Universal Peptide Hydrogel for Scalable Physiological Formation and Bioprinting of 3D Spheroids from Human Induced Pluripotent Stem Cells

Human induced pluripotent stem cells (hiPSCs) are used for drug discoveries, disease modeling and show great potential for human organ regeneration. 3D culture methods have been demonstrated to be an advanced approach compared to the traditional monolayer (2D) method. Here, a self-healing universal peptide hydrogel is reported for manufacturing physiologically formed hiPSC spheroids. With 100 000 hiPSCs encapsulated in 500 µL hydrogel, ≈50 000 spheroids mL⁻¹ (diameter 20–50 µm) are generated in 5 d. The spheroids in the universal peptide hydrogel are viable (85–96%) and show superior pluripotency and differentiation potential based on multiple biomarkers. Cell performance is influenced by the degradability of the hydrogel but not by gel strength. Without postprinting crosslinking aided by UV or visible lights or chemicals, various patterns are easily extruded from a simple star to a kidney-like organ shape using the universal peptide hydrogel bioink showing acceptable printability. A 20.0 × 20.0 × 0.75 mm³ sheet is finally printed with the universal peptide hydrogel bioink encapsulating hiPSCs and cultured for multiple days, and the hiPSC spheroids are physiologically formed and well maintained.     

Photocurable 3D Printing of High Toughness and Self-Healing Hydrogels for Customized Wearable Flexible Sensors

Currently, most customized hydrogels can only be processed via extrusion-based 3D printing techniques, which is limited by printing efficiency and resolution. Here, a simple strategy for the rapid fabrication of customized hydrogels using a photocurable 3D printing technique is presented. This technique has been rarely used because the presence of water increases the molecular distance between the polymer chains and reduces the monomer polymerization rate, resulting in the failure of rapid solid-liquid separation during printing. Although adding cross-linkers to printing inks can effectively accelerate 3D cross-linked network formation, chemical cross-linking may result in reduced toughness and self-healing ability of the hydrogel. Therefore, an interpenetrated-network hydrogel based on non-covalent interactions is designed to form physical cross-links, affording fast solid-liquid separation. Poly(acrylic acid (AA)-N-vinyl-2-pyrrolidone (NVP)) and carboxymethyl cellulose (CMC) are cross-linked via Zn²⁺-ligand coordination and hydrogen bonding; the resulting mixed AA-NVP/CMC solution is used as the printing ink. The printed poly(AA-NVP/CMC) hydrogel exhibited high tensile toughness (3.38 MJ m⁻³) and superior self-healing ability (healed stress: 81%; healed strain: 91%). Some objects like manipulator are successfully customized by photocurable 3D printing using hydrogels with high toughness and complex structures. This high-performance hydrogel has great potential for application in flexible wearable sensors.     

Void‐Free 3D Bioprinting for In Situ Endothelialization and Microfluidic Perfusion

Two major challenges of 3D bioprinting are the retention of structural fidelity and efficient endothelialization for tissue vascularization. Both of these issues are addressed by introducing a versatile 3D bioprinting strategy, in which a templating bioink is deposited layer‐by‐layer alongside a matrix bioink to establish void‐free multimaterial structures. After crosslinking the matrix phase, the templating phase is sacrificed to create a well‐defined 3D network of interconnected tubular channels. This void‐free 3D printing (VF‐3DP) approach circumvents the traditional concerns of structural collapse, deformation, and oxygen inhibition, moreover, it can be readily used to print materials that are widely considered “unprintable.” By preloading endothelial cells into the templating bioink, the inner surface of the channels can be efficiently cellularized with a confluent endothelial layer. This in situ endothelialization method can be used to produce endothelium with a far greater cell seeding uniformity than can be achieved using the conventional postseeding approach. This VF‐3DP approach can also be extended beyond tissue fabrication and toward customized hydrogel‐based microfluidics and self‐supported perfusable hydrogel constructs.     

Fluidic Infiltrative Assembly of 3D Hydrogel with Heterogeneous Composition and Function

3D hydrogels are powerful, multifunctional materials that are poised to become a building block in next-generation systems. Modern schemes to print complex 3D hydrogels are advancing rapidly; however, they possess several limitations including—but not limited to—polymer incompatibility or difficulty in imparting continuous heterogeneity in composition or function. Here, a simple strategy of synthesizing programmable hydrogel systems with tunable form and function in 3D is presented. This approach utilizes commercially available stereolithographic printer/resin to fabricate high-resolution molds followed by the programmed infiltration and gelation of hydrogel prepolymer. This mold is then sacrificed to yield 3D, multifunctional hydrogels exhibiting user-defined heterogeneity. The approach is compatible with numerous in-situ gelling polymers and modifiers ranging from interpenetrating networks of organic or synthetic polymers to functional materials possessing dense concentrations of nanomaterials or fluorescent markers. Accessible and versatile, this approach allows the fabrication of complex, multimaterial constructs with tunable 3D environmental responses inaccessible to well-established hydrogel 3D printing methods.     

3D Printing Personalized,Photocrosslinkable Hydrogel Wound Dressings for the Treatment of Thermal Burns

Considering the variations in burns depending on the circumstances that caused them, the need for personalized medicine and care for burn victims is vital to ensure that optimal treatment is provided. With the level of accuracy and customization that 3D printing brings as a technology, there is potential in its use to fabricate wound dressings that can provide better treatment for burn patients, provided that the material of choice has good printability and can be customized while facilitating wound healing. In this study, the versatility of chitosan methacrylate as said material to be used to fabricate customizable wound dressings via 3D printing is investigated. Synthesized chitosan methacrylate is evaluated to be printable, biodegradable, and biocompatible during wound healing. Various drugs relevant to the treatment of burns are then loaded and different multimaterial wound dressing designs containing different dosages are fabricated via 3D printing. The incorporation of said drugs does not significantly affect the printability of chitosan methacrylate, and the incorporation of antimicrobial agents significantly improves its antimicrobial capabilities. Through in vivo models, these variations in wound dressing designs have good wound healing properties and do not cause any adverse effects in the process.     

A Bionic-Gill 3D Hydrogel Evaporator with Multidirectional Crossflow Salt Mitigation and Aquaculture Applications

In recent years, interfacial solar-driven steam generation has gained huge attention as a sustainable and energy-efficient technology. However, salt scaling on and inside the evaporator structure induced by insufficient ion distribution control will lower the evaporation performance and hinder the stability and durability of evaporators. Herein, inspired by the highly efficient salt-expelling property of the gill filaments of large yellow croaker, a bionic-gill 3D hydrogel evaporator is proposed with fabulous multidirectional ion migration controllability. A 3D structure composed of arrayed beaded hollow columns with beaded hollow holes inspired by gill filaments ensuring longitudinal ion backflow and the peristome-mimetic arrayed grooves of microcavities ensuring lateral ion advection is designed and constructed to achieve fabulous multidirectional crossflow salt ion migration, which ensures high evaporation performance for pure water (an evaporation rate of 2.53 kg m⁻² h⁻¹ with an energy efficiency of 99.3%) as well as for high salinity brine (2.11 kg m⁻² h⁻¹ for 25.0 wt.% NaCl solution), with no salt crystallizing after long-term use. Furthermore, the 3D hydrogel evaporator has excellent chemical stability, mechanical properties, folding-irrelevant evaporation performance, and portability so that it can be used for the preliminary desalination of breeding wastewater through the proposed self-circulation koi aquaculture system.     

A 3D Printable and Bioactive Hydrogel Scaffold to Treat Traumatic Brain Injury

Traumatic brain injury accompanied by intracranial hypertension remains one of the most fatal injuries worldwide. Usually, patients must undergo two surgeries, craniectomy and cranioplasty, to reduce the intracranial pressure and then repair the skull. Traditional biomaterials, such as autologous bones and titanium meshes, which have poor stretchability and very high Young's modulus values up to hundreds of GPa, tend to constrain intracranial tissue and cannot be implanted directly after craniectomy. Thus far, finding elastic and degradable biomaterials to be immediately implanted after a craniectomy has remained a great challenge, which should not only repair cranial defects but also avoid secondary surgery and reduce the risk of complications, has remained a great challenge. Herein, a 3D printable bioactive hydrogel scaffold with high elasticity that can protect brain tissue, adapt to intracranial pressure changes, allow for the transport of nutrients and the proliferation and osteogenic differentiation of bone mesenchymal stem cells is presented. As indicated by in vivo experiments, the hydrogel scaffold helps to treat traumatic brain injury within 8 weeks and degrades safely. With these advantages, this material shows the potential to open up new horizons for cranioplasty and to help patients survive traumatic brain injury.     

3D Printing Hydrogel Scaffolds with Nanohydroxyapatite Gradient to Effectively Repair Osteochondral Defects in Rats

Osteochondral (OC) defects pose an enormous challenge with no entirely satisfactory repair strategy to date. Herein, a 3D printed gradient hydrogel scaffold with a similar structure to that of OC tissue is designed, involving a pure hydrogel-based top cartilage layer, an intermediate layer for calcified cartilage with 40% (w w⁻¹) nanohydroxyapatite (nHA) and 60% (w w⁻¹) hydrogel, and a 70/30% (w w⁻¹) nHA/hydrogel-based bottom subchondral bone layer. This study is conducted to evaluate the efficacy of the scaffold with nHA gradients in terms of its ability to promote OC defect repair. The fabricated composites are evaluated for physicochemical, mechanical, and biological properties, and then implanted into the OC defects in 56 rats. Overall, bone marrow stromal cells (BMSCs)-loaded gradient scaffolds exhibit superior repair results as compared to other scaffolds based on gross examination, micro-computed tomography (micro-CT), as well as histologic and immunohistochemical analyses, confirming the ability of this novel OC graft to facilitate simultaneous regeneration of cartilage-subchondral bone.     

A Hierarchical 3D Graft Printed with Nanoink for Functional Craniofacial Bone Restoration

An ideal craniofacial bone repair graft shall not only focus on the repair ability but also the regeneration of natural architecture with occlusal loads-related function restoration. However, such functional bone tissue engineering scaffold has rarely been reported. Herein, a hierarchical 3D graft is proposed for rebuilding craniofacial bone with both natural structure and healthy biofunction reconstruction. Inspired by the bone healing process, an organic–inorganic nanoink with ultrasmall calcium phosphate oligomers and bone morphogenetic protein-2 incorporated is developed for spatiotemporal guidance of new bone. Based on such homogeneous nanoink, a biomimetic graft, including a cortical layer containing Haversian system, and a cancellous layer featured with triply periodic minimum surface macrostructures, is fabricated via projection-based 3D printing method, and the layers are loaded with distinct concentrations of bioactive factors for regenerating new bone with gradient density. The graft exhibits excellent osteogenic and angiogenic potential in vitro, and accelerates revascularization and reconstructs neo-bone with original morphology in vivo. Benefiting from such natural architecture, loading force is widely transferred with reduced stress concentration around the inserted dental implant. Taken from native physiochemical and structural cues, this wstudy provides a novel strategy for functional tissue engineering through designing function-oriented biomaterials.     

3D Fluorescent Hydrogel Origami for Multistage Data Security Protection

Current fluorescence‐based anti‐counterfeiting strategies primarily encode information onto single 2D planes and underutilize the possibility of encrypting data inside 3D structures to achieve multistage data security. Herein, a fluorescent‐hydrogel‐based 3D anti‐counterfeiting platform is demonstrated, which extends data encryption capability from single 2D planes to complex 3D hydrogel origami geometries. The materials are based on perylene‐tetracarboxylic‐acid‐functionalized gelatin/poly(vinyl alcohol) hydrogels, which simultaneously show Fe³⁺‐responsive fluorescence quenching, borax‐triggered shape memory, and self‐healing properties. By employing an origami technique, various complex 3D hydrogel geometries are facilely fabricated. On the basis of these results, a 3D anti‐counterfeiting platform is demonstrated, in which the data printed by using Fe³⁺ as the ink are safely protected inside complex 3D hydrogel origami structures. In this way, the encrypted data cannot be read until after specially predesigned procedures (both the shape recovery and UV light illumination actions), indicating higher‐level information security than the traditional 2D counterparts. This facile and general strategy opens up the possibility of utilizing 3D fluorescent hydrogel origami for data information encryption and protection.     

Synthetic Bone-Like Structures Through Omnidirectional Ceramic Bioprinting in Cell Suspensions

The integration of hierarchical structure, chemistry, and functional activity within tissue-engineered scaffolds is of great importance in mimicking native bone tissue. Bone is a highly mineralized tissue which forms at ambient conditions by continuous crystallization of the mineral phase within an organic matrix in the presence of bone residing cells. Despite recent advances in the biofabrication of complex engineered tissues, replication of the heterogeneity of bone microenvironments has been a major challenge in constructing biomimetic bone scaffolds. Herein, inspired by the bone biomineralization process, the first example of bone mimicking constructs by 3D writing of a novel apatite-transforming ink in a supportive microgel matrix with living cells is demonstrated. Using this technique, complex bone-mimicked constructs are made at room temperature without requiring invasive chemicals, radiation, or postprocessing steps. This study demonstrates that mineralized constructs can be deposited within a high density of stem cells, directing the cellular organization, and promoting osteogenesis in vitro. These findings offer a new strategy for fabrication of bone mimicking constructs for bone tissue regeneration with scope to generate custom bone microenvironments for disease modeling, multicellular delivery, and in vivo bone repair.     

Development of Thermoinks for 4D Direct Printing of Temperature-Induced Self-Rolling Hydrogel Actuators

4D printing has emerged as an important technique for fabricating 3D objects from programmable materials capable of time-dependent reshaping. In the present investigation, novel 4D thermoinks composed of laponite (LAP), an interpenetrating network of poly(N-isopropylacrylamide) (PNIPAAm), and alginate (ALG) are developed for direct printing of shape-morphing structures. This approach consists of the design and fabrication of 3D honeycomb-patterned hydrogel discs self-rolling into tubular constructs under the stimulus of temperature. The shape morphing behavior of hydrogels is due to shear-induced anisotropy generated via 3D printing. The compositionally tunable hydrogel discs can be programmed to exhibit different actuation behaviors at different temperatures. Upon immersion in 12 °C water, singly crosslinked sheets roll up into a tubular construct. When transferred to 42 °C water, the tubes first rapidly unfold and then slightly curve up in the opposite direction. Through a dual photocrosslinking of PNIPAAm, it is possible to inverse temperature-dependent shape morphing and induce self-folding at higher and unrolling at lower temperatures. The extensive self-assembling motion is essential to developing thermal actuators with broad applications in, e.g., soft robotics and active implantology, whereas controllable self-rolling of planar hydrogels is of the highest interest to biomedical engineering as it allows for effective fabrication of hollow tubes.     

3D Printing of Strong and Tough Double Network Granular Hydrogels

Many soft natural tissues display a fascinating set of mechanical properties that remains unmatched by manmade counterparts. These unprecedented mechanical properties are achieved through an intricate interplay between the structure and locally varying the composition of these natural tissues. This level of control cannot be achieved in soft synthetic materials. To address this shortcoming, a novel 3D printing approach to fabricate strong and tough soft materials is introduced, namely double network granular hydrogels (DNGHs) made from compartmentalized reagents. This is achieved with an ink composed of microgels that are swollen in a monomer-containing solution; after the ink is additive manufactured, these monomers are converted into a percolating network, resulting in a DNGH. These DNGHs are sufficiently stiff to repetitively support tensile loads up to 1.3 MPa. Moreover, they are more than an order of magnitude tougher than each of the pure polymeric networks they are made from. It is demonstrated that this ink enables printing macroscopic, strong, and tough objects, which can optionally be rendered responsive, with high shape fidelity. The modular and robust fabrication of DNGHs opens up new possibilities to design adaptive, strong, and tough hydrogels that have the potential to advance, for example, soft robotic applications.     

3D Printing of Hydrogels for Stretchable Ionotronic Devices

In the booming development of flexible electronics represented by electronic skins, soft robots, and human–machine interfaces, 3D printing of hydrogels, an approach used by the biofabrication community, is drawing attention from researchers working on hydrogel-based stretchable ionotronic devices. Such devices can greatly benefit from the excellent patterning capability of 3D printing in three dimensions, as well as the free design complexity and easy upscale potential. Compared to the advanced stage of 3D bioprinting, 3D printing of hydrogel ionotronic devices is in its infancy due to the difficulty in balancing printability, ionic conductivity, shape fidelity, stretchability, and other functionalities. In this review, a guideline is provided on how to utilize the power of 3D printing in building high-performance hydrogel-based stretchable ionotronic devices mainly from a materials’ point of view, highlighting the systematic approach to balancing the printability, printing quality, and performance of printed devices. Various 3D printing methods for hydrogels are introduced, and then the ink design principles, balancing printing quality, printed functions, such as elastic conductivity, self-healing ability, and device (e.g., flexible sensors, shape-morphing actuators, soft robots, electroluminescent devices, and electrochemical biosensors) performances are discussed. In conclusion, perspectives on the future directions of this exciting field are presented.     

A Multiresponsive Anisotropic Hydrogel with Macroscopic 3D Complex Deformations

As one of the most promising smart materials, stimuli‐responsive polymer hydrogels (SPHs) can reversibly change volume or shape in response to external stimuli. They thus have shown promising applications in many fields. While considerable progress of 2D deformation of SPHs has been achieved, the realization of 3D or even more complex deformation still remains a significant challenge. Here, a general strategy towards designing multiresponsive, macroscopically anisotropic SPHs (MA‐SPHs) with the ability of 3D complex deformations is reported. Through a local UV‐reduction of graphene oxide sheets (GOs) with a patterned fashion in the GO‐poly(N‐isopropylacrylamide) (GO‐PNIPAM) composite hydrogel sheet, MA‐SPHs can be achieved after the introduction of a second poly(methylacrylic acid) network in the unreduced part of GO‐PNIPAM hydrogel sheet. The resulting 3D MA‐SPHs can provide remote‐controllable light‐driven, as well as thermo‐, pH‐, and ionic strength‐triggered multiresponsive 3D complex deformations. Approaches in this study may provide new insights in designing and fabricating intelligent soft materials for bioinspired applications.     

3D Electrophoresis‐Assisted Lithography (3DEAL): 3D Molecular Printing to Create Functional Patterns and Anisotropic Hydrogels

The ability to easily generate anisotropic hydrogel environments made from functional molecules with microscale resolution is an exciting possibility for the biomaterials community. This study reports a novel 3D electrophoresis‐assisted lithography (3DEAL) platform that combines elements from proteomics, biotechnology, and microfabrication to print well‐defined 3D molecular patterns within hydrogels. The potential of the 3DEAL platform is assessed by patterning immunoglobulin G, fibronectin, and elastin within nine widely used hydrogels and characterizing pattern depth, resolution, and aspect ratio. Furthermore, the technique's versatility is demonstrated by fabricating complex patterns including parallel and perpendicular columns, curved lines, gradients of molecular composition, and patterns of multiple proteins ranging from tens of micrometers to centimeters in size and depth. The functionality of the printed molecules is assessed by culturing NIH‐3T3 cells on a fibronectin‐patterned polyacrylamide‐collagen hydrogel and selectively supporting cell growth. 3DEAL is a simple, accessible, and versatile hydrogel‐patterning platform based on controlled molecular printing that may enable the development of tunable, chemically anisotropic, and hierarchical 3D environments.     

3D Particle-Free Printing of Biocompatible Conductive Hydrogel Platforms for Neuron Growth and Electrophysiological Recording

Electrically conductive 3D periodic microscaffolds are fabricated using a particle-free direct ink writing approach for use as neuronal growth and electrophysiological recording platforms. A poly (2-hydroxyethyl methacrylate)/pyrrole ink, followed by chemical in situ polymerization of pyrrole, enables hydrogel printing through nozzles as small as 1 µ m. These conductive hydrogels can pattern complex 2D and 3D structures and have good biocompatibility with test cell cultures ( ≈ 94.5% viability after 7 days). Hydrogel arrays promote extensive neurite outgrowth of cultured Aplysia californica pedal ganglion neurons. This platform allows extracellular electrophysiological recording of steady-state and stimulated electrical neuronal activities. In summation, this 3D conductive ink printing process enables the preparation of biocompatible and micron-sized structures to create customized in vitro electrophysiological recording platforms.     

3D Printing of Multifunctional Hydrogels

3D printing technology has been widely explored for the rapid design and fabrication of hydrogels, as required by complicated soft structures and devices. Here, a new 3D printing method is presented based on the rheology modifier of Carbomer for direct ink writing of various functional hydrogels. Carbomer is shown to be highly efficient in providing ideal rheological behaviors for multifunctional hydrogel inks, including double network hydrogels, magnetic hydrogels, temperature‐sensitive hydrogels, and biogels, with a low dosage (at least 0.5% w/v) recorded. Besides the excellent printing performance, mechanical behaviors, and biocompatibility, the 3D printed multifunctional hydrogels enable various soft devices, including loadable webs, soft robots, 4D printed leaves, and hydrogel Petri dishes. Moreover, with its unprecedented capability, the Carbomer‐based 3D printing method opens new avenues for bioprinting manufacturing and integrated hydrogel devices.     

Combining 3D Printing with Electrospinning for Rapid Response and Enhanced Designability of Hydrogel Actuators

Porous structures have emerged as a breakthrough of shape‐morphing hydrogels to achieve a rapid response. However, these porous actuators generally suffer from a lack of complexity and diversity in obtained 3D shapes. Herein, a simple yet versatile strategy is developed to generate shape‐morphing hydrogels with both fast deformation and enhanced designability in 3D shapes by combining two promising technologies: electrospinning and 3D printing. Elaborate patterns are printed on mesostructured stimuli‐responsive electrospun membranes, modulating in‐plane and interlayer internal stresses induced by swelling/shrinkage mismatch, and thus guiding morphing behaviors of electrospun membranes to adapt to changes of the environment. With this strategy, a series of fast deformed hydrogel actuators are constructed with various distinctive responsive behaviors, including reversible/irreversible formations of 3D structures, folding of 3D tubes, and formations of 3D structures with multi low‐energy states. It is worth noting that although poly(N‐isopropyl acrylamide) is chosen as the model system in the present research, our strategy is applicable to other stimuli‐responsive hydrogels, which enriches designs of rapid deformed hydrogel actuators.