Multi-length scale bioprinting towards simulating microenvironmental cues

Fabrication, sustainability, and key performance indicators of bioelectronics via fiber building blocks

Bioelectronics provide efficient information exchange between living systems and man-made devices, acting as a vital bridge in merging the domains of biology and technology. Using functional fibers as building blocks, bioelectronics could be hierarchically assembled with vast design possibilities across different scales, enhancing their application-specific biointegration, ergonomics, and sustainability. In this work, the authors review recent developments in bioelectronic fiber elements by reflecting on their fabrication approaches and key performance indicators, including the life cycle sustainability, environmental electromechanical performance, and functional adaptabilities. By delving into the challenges associated with physical deployment and exploring innovative design strategies for adaptability, we propose avenues for future development of bioelectronics via fiber building blocks, boosting the potential of "Fiber of Things" for market-ready bioelectronic products with minimized environmental impact.

Harnessing Macromolecular Chemistry to Design Hydrogel Micro- and Macro-Environments

Cell encapsulation within three-dimensional hydrogels is a promising approach to mimic tissues. However, true biomimicry of the intricate microenvironment, biophysical and biochemical gradients, and the macroscale hierarchical spatial organizations of native tissues is an unmet challenge within tissue engineering. This review provides an overview of the macromolecular chemistries that have been applied toward the design of cell-friendly hydrogels, as well as their application toward controlling biophysical and biochemical bulk and gradient properties of the microenvironment. Furthermore, biofabrication technologies provide the opportunity to simultaneously replicate macroscale features of native tissues. Biofabrication strategies are reviewed in detail with a particular focus on the compatibility of these strategies with the current macromolecular toolkit described for hydrogel design and the challenges associated with their clinical translation. This review identifies that the convergence of the ever-expanding macromolecular toolkit and technological advancements within the field of biofabrication, along with an improved biological understanding, represents a promising strategy toward the successful tissue regeneration.

Biofabrication methods for reconstructing extracellular matrix mimetics

In the human body, almost all cells interact with extracellular matrices (ECMs), which have tissue and organ-specific compositions and architectures. These ECMs not only function as cellular scaffolds, providing structural support, but also play a crucial role in dynamically regulating various cellular functions. This comprehensive review delves into the examination of biofabrication strategies used to develop bioactive materials that accurately mimic one or more biophysical and biochemical properties of ECMs. We discuss the potential integration of these ECM-mimics into a range of physiological and pathological in vitro models, enhancing our understanding of cellular behavior and tissue organization. Lastly, we propose future research directions for ECM-mimics in the context of tissue engineering and organ-on-a-chip applications, offering potential advancements in therapeutic approaches and improved patient outcomes.

Biointerface Fiber Technology from Electrospinning to Inflight Printing

Building two-dimensional (2D) and three-dimensional (3D) micro- and nanofibril structures with designable patterns and functionalities will offer exciting prospects for numerous applications spanning from permeable bioelectronics to tissue engineering scaffolds. This Spotlight on Applications highlights recent technological advances in fiber printing and patterning with functional materials for biointerfacing applications. We first introduce the current state of development of micro- and nanofibers with applications in biology and medical wearables. We then describe our contributions in developing a series of fiber printing techniques that enable the patterning of functional fiber architectures in three dimensions. These fiber printing techniques expand the material library and device designs, which underpin technological capabilities from enabling fundamental studies in cell migration to customizable and ecofriendly fabrication of sensors. Finally, we provide an outlook on the strategic pathways for developing the next-generation bioelectronics and "Fiber-of-Things" (FoT) using nano/micro-fibers as architectural building blocks.

Rheological and Biological Impact of Printable PCL-Fibers as Reinforcing Fillers in Cell-Laden Spider-Silk Bio-Inks

The development of bio-inks capable of being 3D-printed into cell-containing bio-fabricates with sufficient shape fidelity is highly demanding. Structural integrity and favorable mechanical properties can be achieved by applying high polymer concentrations in hydrogels. Unfortunately, this often comes at the expense of cell performance since cells may become entrapped in the dense matrix. This drawback can be addressed by incorporating fibers as reinforcing fillers that strengthen the overall bio-ink structure and provide a second hierarchical micro-structure to which cells can adhere and align, resulting in enhanced cell activity. In this work, the potential impact of collagen-coated short polycaprolactone-fibers on cells after being printed in a hydrogel is systematically studied. The matrix is composed of eADF4(C16), a recombinant spider silk protein that is cytocompatible but non-adhesive for cells. Consequently, the impact of fibers could be exclusively examined, excluding secondary effects induced by the matrix. Applying this model system, a significant impact of such fillers on rheology and cell behavior is observed. Strikingly, it could be shown that fibers reduce cell viability upon printing but subsequently promote cell performance in the printed construct, emphasizing the need to distinguish between in-print and post-print impact of fillers in bio-inks.

Microcapillary cell extrusion deposition with picolitre dispensing resolution

Extrusion-based cell deposition has become a prominent technique for expanding bioprinting applications. However, the associated print resolution in the order of nanolitre or above has been a limiting factor. The demand for improving print resolution towards the scale of a single cell has driven the development of precision nozzle extrusion, although the benefits gained remain ambiguous. Here, aided by in situ imaging, we investigated the dynamics of cell organisation through an extrusion-based microcapillary tip with picolitre precision through in-air or immersion deposition. The microcapillary extrusion setup, termed 'Picodis', was demonstrated by generating droplets of colouring inks immersed in an immiscible medium. Next, using 3T3 fibroblast cells as an experimental model, we demonstrated the deposition of cell suspension, and pre-aggregated cell pellets. Then, the dynamic organisation of cells within the microcapillary tip was described, along with cell ejection and deposition upon exiting the tip opening. The vision-assisted approach revealed that when dispersed in a culture medium, the movements of cells were distinctive based on the flow profiles and were purely driven by laminar fluid flow within a narrow tip. The primary process limitations were cell sedimentation, aggregation and compaction, along with trapped air bubbles. The use of picolitre-level resolution microcapillary extrusion, although it provides some level of control for a small number of cells, does not necessarily offer a reliable method when a specified number of cells are required. Our study provides insights into the process limitations of high-resolution cell ink extrusion, which may be useful for optimising biofabrication processes of cell-laden constructs for biomedical research.

Supplementary information

The online version contains supplementary material available at 10.1007/s42242-022-00205-3.

Stereolithographic Visible-Light Printing of Poly(l-glutamic acid) Hydrogel Scaffolds

Bioprinting is a promising fabrication technique aimed at developing biologically functional, tissue-like constructs for various biomedical applications. Among the different bioprinting approaches, vat polymerization-based techniques offer the highest feature resolution compared to more commonly used extrusion-based methods and therefore have greater potential to be utilized for printing complex hierarchical tissue architectures. Although significant efforts have been directed toward harnessing digital light processing techniques for high-resolution bioprinting, the use of stereolithography (SLA) setups for producing distinct hydrogel filaments smaller than 20 μm has received less attention. Improving the bioprinting resolution is still a technical challenge that must consider both the practical limitations of the bioprinter apparatus and the formulation of the cytocompatible bioresin. In this study, we developed a novel bioresin compatible with SLA and capable of printing high-resolution features. This resin, composed of a biosynthetic polypeptide poly(l-glutamic acid) functionalized with tyramine moieties (PLGA-Tyr), was crosslinked using a visible-light photoinitiator system. Varying concentrations of PLGA-Tyr and the co-photoinitiator were evaluated for the hydrogel system's gelation ability, swelling characteristics, degradation profiles, mechanical properties, and cell viability post-encapsulation. This study introduces a custom-built, cost-effective, visible-light SLA bioprinting system named the "MicroNC". Using the newly developed visible-light bioresin, we demonstrated for the first time the ability to fabricate hydrogel scaffolds with well-resolved filaments (less than 8 μm in width) capable of supporting cell viability and proliferation and directing cellular morphology at the single-cell level for up to 14 days. Overall, these experiments have underscored the exciting potential of using the visible-light-photoinitiated PLGA-Tyr material system for developing physiologically relevant in vitro hydrogel scaffolds with feature resolutions comparable to the dimensions of individual human cells for a wide range of biomedical applications.

Cancer cell migration on straight, wavy, loop and grid microfibre patterns

Cell migration plays an important role in physiological and pathological processes where the fibrillar morphology of extracellular matrices (ECM) could regulate the migration dynamics. To mimic the morphological characteristics of fibrillar matrix structures, low-voltage continuous electrospinning was adapted to construct straight, wavy, looped and gridded fibre patterns made of polystyrene (of fibre diameter ca. 3 μm). Cells were free to explore their different shapes in response to the directly-adhered fibre, as well as to the neighbouring patterns. For all the patterns studied, analysing cellular migration dynamics of MDA-MB-231 (a highly migratory breast cancer cell line) demonstrated two interesting findings: first, although cells dynamically adjust their shapes and migration trajectories in response to different fibrillar environments, their average step speed is minimally affected by the fibre global pattern; secondly, a switch in behaviour was observed when the pattern features approach the upper limit of the cell body's minor axis, reflecting that cells' ability to divert from an existing fibre track is limited by the size along the cell body's minor axis. It is therefore concluded that the upper limit of cell body's minor axis might act as a guide for the design of microfibre patterns for different purposes of cell migration.

3D printed biomimetic cochleae and machine learning co-modelling provides clinical informatics for cochlear implant patients

Cochlear implants restore hearing in patients with severe to profound deafness by delivering electrical stimuli inside the cochlea. Understanding stimulus current spread, and how it correlates to patient-dependent factors, is hampered by the poor accessibility of the inner ear and by the lack of clinically-relevant in vitro, in vivo or in silico models. Here, we present 3D printing-neural network co-modelling for interpreting electric field imaging profiles of cochlear implant patients. With tuneable electro-anatomy, the 3D printed cochleae can replicate clinical scenarios of electric field imaging profiles at the off-stimuli positions. The co-modelling framework demonstrated autonomous and robust predictions of patient profiles or cochlear geometry, unfolded the electro-anatomical factors causing current spread, assisted on-demand printing for implant testing, and inferred patients' in vivo cochlear tissue resistivity (estimated mean = 6.6 kΩcm). We anticipate our framework will facilitate physical modelling and digital twin innovations for neuromodulation implants.

Enhancing Peptide Biomaterials for Biofabrication

Biofabrication using well-matched cell/materials systems provides unprecedented opportunities for dealing with human health issues where disease or injury overtake the body’s native regenerative abilities. Such opportunities can be enhanced through the development of biomaterials with cues that appropriately influence embedded cells into forming functional tissues and organs. In this context, biomaterials’ reliance on rigid biofabrication techniques needs to support the incorporation of a hierarchical mimicry of local and bulk biological cues that mimic the key functional components of native extracellular matrix. Advances in synthetic self-assembling peptide biomaterials promise to produce reproducible mimics of tissue-specific structures and may go some way in overcoming batch inconsistency issues of naturally sourced materials. Recent work in this area has demonstrated biofabrication with self-assembling peptide biomaterials with unique biofabrication technologies to support structural fidelity upon 3D patterning. The use of synthetic self-assembling peptide biomaterials is a growing field that has demonstrated applicability in dermal, intestinal, muscle, cancer and stem cell tissue engineering.

Efficacy of Large Groove Texture on Rat Sciatic Nerve Regeneration In Vivo Using Polyacrylonitrile Nerve Conduits

Physical guidance cues play an important role in enhancing the efficiency of nerve conduits for peripheral nerve injury repair. However, very few in vivo investigations have been performed to evaluate the repair efficiency of nerve conduits with micro-grooved inner textures. In this study, polyacrylonitrile nerve conduits were prepared using dry-jet wet spinning, and micro-grooved textures were incorporated on the inner surface. The nerve conduits were applied to treat 10 mm sciatic nerve gaps in Sprague-Dawley (SD) rats. Sixteen weeks following implantation, nerve function was evaluated based on heat sensory tests, electrophysiological assessments and gastrocnemius muscle mass measurements. The thermal latency reaction and gastrocnemii weight of SD rats treated with grooved nerve conduits were almost 25% faster and 60% heavier than those of SD rats treated with smooth nerve conduits. The histological and immunohistochemical stain analyses showed the repair capacity of inner grooved conduits was found to be similar to that of autografts. These results suggest that grooved nerve conduits with groove width larger than 300 μm significantly improve peripheral nerve regeneration by introducing physical guidance cues. The obtained results can support the design of nerve conduits and lead to the improvement of nerve tissue engineering strategies.

Printability assessment of psyllium husk (isabgol)/gelatin blends using rheological and mechanical properties

The primary goal of this study is to highlight the rheological and mechanical properties of a new blend composed of naturally-derived hydrogel materials- psyllium husk (PH) and gelatin (G) for its potential use in three-dimensional (3D) printing technology. The mixtures were prepared at various weight ratios of 100PH, 75PH + 25G and 50PH + 50G. A suitable selection of the printable ink was made based on the preliminary screening steps of manual filament drop test and layer stacking by 3D printing. Printing of the common features such as hexagon and square grids helped evaluating shape fidelity of the chosen ink. Although 50PH + 50G blend was found meeting most of the criteria for an ideal 3D printable ink, rheological and mechanical characterizations have been performed for all the ratios of polymeric blends. This study documents the correlation between various factors of rheology that should be taken into account while categorizing any biomaterial as a printable ink. Yield stress was measured as 18.59 ± 4.21 Pa, 268.74 ± 13.56 Pa and 109.16 ± 9.85 Pa for 50PH + 50G, 75PH + 25G and 100PH, respectively. Similarly, consistency index (K) and flow index (n) were calculated using the power law equation and found as 49.303 ± 4.17, 530.59 ± 10.92, 291.82 ± 10.53 and 0.275 ± 0.04, 0.05 ± 0.005, 0.284 ± 0.04 for 50PH + 50G, 75PH + 25G and 100PH, respectively. The loss modulus (G″) was observed dominating over storage modulus (G') for 50PH + 50G, that depicts its liquid-like property; whereas storage modulus (G') was found dominating in case of 75PH + 25G and 100PH, indicating their solid-like characteristics. In addition, the loss tangent value (tan δ) of 50PH + 50G was observed exceeding unity (1.05), supporting its plastic behavior, unlike 75PH + 25G (0.5) and 100PH (0.33) whose loss tangent values were estimated less than unity revealing their elastic behavior. Also, 50PH + 50G was found to have the highest mechanical strength amongst the three blends with a Young's modulus of 9.170 ± 0.0881 kPa.

Manufacturing micropatterned collagen scaffolds with chemical-crosslinking for development of biomimetic tissue-engineered oral mucosa

The junction between the epithelium and the underlying connective tissue undulates, constituting of rete ridges, which lack currently available soft tissue constructs. In this study, using a micro electro mechanical systems process and soft lithography, fifteen negative molds, with different dimensions and aspect ratios in grid- and pillar-type configurations, were designed and fabricated to create three-dimensional micropatterns and replicated onto fish-scale type I collagen scaffolds treated with chemical crosslinking. Image analyses showed the micropatterns were well-transferred onto the scaffold surfaces, showing the versatility of our manufacturing system. With the help of rheological test, the collagen scaffold manufactured in this study was confirmed to be an ideal gel and have visco-elastic features. As compared with our previous study, its mechanical and handling properties were improved by chemical cross-linking, which is beneficial for grafting and suturing into the complex structures of oral cavity. Histologic evaluation of a tissue-engineered oral mucosa showed the topographical microstructures of grid-type were well-preserved, rather than pillar-type, a well-stratified epithelial layer was regenerated on all scaffolds and the epithelial rete ridge-like structure was developed. As this three-dimensional microstructure is valuable for maintaining epithelial integrity, our micropatterned collagen scaffolds can be used not only intraorally but extraorally as a graft material for human use.

Integrated 3D bioprinting-based geometry-control strategy for fabricating corneal substitutes

BACKGROUND

The shortage of donor corneas is a severe global issue, and hence the development of corneal alternatives is imperative and urgent. Although attempts to produce artificial cornea substitutes by tissue engineering have made some positive progress, many problems remain that hamper their clinical application worldwide. For example, the curvature of tissue-engineered cornea substitutes cannot be designed to fit the bulbus oculi of patients.

OBJECTIVE

To overcome these limitations, in this paper, we present a novel integrated three-dimensional (3D) bioprinting-based cornea substitute fabrication strategy to realize design, customized fabrication, and evaluation of multi-layer hollow structures with complicated surfaces.

METHODS

The key rationale for this method is to combine digital light processing (DLP) and extrusion bioprinting into an integrated 3D cornea bioprinting system. A designable and personalized corneal substitute was designed based on mathematical modelling and a computer tomography scan of a natural cornea. The printed corneal substitute was evaluated based on biomechanical analysis, weight, structural integrity, and fit.

RESULTS

The results revealed that the fabrication of high water content and highly transparent curved films with geometric features designed according to the natural human cornea can be achieved using a rapid, simple, and low-cost manufacturing process with a high repetition rate and quality.

CONCLUSIONS

This study demonstrated the feasibility of customized design, analysis, and fabrication of a corneal substitute. The programmability of this method opens up the possibility of producing substitutes for other cornea-like shell structures with different scale and geometry features, such as the glomerulus, atrium, and oophoron.

4D Printing of High-Performance Thermal-Responsive Liquid Metal Elastomers Driven by Embedded Microliquid Chambers

Four-dimensional (4D) printing of swellable materials have been viewed as an ideal approach to build shape morphing architectures. However, there is less variety in high-performance swellable materials, limiting its development. To address this challenge, we proposed a new strategy for designing high-performance thermal-responsive swellable materials. The reversible liquid-vapor phase change of embedded low boiling point liquid chambers and functional liquid metal fillers endows the designed elastomer with the reversible thermal-responsive swellable property with high stability, fast response speed, and large equilibrium deformation. Notably, liquid metal fillers play a crucial role in improving the thermal-responsive property via improving the thermal conductivity and fracture toughness and decreasing the stiffness. To demonstrate the feasibility of constructing shape morphing architectures with proposed thermal-responsive liquid metal elastomers, typical bilayer structures were printed and investigated. By altering the key design parameters, the response speed and equilibrium deformation can be adjusted as needed. Therefore, complex shape morphing architectures can be printed. This study could provide a new avenue to design swellable material systems for 4D printing of shape morphing architectures.

3D bioprinting for artificial cornea: Challenges and perspectives

Corneal disease is one of the most important causes of blindness worldwide. Currently, the dominating treatment of corneal blindness is corneal transplantation. However, the main source of cornea for transplantation is based on donations which is far from enough to meet the requirement (less than 1:70 of cases). The severe shortage of donor cornea promotes the studies of effective corneal alternatives. However, many problems remain and can't be solved in current researches, such as original geometry reconstruction and ocular optical function restoring. 3D bioprinting can be a promising approach for corneal substitution. The advantages of this technology in corneal regeneration enable personalized corneal implant and single or multi-layer corneal equivalents with controllable structure and designed refractive ability. In this review, the progress, applications and limitations of most influential works among current keratoprosthesis and tissue-engineering cornea researches are discussed. Then the applications of 3D bioprinting in manufacturing multi-layered structures and surface are mentioned. Further, the potential, advantages in current research of 3D bioprinting single or multi-layer corneal equivalents and alternatives are discussed. Finally, an insight into the technical challenges and prospective facing the future research of 3D bioprinting corneal alternatives in vivo and in vitro is provided.

Fabrication of Designable and Suspended Microfibers via Low-Voltage 3D Micropatterning

Building two-dimensional (2D) and three-dimensional (3D) fibrous structures in the micro- and nanoscale will offer exciting prospects for numerous applications spanning from sensors to energy storage and tissue engineering scaffolds. Electrospinning is a well-suited technique for drawing micro- to nanoscale fibers, but current methods of building electrospun fibers in 3D are restrictive in terms of printed height, design of macroscopic fiber networks, and choice of polymer. Here, we combine low-voltage electrospinning and additive manufacturing as a method to pattern layers of suspended mesofibers. Layers of fibers are suspended between 3D-printed supports in situ in multiple fiber layers and designable orientations. We examine the key working parameters to attain a threshold for fiber suspension, use those behavioral observations to establish a "fiber suspension indicator", and demonstrate its utility through design of intricate suspended fiber architectures. Individual fibers produced by this method approach the micrometer/submicrometer scale, while the overall suspended 3D fiber architecture can span over a centimeter in height. We demonstrate an application of suspended fiber architectures in 3D cell culture, utilizing patterned fiber topography to guide the assembly of suspended high-cellular-density structures. The solution-based fiber suspension patterning process we report offers a unique competence in patterning soft polymers, including extracellular matrix-like materials, in a high resolution and aspect ratio. The platform could thus offer new design and manufacturing capabilities of devices and functional products by incorporating functional fibrous elements.