Replicating the intricate hierarchical architecture of natural vascular networks, especially at capillary-scale resolution, remains a pivotal challenge in organ fabrication. Here we present a machine learning-enhanced hybrid bioprinting strategy that combines high-resolution aerosol jet printing of sacrificial materials and high-throughput extrusion printing of tissue matrices. This integrated approach enables sub-10 μm of resolution, achieving capillary-like channels and allowing on-demand modulation of vessel diameters in real time. Constrained Bayesian optimization rapidly identify optimal printing parameters, ensuring reliable, high-fidelity attainment of target channel sizes without exhaustive trial-and-error. This streamlined workflow supports the fabrication from 1D conduits to 3D multibranch hierarchical networks with tunable geometries. Endothelial cells seeded into these channels form continuous, functional monolayers, significantly reducing permeability while maintaining high cell viability and proliferation. By transcending the resolution limits of conventional sacrificial printing, this bioprinting method establishes a new route for producing biomimetic vasculature. Its unique combination of rapid optimization, real-time tunability, and microcapillary-scale precision holds exceptional promise for tissue engineering, regenerative medicine, and drug discovery.

Tissue expanders are widely used for organ and tissue reconstruction surgery. Hydrogel tissue expanders that swell in a biofluid are promising for minimally invasive operations. Nevertheless, clinical applications of existing hydrogel tissue expanders exhibit simple geometries, rapid swelling and insufficient mechanical performances. Here we develop a negatively charged polyelectrolyte hydrogel ink for light-based printing and prolonged yet large expansive profiles for surface organ and tissue reconstruction. This 4D-printed hydrogel tissue expander can be moulded in architecturally sophisticated constructs that adapt to the environment and show favourable mechanics without the need of external triggers for expansion. The ionization degree of the polyelectrolyte hydrogels is tunable by pH value in the surrounding medium and allows a volume equilibrium-swelling up to 10-30 times. In a rabbit model, we use the tissue expander for reconstruction of human-size ears and breasts, highlighting their multifold advantages over existing clinically adopted methods and thus future translation potentials.

Carbon fiber-reinforced polymer (CFRP), valued for its exceptional strength-to-weight ratio, is extensively used in aerospace, automotive, and wind turbine applications. Due to environmental concerns over waste disposal and the high cost of carbon fibers, efficient CFRP recycling is highly desirable. Chemical recycling via solvolysis offers the advantages of mild conditions and complete recovery of both the polymer matrix and carbon fibers. However, conventional mild-condition solvolysis suffers from slow epoxy degradation, hindered by the intrinsic chemical resistance of the matrix. Here, we present an approach to accelerate epoxy decomposition through the synergistic effects of hybrid-solvent solvolysis and mechanical agitation. The hybrid solvent, comprising a reactive smallmolecule alcohol and an inert good solvent, promotes rapid and selective anhydride-epoxy network cleavage via transesterification with an embedded catalyst. An experimentally validated diffusion-reaction model was used to study the effect of solvent mixing ratios on residual mass and size evolution of cube samples during epoxy decomposition. Additionally, a gel-swollen layer model was developed, and gel and swollen layer thicknesses were first measured to understand the epoxy decomposition mechanism. The solvent mixing ratios influence the degradation rate by balancing the diffusivity and reaction rates and altering the gel-swollen layer thicknesses. Furthermore, mechanical agitation enhances effective diffusivity by accelerating erosion of the gel layer from epoxy surfaces. Applying this agitation-assisted hybrid-solvent solvolysis to the epoxy matrix and its CFRP reduces decomposition time by up to 75 %. This work demonstrates a versatile, cost-effective, and energy-efficient approach for accelerating the recycling of epoxy thermosets and composites, offering potential for sustainable, scalable industrial implementation.

Liquid crystal elastomers (LCEs) offer significant promise as soft actuator materials, but their potential has not yet been fully explored for 4D printing applications. Most existing studies rely on extrusion-based printing methods, which offer limited resolution and impose constraints on fabricating intricate, free-standing structures. Moreover, it remains a significant challenge to design and spatially control liquid crystal orientation within complex 3D structures to achieve desired shape transformations. To address these challenges, this study introduces a 4D printing strategy that combines two-stage UV-curable LCEs with vat photopolymerization-based 3D printing, such as digital light processing (DLP). The LCE can be initially printed into complex geometries with high precision, followed by a post-printing programming step in which mechanical deformation is applied to the printed structure to define the desired shape. A subsequent thermal treatment forms covalent linkages to lock the programmed configuration. The resulting structures can reversibly transition between the printed and programmed configurations upon temperature change. This 4D printing strategy is shown to overcome key limitations of current approaches and significantly broaden the design space and functional potential of programmable shape-changing structures for various applications, including mechanically active metamaterials, morphing architecture, and soft robotics.

Volumetric printing is an emerging additive manufacturing technique that builds 3D constructs with enhanced printing speed and surface quality by forgoing the stepwise ink renewal. Existing volumetric printing techniques almost exclusively rely on light energy to trigger photopolymerization in transparent inks, limiting the material choice, build size, cell density and in vivo printability. Sonicated ink (or sono-ink) and focused-ultrasound (FUS) writing have been developed for deep-penetration acoustic volumetric printing (DAVP) within optically scattering media and beneath soft tissues. This technology uses rapid sono-thermal heating to induce material solidification at the FUS focal region, constructing 3D objects without the need for a build platform. Here, we describe two procedures necessary to achieve DAVP. First, we provide a step-by-step guide for preparing and characterizing multicomponent viscoelastic self-enhancing sono-inks. The lower critical solution temperature polymers are synthesized as a phase-transition reversible acoustic absorber to formulate the sono-inks. We characterize the rheological, acoustic and cytocompatibility properties of the sono-inks. We then detail the procedure for building a 3D FUS printer by integrating an FUS transducer with a 3D printing platform. The development of the 3D FUS printer needs basic knowledge of the ultrasound system, FUS physics and volumetric printing. Using the sono-inks and the 3D FUS printer, we further provide guidance to evaluate the sono-thermal heating effect and characterize the volumetric printing resolutions. We demonstrate the printing of volumetric constructs through optically scattering materials such as centimeter-thick biological tissues. The procedures require ~470 h to complete. The preparation of multicomponent viscoelastic self-enhancing sono-inks, synthesized as phase-transition reversible acoustic absorbers, enables acoustic volumetric printing beneath diverse tissue types in optically scattering media.

In this study, we present the development of a cryobioink designed to fabricate anisotropic scaffolds that support both neural and muscle cell-alignment. Given the critical role of cellular organization in nerve fibers and neuromuscular junctions, we employed a vertical cryobioprinting-enabled ice-templating technique to create scaffolds with aligned microchannels. These channels facilitated cell-alignment, which is important in modeling neural and neuromuscular tissues. By integrating hyaluronic acid-methacrylate (HAMA) with gelatin methacryloyl and the necessary cryoprotective agent melezitose, we showcased that the cryobioink could preserve cell viability during freezing/thawing processes, even at low temperatures employed during cryobioprinting. We optimized HAMA concentration to enhance neural cell viability and alignment, and successfully constructed anisotropic scaffolds featuring distinct sections that contained muscle and neural cells, establishing a model for neuromuscular junctions. The resulting models provide a versatile platform for studying nerve fibers and neuromuscular dysfunctions, offering potential advancements in neural regeneration research.

Fourth-dimensional (4D) printing has progressed tremendously since its first conceptualization in 2013. 4D printing is an emerging branch of three-dimensional (3D) printing that allows printed parts to change their shapes and properties as a function of time under external stimuli. It has revolutionized the fabrication of smart polymer composites with customized geometry and programmed dynamic functions for expanding engineering and healthcare applications. This review provides a comprehensive overview of recent advances in the 4D printing of polymer composites, emphasizing three pivotal areas: 3D printing methodologies, smart material design, and their healthcare applications. We start with 3D printing techniques, encompassing traditional methods, multimaterial printing approaches, and other emerging technologies for functional polymer systems. We discuss the molecular engineering of shape-shifting smart polymers, including shape memory polymers, liquid crystal elastomers, magnetoactive soft materials, and hydrogel composites. The structural design strategies and modeling-guided design of smart materials are also covered. We summarize the emerging healthcare applications of 4D-printed polymer composites in medical devices, soft robotics, wearables, drug delivery, and tissue repair/regeneration. Finally, challenges, opportunities, and future directions are highlighted in material design and printing techniques for 4D printing to advance next-generation healthcare solutions.

Volumetric 3D printing enables the rapid fabrication of centimetre-scale objects, with the fastest techniques requiring only a few seconds. Having emerged during the past 7 years, this new family of technologies is posed to revolutionize additive manufacturing, fabricating objects and functional parts in a layerless fashion directly within a vat of material in response to optical and acoustic fields. Modern volumetric 3D printing methods are overcoming many challenges inherent to conventional layer-by-layer approaches, the standard in research and industry for the past 40 years. This Review focuses on identifying upcoming challenges and research directions in materials chemistry and process engineering to move volumetric 3D printing from its infancy to its broader adoption. Recent advances include the development of techniques based on optical tomography, light and acoustic holography, xolography, multiwavelength and upconversion-mediated printing, as well as the introduction of materials with custom-designed properties. Promising applications in the development of optical and photonic components, rapid prototyping, soft robotics and bioprinting of living cells are discussed along with a vision for the evolution of volumetric manufacturing towards a broadly accessible technology platform. Volumetric 3D printing is an emerging set of technologies enabling layerless, fast fabrication of complex, multicomponent objects. This Review explores challenges in materials design and process engineering, highlighting future directions for the widespread adoption and novel applications of these technologies.

Bioprinting has facilitated tissue engineering by enabling the fabrication of biologically and physiologically relevant 3D constructs. However, conventional bioprinting techniques often lack precise control over the spatial organization of cells within bioprinted structures. Acoustics, on the other hand, offers a powerful tool for non-contact, label-free, high-precision cell manipulation but is inherently limited in its ability to create complex volumetric architectures. The integration of these two technologies, termed acoustic bioprinting, holds significant promise for advancing biofabrication. In this review, the synergistic potential of acoustics in enhancing three primary bioprinting modalities-droplet, light-polymerization, and extrusion-is analyzed. The ways in which acoustic fields can improve cell patterning, alignment, and bioink-manipulation-leading to more biomimetic constructs with enhanced physiological properties-are dicussed. Additionally, novel ultrasound-polymerization-based bioprinting technologies that leverage cavitation, sono-thermal effects, and liposome-mediated polymerization to enable deep penetration biofabrication, expanding the scope of bioprinting beyond conventional methods, are explored. By leveraging the strengths of both bioprinting and acoustics, this review highlights emerging strategies that can shape the next generation of biofabrication, offering innovative solutions for tissue engineering and regenerative medicine.

Vat photopolymerization (VPP) is one of the most successful additive manufacturing modalities, offering high printing resolution and a wide selection of photo-resins for applications in aerospace, electronics, soft robotics, and biomedical devices. However, conventional photo-resins, primarily derived from fossil resources, present sustainability challenges. They often rely on short-chain oligomers that form brittle, dense polymer networks, limiting their performance in high-demand applications, especially for elastomeric materials. In this study, we developed an ammonia-free natural rubber latex-based photo-resin featuring an ultra-high molecular weight polymer with low viscosity (<10 Pa.s) and rapid curing speed (similar to 11 s, corresponding to gelling point), making it highly suitable for VPP. The printed green parts underwent a two-step process of crosslinking and coagulation, resulting in semi-interpenetrating polymer networks with unique structural properties. Two curing intensities were investigated: 18 and 35 mW/cm(2). Lower intensity resulted in lower 9 x 10(-5) mol/cm(3) in crosslinked density and higher intensity, 1.4 x 10(-4) mol/cm(3) in crosslink density. We systematically investigated multi-scale structure-property relationships using spin-lattice (T1) and spin-spin (T2) relaxation analysis via inversion recovery and Carr-Purcell-Meiboom-Gill. 18 mW/cm(2) with 30 s of curing and drying resulted in two regimens of motion for Rubber polymer with intermediate crosslinking density and intermediate entanglements dominating the network. Also, 35 mW/cm(2) with 30 s of curing and drying resulted in two regimes of Rubber polymer: however, one with a higher crosslinked and a mobile polymer phase. Optimized curing parameters enabled the fabrication of highly stretchable elastomers with 5-7.8 MPa tensile strengths and breaking strains of 750%-900%. These results highlight the potential of biomass-based photo-resins to advance sustainable 3D printing technologies. Furthermore, we demonstrated the feasibility of this formulation by printing complex geometries using a commercially available SLA printer.

Conventional hydrogel-based bioprinting methods often suffer from insufficient cell densities, which may limit crucial cell-cell interactions and impair overall tissue functions. Here, we present an approach that modifies cell membranes with acrylate bonds, allowing living cells at physiological densities (up to ∼109 cells mL-1) to serve directly as bioinks, demonstrating photoactivated bioprinting through digital light processing using purely cellular bioinks. Our cell-dense bioinks (CLINKs) rapidly produce tissue constructs that closely mimic native tissues, characterized by strong structural relevancy and robust functionality. The high cellularity and living nature of CLINKs enable the creation of advanced biological models such as connected neural circuits and rhythmically contracting mini-hearts derived entirely from stem cells, effectively capturing essential native-like behaviors. Implants created through this method showcase the capacity to integrate with the host, thereby promoting regeneration. Our CLINK technology holds substantial promise in tissue biofabrication, opening alternative avenues for biomedical applications.

The human body relies on modular assembly for realizing its functions. Here the development of a bioinspired cryo(bio)printing‐based method is reported to fabricate shelf‐ready, storable modular scaffolds toward scalable tissue assembly. The mechanism lies in that the cryo(bio)printed modular scaffolds are first assembled into the final hierarchy in their frozen state, which can be subsequently bonded together into an integral piece by contacting each other at the interface during the melting and photocrosslinking processes. This method not only addresses the height limitation associated with the recently developed cryo(bio)printing technology by enabling scalable tissue fabrication through modular assembly, but also allows generating tissue constructs of same or dissimilar materials to fit defects of different scales and shapes, thus providing more precision treatment. Cellular evaluations on the cryobioprinted modular hydrogels validate cell viability, spreading, and differentiation following assembly. The chick ex ovo and rat subcutaneous implantation assays further confirm the potential of direct in vivo assembly using shelf‐ready cryobioprinted modular tissue constructs.

Bone tissue engineering strategies face considerable challenges owing to the complexity of the bone. This study introduces an unconventional method based on multimaterial 3D coextrusion-(bio)printing to address some of these challenges. The strategy enables simultaneous (bio)printing of a poly(lactic-co-glycolic acid)-based polymer ink laden with hydroxyapatite and a gelatin methacryloyl (GelMA)-based hydrogel bioink containing mesenchymal stem cells (MSCs). The resulting composite scaffolds exhibited a trabecular bone-like porosity and favorable compressive properties, surpassing those of GelMA alone. The (bio)printed MSCs demonstrated favorable viability, proliferation, morphology, and differentiation. This converged approach in multimaterial (bio)printing has the potential to transform the field of bone tissue engineering, offering a more efficient and effective way to regenerate skeletal tissues.

Decellularized extracellular matrix (dECM)-based hydrogels are widely applied to additive biomanufacturing strategies for relevant applications. The extracellular matrix components and growth factors of dECM play crucial roles in cell adhesion, growth, and differentiation. However, the generally poor mechanical properties and printability have remained as major limitations for dECM-based materials. In this study, heart-derived dECM (h-dECM) and meniscus-derived dECM (Ms-dECM) bioinks in their pristine, unmodified state supplemented with the photoinitiator system of tris(2,2-bipyridyl) dichlororuthenium(II) hexahydrate and sodium persulfate, demonstrate cytocompatibility with volumetric bioprinting processes. This recently developed bioprinting modality illuminates a dynamically evolving light pattern into a rotating volume of the bioink, and thus decouples the requirement of mechanical strengths of bioprinted hydrogel constructs with printability, allowing for the fabrication of sophisticated shapes and architectures with low-concentration dECM materials that set within tens of seconds. As exemplary applications, cardiac tissues are volumetrically bioprinted using the cardiomyocyte-laden h-dECM bioink showing favorable cell proliferation, expansion, spreading, biomarker expressions, and synchronized contractions; whereas the volumetrically bioprinted Ms-dECM meniscus structures embedded with human mesenchymal stem cells present appropriate chondrogenic differentiation outcomes. This study supplies expanded bioink libraries for volumetric bioprinting and broadens utilities of dECM toward tissue engineering and regenerative medicine.

4D printing has attracted widespread attention due to its ability to fabricate complex structures capable of responding to specific stimuli. However, existing smart materials used for 4D printing, such as conventional hydrogels, liquid crystal elastomers, and shape-memory polymers, are constrained in their ability to fulfill both mechanical ultrastrength and ultrastretchability requirements simultaneously due to their water saturation or uneven network structures. This study presents 4D printing of liquid-free ionic elastomers (LFIEs) that is achieved through photopolymerization of the acrylic acid-tetramethylammonium chloride-polymerizable deep eutectic solvent. The transition temperature (Tt) of the high-fracture-strength LFIEs is adjusted by introducing phenylphosphoric acid, harboring multiple hydrogen-bonds, leading to improved molecular chain mobility through softening of the chains. Consequently, the 4D-printed LFIEs retain their high strength while achieving ultrastretchable properties even under elevated temperatures during heating. Moreover, the 4D-printed LFIEs demonstrate both a swift response time and a high fixation rate even after undergoing repeated transformation cycles. These outstanding capabilities exhibit by LFIEs present a reliable strategy for advancing their applications in diverse fields, such as smart furniture, robotics, and intelligent manufacturing with remote monitoring, and beyond. 4D printing of liquid-free ionic elastomers is demonstrated, achieving high strengths while maintaining ultrastretchable properties even under elevated temperatures during heating, in addition to both a swift response time and a high fixation rate under repeated transformation cycles. These capabilities present a reliable strategy for advancing their applications in diverse fields, such as smart furniture, robotics, and intelligent manufacturing with remote monitoring, and beyond. image

With the rapid development and popularization of additive manufacturing, different technologies, including, but not limited to, extrusion-, droplet-, and vat-photopolymerization-based fabrication techniques, have emerged that have allowed tremendous progress in three-dimensional (3D) printing in the past decades. Bioprinting, typically using living cells and/or biomaterials conformed by different printing modalities, has produced functional tissues. As a subclass of vat-photopolymerization bioprinting, digital light processing (DLP) uses digitally controlled photomasks to selectively solidify liquid photocurable bioinks to construct complex physical objects in a layer-by-layer manner. DLP bioprinting presents unique advantages, including short printing times, relatively low manufacturing costs, and decently high resolutions, allowing users to achieve significant progress in the bioprinting of tissue-like complex structures. Nevertheless, the need to accommodate different materials while bioprinting and improve the printing performance has driven the rapid progress in DLP bioprinters, which requires multiple pieces of knowledge ranging from optics, electronics, software, and materials beyond the biological aspects. This raises the need for a comprehensive review to recapitulate the most important considerations in the design and assembly of DLP bioprinters. This review begins with analyzing unique considerations and specific examples in the hardware, including the resin vat, optical system, and electronics. In the software, the workflow is analyzed, including the parameters to be considered for the control of the bioprinter and the voxelizing/slicing algorithm. In addition, we briefly discuss the material requirements for DLP bioprinting. Then, we provide a section with best practices and maintenance of a do-it-yourself DLP bioprinter. Finally, we highlight the future outlooks of the DLP technology and their critical role in directing the future of bioprinting. The state-of-the-art progress in DLP bioprinter in this review will provide a set of knowledge for innovative DLP bioprinter designs.

Scalable Tissue Fabrication In article number 2309173, Yu Shrike Zhang and co-workers present a cryo(bio)printing method to fabricate shelf-ready, storable modules toward scalable tissue assembly, where cryo(bio)printed modular constructs are first assembled into the final hierarchy in their frozen state, and subsequently bonded together into an integral piece during the melting and crosslinking processes, addressing the height limitation associated with cryo(bio)printing and allowing generating tissue constructs of same or dissimilar materials.

With the development of fully printed electronics, soft sensors are in demand in various fields, such as wearable electronics, soft machines, etc. Most soft resistive sensors are made of conductive elements dispersed in a viscoelastic polymer binder, exhibiting resistive viscoelastic behavior. The resistance of soft resistive sensors is time-dependent due to the viscoelastic response of polymer binder and structural rearrangement of the conductive pathway. In this paper, experiments and theoretical modeling are used to study the resistive viscoelastic behavior of printed silver wires. The printed silver wire belongs to conductive polymer composites (CPCs) consisting of conductive silver-nanoparticle pathways in an elastic polymer binder. Based on tunneling theory, a multi-branch model is developed to capture the resistance variation of the printed silver wire under mechanical loading. Our experiment-validated model uses only a single set of parameters to predict the resistive relaxation behaviors of CPCs under different strain and different loading rates. Moreover, we demonstrated this numerical model could describe the resistance response under complex loading conditions, such as cyclic loading, similar to the sensor's working condition. The multi-branch model can be extended to any other soft resistive sensor, such as a strain sensor, and provide a new avenue to calibrate these soft sensors.

Fiber reinforced polymer composites (FRPCs) are valued for their high strength and lightweight and have found applications from aerospace to renewable energy. Additive manufacturing (AM, or 3D printing) of FRPCs is of great interest in recent years due to the manufacturing flexibility offered by AM. Direct ink write (DIW) 3D printing is a popular choice for FRPC 3D printing due to its low cost and open framework for broad material choices. However, previously explored techniques for AM of FRPCs are hindered by heavy reliance on fiber orientation dictated by extrusion toolpath and often require specialized hardware to print FRPCs with low viscosity and long cure time thermoset epoxy matrices. In this paper, we introduce a single-stream hybrid DIW AM technique for the creation of mechanically robust FRPC functional structures where the matrix is DIW 3D printed, and pre-epoxy impregnated (prepreg) woven carbon fiber (CF) fabrics are robotically placed. Additionally, functional components, such as conductive elements, can be readily integrated. We investigated the impact of prepreg woven CF reinforcement on a two-stage photo-thermal thermoset resin matrix on the mechanical characteristics of manufactured FRPCs. We also combined these efforts to fabricate functional structures, including strain sensors for in-situ deformation monitoring, heating elements, and embedded light sensors. This study found that the proposed single-stream hybrid DIW AM process could be a facile approach to fabricate high-strength FRPC functional structures.