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Electrochemical impedance spectroscopy (EIS), one of the techniques for electrochemical analysis, has been used for a wide range of applications especially chemical- and bio-sensing. Besides, EIS is one of the few techniques that has been proven useful for membrane fouling detection. Electrodes are some of the most essential components for analytes detection applications with EIS. With the advances in printing technology, the fabrication of advanced printed electrode systems with miniature designs and improved sensitivity for electrochemical sensing is made possible. This review addresses recent advances in printed electrodes for electrochemical impedance spectroscopy with the emphasis placed on discussing the use of printed electrodes for membrane fouling detection. Common electrode designs and materials for electrochemical impedance spectroscopy are discussed, along with practical examples of these printed electrodes. The commonly used printing techniques for the fabrication of printed electrodes are also deliberated. The successful realization of printed electrodes for EIS requires careful design of electrodes, proper selection of electrode materials, as well as optimization of the printing process. It is expected that printed electrodes will be widely accepted for EIS sensing applications and will facilitate the creation of low-cost high-performance sensing devices. Economic Development Board (EDB) National Research Foundation (NRF) Public Utilities Board (PUB) Accepted version This research is supported by PUB, Singapore’s National Water Agency under its Urban Solutions & Sustainability (Competitive Research Programme (CRP)(Water) Scheme, PUB-1804-0075) and the National Research Foundation, Prime Minister’s Oce, Singapore under its Medium-Sized Centre funding scheme. The financial support of the Economic Development Board (EDB) of Singapore to the Singapore Membrane Technology Centre (SMTC), Nanyang Environment & Water Research Institute (NEWRI) is greatly appreciated

Silver nanoparticle based microelectrodes embedded between layers of hydrogel material were successfully fabricated. 3D bioprinting is employed to print the entire bioelectronics platform comprising of conducting silver ink and Gelatin methacryloyl (GelMA) hydrogel. The additive manufacturing technique of bioprinting gives design freedom for the circuit, saves material and shortens the time to fabricate the bioelectronics platform. The silver platform shows excellent electrical conductivity, structural flexibility and stability in wet environment. It is tested for biocompatibility using C2C12 murine myoblasts cell line. The work demonstrates the potential of the fabricated platform for the realization of practical bioelectronic devices. NRF (Natl Research Foundation, S’pore) Published version

Bioelectronics platforms are gaining widespread attention as they provide a template to study the interactions between biological species and electronics. Decoding the effect of the electrical signals on the cells and tissues holds the promise for treating the malignant tissue growth, regenerating organs and engineering new-age medical devices. This work is a step forward in this direction, where bio- and electronic materials co-exist on one platform without any need for post processing. We fabricate a freestanding and flexible hydrogel based platform using 3D bioprinting. The fabrication process is simple, easy and provides a flexible route to print materials with preferred shapes, size and spatial orientation. Through the design of interdigitated electrodes and heating coil, the platform can be tailored to print various circuits for different functionalities. The biocompatibility of the printed platform is tested using C2C12 murine myoblasts cell line. Furthermore, normal human dermal fibroblasts (primary cells) are also seeded on the platform to ascertain the compatibility. NRF (Natl Research Foundation, S’pore)

The use of hydrogels enables the encapsulation and patterning of living cells within 3-D environment that closely mimic the physical, chemical, and biological aspects of the native extracellular matrix (ECM). This chapter first reviews the developments, key characteristics, and applications of some commonly used and emerging hydrogels. It then discusses the compatibility between hydrogels and the bioprinting processes and highlights some of the recent emerging trends in hydrogel research.

Over the years, the field of bioprinting has attracted attention for its highly automated fabrication system that enables the precise patterning of living cells and biomaterials at pre-defined positions for enhanced cell-matrix and cell-cell interactions. Notably, vat polymerization (VP)-based bioprinting is an emerging bioprinting technique for various tissue engineering applications due to its high fabrication accuracy. Particularly, different photo-initiators (PIs) are utilized during the bioprinting process to facilitate the crosslinking mechanism for fabrication of high-resolution complex tissue constructs. The advancements in VP-based printing have led to a paradigm shift in fabrication of tissue constructs from cell-seeding of tissue scaffolds (non-biocompatible fabrication process) to direct bioprinting of cell-laden tissue constructs (biocompatible fabrication process). This paper, presenting a first-time comprehensive review of the VP-based bioprinting process, provides an in-depth analysis and comparison of the various biocompatible PIs and highlights the important considerations and bioprinting requirements. This review paper reports a detailed analysis of its printing process and the influence of light-based curing modality and PIs on living cells. Lastly, this review also highlights the significance of VP-based bioprinting, the regulatory challenges and presents future directions to transform the VP-based printing technology into imperative tools in the field of tissue engineering and regenerative medicine. The readers will be informed on the current limitations and achievements of the VP-based bioprinting techniques. Notably, the readers will realize the importance and value of highly-automated platforms for tissue engineering applications and be able to develop objective viewpoints towards this field. Accepted version

Spatial distribution of biological cells plays a key role in tissue engineering for reconstituting the cellular microenvironment, and recently, acoustofluidics are explored as a viable tool for controlling structures in tissue fabrication because of its good biocompatibility, low-power consumption, automation capability, nature of non-invasive, and non-contact. Herein, a reusable acoustofluidic device is developed using surface acoustic waves for manipulating microparticles/cells to form a 3D matrix pattern inside a scaffold-based hydrogel contained in a millimetric chamber. The 3D patterned and polymerized hydrogel construct can be easily and safely removed from the chamber using a proposed lifting technique, which prevent any physical damages or contaminations and promote the reusability of the chamber. The generated 3D patterns of microparticles and cells are numerically studied using a finite-element method, which is well validated by the experimental results. The proposed acoustofluidic device is a useful tool for in vitro engineering 3D scaffold-based artificial tissues for drug and toxicity testing and building organs-on-chip applications. Ministry of Education (MOE) Nanyang Technological University The authors gratefully acknowledge the support of 1) Nanyang Technological University and the Ministry of Education of Singapore through a Ph.D. Scholarship and AcRF Tier 1 research grant (RG 96/18); 2) the UK Engineering and Physical Sciences Research Council (EPSRC) grants (EP/P018998/1); and 3) Special Interesting Group of Acoustofluidics funded by UK Fluids Network (EP/N032861/1).

Bioprinting is an emerging technology that allows the assembling of both living and non-living biological materials into an ideal complex layout for further tissue maturation. Bioprinting aims to produce engineered tissue or organ in a mechanized, organized, and optimized manner. Various biomaterials and techniques have been utilized to bioprint biological constructs in different shapes, sizes and resolutions. There is a need to systematically discuss and analyze the reported strategies employed to fabricate these constructs. We identified and discussed important design factors in bioprinting, namely shape and resolution, material heterogeneity, and cellular-material remodeling dynamism. Each design factors are represented by the corresponding process capabilities and printing parameters. The process-design map will inspire future biomaterials research in these aspects. Design considerations such as data processing, bio-ink formulation and process selection are discussed. Various printing and crosslinking strategies, with relevant applications, are also systematically reviewed. We categorized them into 5 general bioprinting strategies, including direct bioprinting, in-process crosslinking, post-process crosslinking, indirect bioprinting and hybrid bioprinting. The opportunities and outlook in 3D bioprinting are highlighted. This review article will serve as a framework to advance computer-aided design in bioprinting technologies.

Collagen is the most abundant extracellular matrix protein that is widely used in tissue engineering (TE). There is little research done on printing pure collagen. To understand the bottlenecks in printing pure collagen, it is imperative to understand collagen from a bottom-up approach. Here it is aimed to provide a comprehensive overview of collagen printing, where collagen assembly in vivo and the various sources of collagen available for TE application are first understood. Next, the current printing technologies and strategy for printing collagen-based materials are highlighted. Considerations and key challenges faced in collagen printing are identified. Finally, the key research areas that would enhance the functionality of printed collagen are presented. Nanyang Technological University National Research Foundation (NRF) This research was supported by the National Research Foundation, Prime Minister’s Office, Singapore under its Medium-Sized Centre funding scheme. This work was also supported by NTU start-up grant. This research was conducted in collaboration with HP Inc. and supported/partially supported by the Singapore Government through the Industry Alignment Fund-Industry Collaboration Projects Grant.

Bioprinting is an emerging research field that has attracted tremendous attention for various applications; it offers a highly automated, advanced manufacturing platform for the fabrication of complex bioengineered constructs. Different bio-inks comprising multiple types of printable biomaterials and cells are utilized during the bioprinting process to improve the homology to native tissues and/or organs in a highly reproducible manner. This paper, presenting a first-time comprehensive yet succinct review of microvalve-based bioprinting, provides an in-depth analysis and comparison of different drop-on-demand bioprinting systems and highlights the important considerations for microvalve-based bioprinting systems. This review paper reports a detailed analysis of its printing process, bio-ink properties and cellular components on the printing outcomes. Lastly, this review highlights the significance of drop-on-demand bioprinting for various applications such as high-throughput screening, fundamental cell biology research, in situ bioprinting and fabrication of in vitro tissue constructs and also presents future directions to transform the microvalve-based bioprinting technology into imperative tools for tissue engineering and regenerative medicine.

Fabrication techniques for cardiac tissue engineering have been evolving around scaffold-based and scaf-fold-free approaches. Conventional fabrication approaches lack control over scalability and homogeneous cell distribu-tion. Bioprinting provides a technological platform for controlled deposition of biomaterials, cells, and biological fac-tors in an organized fashion. Bioprinting is capable of alternating heterogeneous cell printing, printing anatomical rele-vant structure and microchannels resembling vasculature network. These are essential features of an engineered cardiac tissue. Bioprinting can potentially build engineered cardiac construct that resembles native tissue across macro to na-noscale.

Microfluidics has found ubiquitous presence in biological applications such as tissue spheroid fabrication and pharmacology investigation. The increasing prevalence and complexity demand a highly adaptable fabrication method for the rapid and convenient production of these microfluidic systems. 3D printing, as an emerging fabrication technique, was investigated in this paper. Microfluidic features were fabricated using two most widely used 3D printing technologies namely the inkjet printing and filament deposition techniques. The printing resolution, accuracy, repeatability, surface roughness, wetting ability, and biocompatibility of the printed microfluidic chips were characterized. The capability of 3D printing was demonstrated by printing a number of microfluidic devices such as rotational flow device and gradient generator. Results showed that 3D printing techniques were successful in making intricate microscale architectures and have the potential of greatly simplifying the manufacturing process.

Hydrogels are 3D networks that have a high water content. They have been widely used as cell carriers and scaffolds in tissue engineering due to their structural similarities to the natural extracellular matrix. Among these, smart hydrogels refer to a group of hydrogels that is responsive to various external stimuli such as pH, temperature, light, electric, and magnetic field. Combining the potential of 3D printing and smart hydrogels is an exciting new paradigm in the fabrication of a functional 3D tissue. In this article, we provide a state-of-the-art review on smart hydrogels and bioprinting. We identify the critical material properties needed for the most commonly used bioprinting techniques, namely extrusion-based, inkjet-based, and laser-based techniques. The latest progress in different smart hydrogel systems and their applications in bioprinting are presented. The challenges of printing these hydrogel systems are also highlighted. Lastly, we present the potentials and the future perspectives of smart hydrogels in 3D bioprinting.