intrinsic difficulties present in producing and optimizing bioelectronics, recent research has focused on utilizing machine learning (ML) to reliably mitigate such issues and aid in process development. This review focuses on the recent developments of integrating ML into bioelectronics, aiding in a multitude of areas, such as material development, fabrication process optimization, and system integration. First, discussing how ML has aided in the material development by identifying complex relationships between process input parameters and desired outputs, such as product design. Second, examine the advancements in ML to accurately optimize fabrication precision and stability for various 3D printing technologies. Third, provide an overview of how ML can greatly assist in the analysis of complex, nonlinear relationships in data obtained from bioelectronics. Lastly, a summary of the challenges present with utilizing ML with bioelectronics and any other developments in this field. Such advancements in the field of bioelectronics and ML could hopefully build a strong foundation for this research field, promoting smart optimization together with effective use of ML to further enhance the effectiveness of such applications.

The first years of an infant’s life represent a sensitive period for neurodevelopment and see the emergence of nascent forms of executive function (EF), which are required to support complex cognition. Few tests exist for measuring EF during infancy, and the available tests require painstaking manual coding of infant behaviour. In modern clinical and research practice, human coders collect data on EF performance by manually labelling video recordings of infant behaviour during toy or social interaction. Besides being extremely time-consuming, video annotation is known to be rater-dependent and subjective. To address these issues, starting from existing cognitive flexibility research protocols, we developed instrumented toys as a new task instrumentation and data collection tool suitable for infant use. A commercially available device comprising a Barometer and Inertial Measurement Unit (IMU) embedded in a 3D-printed lattice structure was used to detect when and how the infant interacts with the toy. The data collected using the instrumented toys provides a rich dataset describing the sequence of toy interaction and individual toy interaction patterns, from which EF-relevant aspects of infant cognition may be inferred. Such a tool potentially provides an objective, reliable, and scalable method of collecting early developmental data in socially interactive contexts.

A hybrid additive manufacturing process is proposed to incorporate piezoresistive sensors into a polyamide matrix for real-time structural health monitoring. 3D-printed samples were tested under tensile, flexural, and indentation testing using various sizes, numbers, and locations of the sensors, while the electrical resistance was recorded in real-time. Under uniaxial tensile loading, strength reached 36 MPa, and the resistance values linearly increased by up to 105% at ∼150% of strain. During flexural testing, the sensor had higher resistance when it was located above the supports rather than under the loading roller. The conductive paths were placed near the plate's top, middle and bottom sections to localise the damage during indentation testing. The energy absorbed during indentation testing reached 34–36 J. Resistance readings of sensors with 1 cm width showed better linearity and sensitivity than their wide (10 cm) counterparts. Three 2D sensors were interconnected to make a 3D sensor. The resistance values gradually increased after each strike during repeated indentation, but the overall strength remained similar. In summary, this work demonstrates a novel approach toward customisable 3D-printed piezoresistive sensors for detecting global or local damage. Agency for Science, Technology and Research (A*STAR) Submitted/Accepted version B.M.G.’s research was funded through the Singapore International Graduate Award (SINGA) from the Agency for Science, Technology and Research (A*STAR).

The process-structure-property (PSP) relationship of continuous carbon fiber reinforced thermoplastics manufactured through fused filament fabrication was investigated for the first time, specifically on mode I Interlaminar Fracture toughness (ILFT). A standard double cantilever beam test is used to evaluate the mode I ILFT. The PSP relationship is established using micro-computed tomography scan and laser optical microscope. This study demonstrates that higher nozzle, bed temperatures, and lower print speed enhances the mode I ILFT. The mode I ILFT is found to be greatly affected by porosity and bond formation between the layers, which are dependent on the process parameters. National Research Foundation (NRF) This research is supported by the National Research Foundation, Prime Minister’s Office, Singapore under its Medium-Sized Centre funding scheme.

A database linking process parameters and material properties for additive manufacturing enables the performance of the material to be determined based on the process parameters, which are useful in the design and fabrication stage of a product. The data, however, are often incomplete as each individual research work focused on certain process parameters and material properties due to the wide range of variables available. Imputation of missing data is thus required to complete the material library. In this work, we attempt to collate the data of Ti6Al4V, a popular alloy used in aerospace and biomedical industries, fabricated using powder bed fusion, or commonly known as selective laser melting (SLM). Various imputation techniques of missing data of the SLM Ti6Al4V dataset, such as the k-nearest neighbor (kNN), multivariate imputation by chained equations, and graph imputation neural network (GINN) are investigated in this article. It was observed that kNN performed better in imputing variables related to process parameters, whereas GINN performed better in variables related to material properties. To further improve the quality of imputation, a strategy to use the median of the imputed values obtained from the three models has resulted in significant improvement in terms of the relative mean square error. Self-organizing map was used to visualize the relationship among the process parameters and the material properties.

Grippers with shape conformance and tunable rigidity are often achieved using a combination of soft and rigid structures made of different materials. These grippers are often known as soft grippers. In this review article, various gripper designs with shape conforming ability and stiffness tunability are discussed. In particular, the discussion mainly focuses on the advantages and limitations of each gripper design in terms of the shape conformance and the ease of manufacturability. Then, various 3D printing technologies that are capable of doing multimaterial printing have been introduced. The potential of multimaterial 3D printing of soft and smart grippers has been discussed. Agency for Science, Technology and Research (A*STAR) National Research Foundation (NRF) Submitted/Accepted version This work is partly supported by the Schaeffler Hub for Advanced Research at NTU, under the ASTAR IAF-ICP Programme ICP1900093.

The ability to design and fabricate lightweight structure is one of the most important aspects for building a drone system. Often, connecting wires are used on the drone system for powering and signal transmission at the expense of the drone’s weight. In this paper, we explore the design and fabrication of a safety cage for drone using design optimization and 3D printing. A comparison between fused deposition modelling (FDM) and selective laser sintering (SLS) 3D printing techniques for the fabrication of thin structures was made, and it was found that SLS is more superior in this aspect. A hybrid 3D printing process combining SLS and aerosol jet printing is proposed for the fabrication of lightweight multifunctional structure with printed circuit for a drone safety cage. The safety cage is designed in such a way that maximizes the production efficiency of SLS printing process. In addition, aerosol jet printing is used to fabricate conformal circuit onto the 3D-printed safety cage structure to replace the conventional connecting wires for weight saving purpose. Lastly, an electrical characterization is conducted to investigate the functionality of the printed conductive traces on the safety cage. Nevertheless, this work demonstrates the streamlining of various 3D printing approaches for the fabrication of multifunctional structures with conformal circuits. National Research Foundation (NRF) Submitted/Accepted version This research is supported by NAMIC Singapore and funded by the National Research Foundation Singapore under its Innovation Cluster Programme.

The use of 3D printing to produce acoustic panels with good mechanical and acoustic properties was investigated in this paper. Various fiber layups of the fiberglass face sheet and core designs were fabricated and tested for their indentation resistance and acoustic absorption performance. It was found that the bidirectional face sheet layup exhibited the best indentation energy absorption recording 4.2 J, which is 37% more than the 45-degree layout and 66% more than the quasi-isotropic layup. The specific energy absorption of the hybrid honeycomb core is the best among the three core designs recording 404 J/kg, which is 56% higher than the corrugated triangle with horizontal beam core (359 J/kg) and 20% higher than double ellipse core (335 J/kg). Computed-Tomography (CT) scan was used to study the fracture behavior of the sandwich structures. It was found that the bidirectional layup exhibited a different failure mode as compared to the 45-degree and quasi-isotropic layup. In terms of the acoustic properties, the face sheets with various layup patterns have a low acoustic absorption coefficient with minimal differences from each other at low frequencies (500 Hz–3000 Hz) and have higher absorption coefficients with greater differences from each other at frequencies between 3000 Hz–6500 Hz. The absorption curve was significantly affected by the design of the core. The orientation of the core also comes into play if the core is asymmetrical. The hybrid honeycomb sandwich structure was the optimal structure among the three designs for balanced indentation resistance and acoustic insulation. National Research Foundation (NRF) This research is supported by the National Research Foundation, Prime Minister's Office, Singapore under its Medium-Sized Centre funding scheme.

Polyjet, a material jetting 3D printing technique, has been widely used for the fabrication of patient-specific anatomical models owing to the toolless fabrication technique and its ability to print multiple materials in a single part. Although the fabrication of anatomical models with high dimensional accuracy has been demonstrated, 3D printed anatomical models with tissue-mimicking properties have not been realized. In this study, a composite layering design was used to tune the shore hardness and compressive modulus of the Polyjet-printed parts in an attempt to mimic the properties of human tissues. 216 specimens (with 72 combinations of design parameters) were printed and tested to develop the material library for the anatomical models. An analytical model was developed to estimate the effective compressive modulus and shore hardness of the composite laminate. A neural network was used to learn the multi-dimensional relationship between the design parameters and mechanical properties. The 5-33-2 network size is found to be the optimum neural network structure with a mean square error of 0.98% for the compressive modulus, lower than the traditional response surface method model. A genetic algorithm was used to search the design space for the most optimum design parameters for the targeted effective shore hardness. Nanyang Technological University National Research Foundation (NRF) Published version This research is supported by the National Research Foundation, Prime Minister’s Office, Singapore under its Medium-Sized Centre funding scheme and the Lee Kong Chian School of Medicine, Nanyang Technological University, Singapore Start-Up Grant.

Fiber reinforced sandwich structures are widely used as structural elements due to their lightweight and high load-bearing capacity. In this paper, effort is taken to fabricate fiber facesheet (0°/90° orientation) and corrugated cores (trapezoidal vertical pillared, core-1 and sinusoidal vertical pillared, core-2) of the sandwich structure using fused filament fabrication and Inkjet printing techniques, respectively. Firstly, the quasi-static indentation properties of the additively manufactured facesheet, cores, and sandwich structures were investigated. In addition, acoustic emission signals were monitored during quasi-static indentation testing and the resulting hits data were correlated with quasi-static indentation test results. The quasi-static indentation test results showed that sinusoidal sandwich structures have the highest load-bearing capacity of 1.79 kN until crack initiation and a total energy-absorption capacity of 20.05 J. Acoustic emission hits recorded in the range of 1–30, 30–100, and above 100 represented crack initiation, crack propagation, and specimen failure, respectively. Lastly, micro-computed tomography analysis was performed on the sandwich structures at intermediate indentation displacements, thus leading to the identification of hard to detect failure points and damage propagation. Fracture surface morphology of the sandwich structures showed that fiber pullout and core shear were the dominant damage mechanisms.

The sintering of printed nanoparticle films is a necessary processing step for most nanoparticle inks to make the printed film functional. The sintering of nanoparticle is usually performed through thermal sintering, photonic sintering, induction sintering, etc. Intense pulsed light (IPL) sintering method is one of the most popular sintering methods for nanoparticle inks due to the fast and effective process, but it may yield mediocre performance if improper sintering parameters are used. In this work, we investigate the correlation between the two factors which are the print passes of aerosol jet printing and the sintering distance of the samples on the effect of the surface morphology and sheet resistance. A contradictory correlation between the two factors was observed and a multi-objective optimization was carried out using machine learning method to identify the most optimum conditions for both factors. We found that multi-objective optimization approach is effective in reducing the conflicting responses, thus the sintered thin film can have low sheet resistance and low surface roughness. This work provides an essential guide for achieving conductive films with electrical conductivity and low surface roughness using IPL sintering process for fast fabrication of multi-layered electronics such as electrochemical electrodes.

Unlike traditional hard grippers, soft robotic grippers are commonly made of soft materials so that the soft grippers can produce motion via elastic deformations of their compliant components. The advantages of compliance allow soft grippers to effectively eliminate shocks caused by hard contact, which usually occurs when a hard robotic gripper manipulates a hard object. Until now, the soft robotic grippers are able to operate numerous objects with irregular geometries and different textures. Besides, with the help of embedded sensors, soft robotic grippers have facilitated the growing automation of many tasks, which are thought to be far too delicate for robotic manipulation. This paper reviews the advancement in soft robotic grippers. The paper first introduces the actuation technologies followed by the design and fabrication techniques. The use of 3D printing techniques in the fabrication of the soft gripper is also discussed. The Review then highlights the challenges and future outlook in the fabrication of soft grippers and sensors. Nanyang Technological University Submitted/Accepted version This work was partly supported by the Schaeffler Hub for Advanced Research at NTU, under the ASTAR IAF-ICP Programme ICP1900093.

While significant progress has been made in understanding laser powder bed fusion (L-PBF) as well as the fabrication of various materials using this technology, there is still limited adoption in the industry. One of the key obstacles identified is the lack of materials that can truly manufacture functional parts directly with L-PBF. This paper covers the emerging research on in-situ alloying and multi-metal processing. A comprehensive overview of the underlying scientific topics behind them is presented. The current state of research and progress from different perspectives (the materials and L-PBF processing parameters) are reviewed in order to provide a basis for follow-up research and development of these approaches. Defects, especially those associated with these two material processing routes, are also elucidated by discussing the mechanisms of their formation, including the main influencing factors, and the tendency for them to occur. Future research trends and potential topics are illustrated. The final part of this paper summarizes findings from this review and outlines the possibility of in-situ alloying and multi-metal processing using L-PBF. National Research Foundation (NRF) Accepted version

This article provides a database of the mechanical properties of additively manufactured polymeric materials fabricated using material extrusion (e.g., fused filament fabrication (FFF)). Mechanical properties available in the literatures are consolidated in table form for different polymeric materials for FFF. Mechanical properties such as tensile, compressive, flexural, interlayer, fatigue, and creep properties are discussed in detail. The effects of printing parameters such as raster angle, infill, and specimen orientation on properties are also provided, together with a discussion of the possible causes (e.g., texture, microstructure changes, and defects) of anisotropy in properties. In addition to that, research gaps are identified which warrant further investigation. NRF (Natl Research Foundation, S’pore) Accepted version

Continuous fiber-reinforced polymer (FRP) composites have been used for many applications to create strong yet lightweight products due to their high strength-to-weight and stiffness-to-weight ratios. Aerospace [1], automotive [2], and sport [3]industries are three of the few industries that have been using FRP composites. The increasing need for prototyping and customization of fiber reinforced polymer composite parts is prompting innovations in new manufacturing processes to realize short manufacturing cycle time and low production cost, which is challenging to accomplish using conventional molding process. Fused filament fabrication (FFF) - a material extrusion additive manufacturing (AM) technique trademarked as fused deposition modelling (FDM) by Stratasys- holds promise to achieve low-cost production on continuous fiber-reinforced thermoplastic (FRTP) composites. In this paper, the FFF technique is employed to fabricate continuous carbon and glass FRTP composites and its microstructural characteristics and the resulting tensile, flexural, and quasi-static indentation characteristics of the printed composites are examined. Additionally, the fracture behavior of each test sample is evaluated and discussed in detail. Agency for Science, Technology and Research (A*STAR) Accepted version This work was supported under the A*STAR TSRP – Industrial Additive Manufacturing Programme by the A*STAR Science & Engineering Research Council (SERC) [grant number 1325504105].

Additive manufacturing (AM) technique can help reduce time, cost, and complexity of the manufactured parts apart from adding functionality. This work explores AM techniques, namely fused deposition modeling (FDM) and PolyJet printing to fabricate a free-form structure with embedded electrical components. Both additive manufacturing technologies are discussed, analyzed, and compared. Another additive manufacturing method is then employed to print the electronic circuitry inside to connect the components. High-resolution X-ray computed tomography (CT) is employed to investigate the dimensional accuracy of the printed parts. This work demonstrates an innovative approach to construct arbitrary 3D objects with fully functional electronic circuits. NRF (Natl Research Foundation, S’pore)

This paper describes the design, fabrication and characterization of a microchannel-based strain sensor using flexible material. The work explores the use of additive manufacturing, also known as 3D printing, to fabricate the sensor in easy and cost-effective way. It is shown that 3D printing can print complex designs with ease and fabricate objects with embedded features. Microchannels with dimension of 500 μm diameter are printed within the sensor structure and filled with conductive silver nanoparticle ink. The fabricated device is checked for printability and design valve adherence using optical microscope and micro-CT. The printed sensor is capable of measuring normal (orthogonal to channels) and in-plane (parallel to channels) tensile forces and is tested using a custom-built test rig.

This chapter reviews the recent progress in 3D printing of fiber-reinforced composites and nanocomposites. Emphasis has been paid to discuss the various 3D printing techniques available for composite material printing. 3D composite printing techniques are categorized based on the state of feedstock material used. 3D printing of composite materials offers several advantages over traditional composite manufacturing such as low-cost processing, design freedom, customization, and material saving. Core and nanofiller materials that are used for printing in each technique are categorized, and their industrial applications are discussed. The current limitations and challenges in printing techniques, materials, and mass production of 3D printed composites are outlined. Moreover discussions are provided for addressing the challenges. The future research direction for developing new materials, nanocomposite 3D printing techniques for future industrial applications are proposed.

This chapter reviews the landscape of polymer/carbon nanotube (CNT) composites materials for applications requiring electrical conductivity. Emphasis has been paid to discuss composite material compositions that can be processed using 3-dimensional (3D) printing techniques. 3D printing offers many advantages over traditional subtractive manufacturing, namely, cost reduction, design innovation, customization, and saving material. The chapter gives an overview of various 3D printing techniques for polymer/CNT composites, their properties and potential application in fields of biomedical, electronic devices, and shielding effects.

Fabricating composite parts with high fiber loading and design flexibility has always been a challenge. In the recent issue of Matter, Shi and co-workers showed that high fiber loading can be achieved using localized in-plane thermal assisted (LITA) 3D printing. Coupled with robotic arm, it provides a promising way to fabricate lightweight and high-performance continuous fiber reinforced thermoset composites with great design flexibility. Is 3D Printing of carbon fiber composite the inevitable revolution for the composite industry? We discuss the challenges and potentials briefly here toward the future of composite manufacturing.

A novel custom-made 3D silicone printer and two-part Ecoflex silicone resins were used to 3D-print standard-shaped silicone coupon and irregular-shaped meniscus structures via a heat-cured extrusion-based method. This article is segmented into three parts: (1) study on the effect of 3D printing parameters on dimensional accuracy and mechanical properties of 3D-printed silicone, (2) reliability and failure analysis of 3D-printed silicone according to ASTM D575 standards under monotonic and cyclic compressive loading, and (3) cytotoxicity of 3D-printed silicone by extraction method according to ISO 10993-12 for different extraction time and extract volume/surface area ratios. Based on analysis using regression method and analysis of variance, we found that the dimensional accuracy of lengths and widths is sensitive to both nozzle diameters and bed temperatures (BTs), while the height is only sensitive to BTs. Failure results were analyzed using the two-parameter Weibull probability distribution model and Weibull regression analysis and revealed that the Weibull modulus had a value greater than 1 in all groups, indicating an increasing failure rate with time for Ecoflex 30 and 50 meniscus implants. Results from quantitative cell proliferative assay exhibit statistically insignificant differences for all samples, pointing to the low cytotoxicity and excellent biocompatibility of printed silicone. Accepted version This research is supported by the National Research Foundation, Prime Minister’s Office, Singapore, under its Medium-Sized Centre funding scheme and by the NTU StartUp Grant.

The complete mechanical properties (tensile, compressive, and shear properties) of the additively manufactured (AM) continuous carbon fiber-reinforced thermoplastic (CFRTP) fabricated using extrusion-based AM technique were investigated and reported. The fracture modes of the AM CFRTP in various mechanical tests were studied and reported. Anisotropic mechanical properties were observed in all mechanical tests, with the fiber direction having the highest strengths and stiffnesses and the across-the-layer direction having the lowest strengths and stiffnesses. A proof of concept topology optimized unmanned aerial vehicle (UAV) landing gear was designed and fabricated using the mechanical properties obtained experimentally. Finite element analysis and compressive tests conducted show that the UAV landing gear structure fabricated using the AM CFRTP was able to survive the most extreme condition during operation. National Research Foundation (NRF) Accepted version The authors would like to express their appreciation to Nanyang Technological University (NTU) High Performance Computing Centre (HPCC) for providing all the necessary supercomputing resources and support. This research is supported by the National Research Foundation, Prime Minister's Office, Singapore under its Medium-Sized Centre funding scheme.

Additive manufacturing (AM) has brought about a revolution in the way we can manufacture complex products with customized features. AM has paved its way in the application areas ranging from aerospace, automotive, consumer to biomedical. AM of composites has attracted special attention due to its promise in improving, modifying, and diversifying the properties of generic materials through introducing reinforcements. This review provides a detailed landscape of fiber‐reinforced composites processed via AM techniques. Different AM processes, various material formulations, and strengths and drawbacks of AM methods are discussed. Emphasis is paid to AM techniques focusing on continuous fibers, as they hold the promise of becoming the next‐generation composite fabrication methodology. The article also tries to identify the potential of AM technology for fiber‐reinforced composites and delves into challenges facing the area. NRF (Natl Research Foundation, S’pore) Accepted version

Unmanned aerial vehicles (UAV) are gaining popularity due to their application in military, private and public sector, especially being attractive for fields where human operator is not required. Light-weight UAVs are more desirable as they have better performance in terms of shorter take-off range and longer flight endurance. However, light weight structures with complex inner features are hard to fabricate using conventional manufacturing methods. The ability to print complex inner structures directly without the need of a mould gives additive manufacturing (AM) an edge over conventional manufacturing. Recent development in composite and multi-material printing opens up new possibilities of printing lightweight structures and novel platforms like flapping wings with ease. This paper explores the impact of additive manufacturing on aerodynamics, structures and materials used for UAVs. The review will discuss state-of-the-art AM technologies for UAVs through innovations in materials and structures and their advantages and limitations. The role of additive manufacturing to improve the performance of UAVs through smart material actuators and multi-functional structures will also be discussed. NRF (Natl Research Foundation, S’pore)

In this study, the 3D printing technique was utilized to manufacture the sandwich composites. Composite filament fabrication based 3D printer was used to print the face-sheet, and inkjet 3D printer was used to print the sandwich core structure. This work aims to study the compressive failure of the sandwich structure manufactured by using these two manufacturing techniques. Two different types of core structures were investigated with the same type of face-sheet configuration. The core structures were printed using photopolymer, while the face-sheet was made using nylon/glass. The out-of-plane compressive strength of the 3D printed sandwich composite structure has been examined in accordance with ASTM standards C365/C365-M and presented in this paper.