Spatial-Selective Volumetric 4D Printing and Single-Photon Grafting of Biomolecules within Centimeter-Scale Hydrogels via Tomographic Manufacturing

Structured Light Projection Using Image Guide Fibers for In Situ Photo-biofabrication

Light-based biofabrication techniques have revolutionized the field of tissue engineering and regenerative medicine. Specifically, the projection of structured light, where the spatial distribution of light is controlled at both macro and microscale, has enabled precise fabrication of complex three dimensional structures with high resolution and speed. However, despite tremendous progress, biofabrication processes are mostly limited to benchtop devices which limit the flexibility in terms of where the fabrication can occur. Here, a Fiber-assisted Structured Light (FaSt-Light) projection apparatus for rapid in situ crosslinking of photoresins is demonstrated. This approach uses image-guide fiber bundles which can project bespoke images at multiple wavelengths, enabling flexibility and spatial control of different photoinitiation systems and crosslinking chemistries and also the location of fabrication. Coupling of different sizes of fibers and different lenses attached to the fibers to project small (several mm) or large (several cm) images for material crosslinking is demonstrated. FaSt-Light allows control over the cross-section of the crosslinked resins and enables the introduction of microfilaments which can further guide cellular infiltration, differentiation, and anisotropic matrix production. The proposed approach can lead to a new range of in situ biofabrication techniques which improve the translational potential of photofabricated tissues and grafts.

Bioxolography Using Diphenyliodonium Chloride and N-Vinylpyrrolidone Enables Rapid High-Resolution Volumetric 3D Printing of Spatially Encoded Living Matter

Light-based volumetric bioprinting enables fabrication of cubic centimeter-sized living materials with micrometer resolution in minutes. Xolography is a light sheet-based volumetric printing technology that offers unprecedented volumetric generation rates and print resolutions for hard plastics. However, the limited solubility and reactivity of current dual-color photoinitiators (DCPIs) in aqueous media have hindered their application for high-resolution bioprinting of living matter. Here, we present a novel three-component formulation that drastically improves photoreactivity and thereby enables high-resolution, rapid, and cytocompatible Xolographic biofabrication of intricately architected yet mechanically robust living materials. To achieve this, various relevant additives are systematically explored, which revealed that diphenyliodonium chloride and N-vinylpyrrolidone strongly enhance D-mediated photoreactivity, as confirmed by dual-color photo-rheology. This enables Xolographic bioprinting of gelatin methacryloyl-based bioresins, producing >1 cm3 constructs at ≈20 µm positive and 125 µm negative resolution within minutes. Multimaterial printing, molecular patterning, and grayscale-mediated mechanical patterning are explored to programmably create intricate, biomimetic, and concentration-controlled architectures. We demonstrate the Bioxolographic printing of various cell types, showing excellent cell viability, compatibility with long-term culture, and ability for nascent protein deposition. These results position Bioxolography as a transformative platform for rapid, scalable, high-resolution fabrication of functional living materials with encoded chemical and mechanical properties.

Multi-material Volumetric Bioprinting and Plug-and-play Suspension Bath Biofabrication via Bioresin Molecular Weight Tuning and via Multiwavelength Alignment Optics

Volumetric Bioprinting (VBP), enables to rapidly build complex, cell-laden hydrogel constructs for tissue engineering and regenerative medicine. Light-based tomographic manufacturing enables spatial-selective polymerization of a bioresin, resulting in higher throughput and resolution than what is achieved using traditional techniques. However, methods for multi-material printing are needed for broad VBP adoption and applicability. Although converging VBP with extrusion bioprinting in support baths offers a novel, promising solution, further knowledge on the engineering of hydrogels as light-responsive, volumetrically printable baths is needed. Therefore, this study investigates the tuning of gelatin macromers, in particular leveraging the effect of molecular weight and degree of modification, to overcome these challenges, creating a library of materials for VBP and Embedded extrusion Volumetric Printing (EmVP). Bioresins with tunable printability and mechanical properties are produced, and a novel subset of gelatins and GelMA exhibiting stable shear-yielding behavior offers a new, single-component, ready-to-use suspension medium for in-bath printing, which is stable over multiple hours without needing temperature control. As a proof-of-concept biological application, bioprinted gels are tested with insulin-producing pancreatic cell lines for 21 days of culture. Leveraging a multi-color printer, complex multi-material and multi-cellular geometries are produced, enhancing the accessibility of volumetric printing for advanced tissue models.

Bio-orthogonal tuning of matrix properties during 3D cell culture to induce morphological and phenotypic changes

Described herein is a protocol for producing a synthetic extracellular matrix that can be modified in situ during three-dimensional cell culture. The hydrogel platform is established using modular building blocks employing bio-orthogonal tetrazine (Tz) ligation with slow (norbornene, Nb) and fast (trans-cyclooctene, TCO) dienophiles. A cell-laden gel construct is created via the slow, off-stoichiometric Tz/Nb reaction. After a few days of culture, matrix properties can be altered by supplementing the cell culture media with TCO-tagged molecules through the rapid reaction with the remaining Tz groups in the network at the gel-liquid interface. As the Tz/TCO reaction is faster than molecular diffusion, matrix properties can be modified in a spatiotemporal fashion simply by altering the identity of the diffusive species and the diffusion time/path. Our strategy does not interfere with native biochemical processes nor does it require external triggers or a second, independent chemistry. The biomimetic three-dimensional cultures can be analyzed by standard molecular and cellular techniques and visualized by confocal microscopy. We have previously used this method to demonstrate how in situ modulation of matrix properties induces epithelial-to-mesenchymal transition, elicits fibroblast transition from mesenchymal stem cells and regulates myofibroblast differentiation. Following the detailed procedures, individuals with a bachelor's in science and engineering fields can successfully complete the protocol in 4-5 weeks. This protocol can be applied to model tissue morphogenesis and disease progression and it can also be used to establish engineered constructs with tissue-like anisotropy and tissue-specific functions.

A Comparative Study between Thiol-Ene and Acrylate Photocrosslinkable Hyaluronic Acid Hydrogel Inks for Digital Light Processing

Photocrosslinkable formulations based on the radical thiol-ene reaction are considered better alternatives than methacrylated counterparts for light-based fabrication processes. This study quantifies differences between thiol-ene and methacrylated crosslinked hydrogels in terms of precursors stability, the control of the crosslinking process, and the resolution of printed features particularized for hyaluronic acid (HA) inks at concentrations relevant for bioprinting. First, the synthesis of HA functionalized with norbornene, allyl ether, or methacrylate groups with the same molecular weight and comparable degrees of functionalization is presented. The thiol-ene hydrogel precursors show storage stability over 15 months, 3.8 times higher than the methacrylated derivative. Photorheology experiments demonstrate up to 4.7-times faster photocrosslinking. Network formation in photoinitiated thiol-ene HA crosslinking allows higher temporal control than in methacrylated HA, which shows long post-illumination hardening. Using digital light processing, 4% w/v HA hydrogels crosslinked with a dithiol allowed printing of 13.5 × 4 × 1 mm3 layers with holes of 100 µm resolution within 2 s. This is the smallest feature size demonstrated in DLP printing with HA-based thiol-ene hydrogels. The results are important to estimate the extent to which the synthetic effort of introducing -ene functions can pay off in the printing step.

Xolography for Biomedical Applications: Dual-Color Light-Sheet Printing of Hydrogels With Local Control Over Shape and Stiffness

Current challenges in tissue engineering include creation of extracellular environments that support and interact with cells using biochemical, mechanical, and structural cues. Spatial control over these cues is currently limited due to a lack of suitable fabrication techniques. This study introduces Xolography, an emerging dual-color light-sheet volumetric printing technology, to achieve control over structural and mechanical features for hydrogel-based photoresins at micro- to macroscale while printing within minutes. A water-soluble photoswitch photoinitiator system and a library of naturally-derived, synthetic, and thermoresponsive hydrogels for Xolography are proposed. Centimeter-scale, 3D constructs with positive features of 20 µm and negative features of ≈100 µm are fabricated with control over mechanical properties (compressive moduli 0.2 kPa-6.5 MPa). Notably, switching from binary to grayscaled light projection enables spatial control over stiffness (0.2-16 kPa). As a proof of concept, grayscaled Xolography is leveraged with thermoresponsive hydrogels to introduce reversible anisotropic shape changes beyond isometric shrinkage. Xolography of viable cell aggregates is finally demonstrated, laying the foundation for cell-laden printing of dynamic, cell-instructive environments with tunable structural and mechanical cues in a fast one-step process. Overall, these innovations unlock unique possibilities of Xolography across multiple biomedical applications.

Integrating microfluidics, hydrogels, and 3D bioprinting for personalized vessel-on-a-chip platforms

Thrombosis, a major cause of morbidity and mortality worldwide, presents a complex challenge in cardiovascular medicine due to the intricacy of clotting mechanisms in living organisms. Traditional research approaches, including clinical studies and animal models, often yield conflicting results due to the inability to control variables in these complex systems, highlighting the need for more precise investigative tools. This review explores the evolution of in vitro thrombosis models, from conventional polydimethylsiloxane (PDMS)-based microfluidic devices to advanced hydrogel-based systems and cutting-edge 3D bioprinted vascular constructs. We discuss how these emerging technologies, particularly vessel-on-a-chip platforms, are enabling researchers to control previously unmanageable factors, thereby offering unprecedented opportunities to pinpoint specific clotting mechanisms. While PDMS-based devices offer optical transparency and fabrication ease, their inherent limitations, including non-physiological rigidity and surface properties, have driven the development of hydrogel-based systems that better mimic the extracellular matrix of blood vessels. The integration of microfluidics with biomimetic materials and tissue engineering approaches has led to the development of sophisticated models capable of simulating patient-specific vascular geometries, flow dynamics, and cellular interactions under highly controlled conditions. The advent of 3D bioprinting further enables the creation of complex, multi-layered vascular structures with precise spatial control over geometry and cellular composition. Despite significant progress, challenges remain in achieving long-term stability, incorporating immune components, and translating these models to clinical applications. By providing a comprehensive overview of current advancements and future prospects, this review aims to stimulate further innovation in thrombosis research and accelerate the development of more effective, personalized approaches to thrombosis prevention and treatment.

Engineered Living Systems Based on Gelatin: Design, Manufacturing, and Applications

Engineered living systems (ELSs) represent purpose-driven assemblies of living components, encompassing cells, biomaterials, and active agents, intricately designed to fulfill diverse biomedical applications. Gelatin and its derivatives have been used extensively in ELSs owing to their mature translational pathways, favorable biological properties, and adjustable physicochemical characteristics. This review explores the intersection of gelatin and its derivatives with fabrication techniques, offering a comprehensive examination of their synergistic potential in creating ELSs for various applications in biomedicine. It offers a deep dive into gelatin, including its structures and production, sources, processing, and properties. Additionally, the review explores various fabrication techniques employing gelatin and its derivatives, including generic fabrication techniques, microfluidics, and various 3D printing methods. Furthermore, it discusses the applications of ELSs based on gelatin in regenerative engineering as well as in cell therapies, bioadhesives, biorobots, and biosensors. Future directions and challenges in gelatin fabrication are also examined, highlighting emerging trends and potential areas for improvements and innovations. In summary, this comprehensive review underscores the significance of gelatin-based ELSs in advancing biomedical engineering and lays the groundwork for guiding future research and developments within the field.

Advances in 4D Bioprinting: The Next Frontier in Regenerative Medicine and Tissue Engineering Applications

4D bioprinting is a critical advancement in tissue engineering and regenerative medicine (TERM), enabling the creation of structures that dynamically respond to environmental stimuli over time. This review investigates various fabrication techniques and responsive materials that are central to these fields. It underscores the integration of multi-material and biocomposite approaches in 4D bioprinting, which is crucial for fabricating complex and functional constructs with heterogeneous properties. Using 4D bioprinting techniques enhances the mimicry of natural tissue characteristics, offering tailored responses and improved integration with biological systems. Furthermore, this study highlights the synergy between 4D bioprinting and tissue engineering and demonstrates the technology's potential for developing tissues and organs. In regenerative medicine, 4D bioprinting's applications extend to creating smart implants and advanced drug delivery systems that adapt to the body's changes, promoting healing and tissue regeneration. Finally, the challenges and future directions of 4D bioprinting are also explored and emphasize its transformative impact on biomedical engineering and the future of healthcare.

Bioprinting of Aptamer-Based Programmable Bioinks to Modulate Multiscale Microvascular Morphogenesis in 4D

Dynamic growth factor presentation influences how individual endothelial cells assemble into complex vascular networks. Here, programmable bioinks are developed that facilitate dynamic vascular endothelial growth factor (VEGF) presentation to guide vascular morphogenesis within 3D-bioprinted constructs. Aptamer's high affinity is leveraged for rapid VEGF sequestration in spatially confined regions and utilized aptamer-complementary sequence (CS) hybridization to tune VEGF release kinetics temporally, days after bioprinting. It is shown that spatial resolution of programmable bioink, combined with CS-triggered VEGF release, significantly influences the alignment, organization, and morphogenesis of microvascular networks in bioprinted constructs. The presence of aptamer-tethered VEGF and the generation of instantaneous VEGF gradients upon CS-triggering restricted hierarchical network formation to the printed aptamer regions at all spatial resolutions. Network properties improved as the spatial resolution decreased, with low-resolution designs yielding the highest network properties. Specifically, CS-treated low-resolution designs exhibited significant vascular network remodeling, with an increase in vessel density(1.35-fold), branching density(1.54-fold), and average vessel length(2.19-fold) compared to non-treated samples. The results suggest that CS acts as an external trigger capable of inducing time-controlled changes in network organization and alignment on-demand within spatially localized regions of a bioprinted construct. It is envisioned that these programmable bioinks will open new opportunities for bioengineering functional, hierarchically self-organized vascular networks within engineered tissues.

3D Bioprinting for Engineered Tissue Constructs and Patient-Specific Models: Current Progress and Prospects in Clinical Applications

Advancements in bioprinting technology are driving the creation of complex, functional tissue constructs for use in tissue engineering and regenerative medicine. Various methods, including extrusion, jetting, and light-based bioprinting, have their unique advantages and drawbacks. Over the years, researchers and industry leaders have made significant progress in enhancing bioprinting techniques and materials, resulting in the production of increasingly sophisticated tissue constructs. Despite this progress, challenges still need to be addressed in achieving clinically relevant, human-scale tissue constructs, presenting a hurdle to widespread clinical translation. However, with ongoing interdisciplinary research and collaboration, the field is rapidly evolving and holds promise for personalized medical interventions. Continued development and refinement of bioprinting technologies have the potential to address complex medical needs, enabling the development of functional, transplantable tissues and organs, as well as advanced in vitro tissue models.

Human breast tissue engineering in health and disease

The human mammary gland represents a highly organized and dynamic tissue, uniquely characterized by postnatal developmental cycles. During pregnancy and lactation, it undergoes extensive hormone-stimulated architectural remodeling, culminating in the formation of specialized structures for milk production to nourish offspring. Moreover, it carries significant health implications, due to the high prevalence of breast cancer. Therefore, gaining insight into the unique biology of the mammary gland can have implications for managing breast cancer and promoting the well-being of both women and infants. Tissue engineering techniques hold promise to narrow the translational gap between existing breast models and clinical outcomes. Here, we provide an overview of the current landscape of breast tissue engineering, outline key requirements, and the challenges to overcome for achieving more predictive human breast models. We propose methods to validate breast function and highlight preclinical applications for improved understanding and targeting of breast cancer. Beyond mammary gland physiology, representative human breast models can offer new insight into stem cell biology and developmental processes that could extend to other organs and clinical contexts.

Bioinks and biofabrication techniques for biosensors development: A review

3D bioprinting technologies and bioink development are enabling significant advances in miniaturized and integrated biosensors. For example, bioreceptors can be immobilized within a porous 3D structure to significantly amplify the signal, while biocompatible and mechanically flexible systems uniquely enable wearable chem- and bio-sensors. This advancement is accelerating translation by enabling the production of high performance, reproducible, and flexible analytical devices. The formulation of the bioink plays a crucial role in determining the bio-functionality of the resulting printed structures, e.g., the porosity that allows the analyte to diffuse through the 3D structure, the affinity and avidity of the receptors, etc. This review explores the next generation of advanced bioinks for biosensor development and provides insights into the latest cutting-edge bioprinting technologies. The bioprinting methods available for biosensor fabrication including inkjet, extrusion, and laser-based bioprinting, are discussed. The advantages and limitations of each method are analysed, and recent advancements in bioprinting technologies are presented. The review then delves into the properties of advanced bioinks, such as biocompatibility, printability, stability, and applicability. Different types of advanced bioinks are explored, including multicomponent, stimuli-responsive, and conductive bioinks. Finally, the next generation of bioinks for biosensors is considered, identifying possible new opportunities and challenges. Overall, this literature review highlights the combined importance of bioink formulation and bioprinting methods for the development of high-performance analytical biosensors.

Multi-Material Volumetric Additive Manufacturing of Hydrogels using Gelatin as a Sacrificial Network and 3D Suspension Bath

Volumetric additive manufacturing (VAM) is an emerging layerless method for the rapid processing of reactive resins into 3D structures, where printing is much faster (seconds) than other lithography and direct ink writing methods (minutes to hours). As a vial of resin rotates in the VAM process, patterned light exposure defines a 3D object and then resin that has not undergone gelation can be washed away. Despite the promise of VAM, there are challenges with the printing of soft hydrogel materials from non-viscous precursors, including multi-material constructs. To address this, sacrificial gelatin is used to modulate resin viscosity to support the cytocompatible VAM printing of macromers based on poly(ethylene glycol) (PEG), hyaluronic acid (HA), and polyacrylamide (PA). After printing, gelatin is removed by washing at an elevated temperature. To print multi-material constructs, the gelatin-containing resin is used as a shear-yielding suspension bath (including HA to further modulate bath properties) where ink can be extruded into the bath to define a multi-material resin that can then be processed with VAM into a defined object. Multi-material constructs of methacrylated HA (MeHA) and gelatin methacrylamide (GelMA) are printed (as proof-of-concept) with encapsulated mesenchymal stromal cells (MSCs), where the local hydrogel properties guide cell spreading behavior with culture.

Light from Afield: Fast, High-Resolution, and Layer-Free Deep Vat 3D Printing

Harnessing light for cross-linking of photoresponsive materials has revolutionized the field of 3D printing. A wide variety of techniques leveraging broad-spectrum light shaping have been introduced as a way to achieve fast and high-resolution printing, with applications ranging from simple prototypes to biomimetic engineered tissues for regenerative medicine. Conventional light-based printing techniques use cross-linking of material in a layer-by-layer fashion to produce complex parts. Only recently, new techniques have emerged which deploy multidirection, tomographic, light-sheet or filamented light-based image projections deep into the volume of resin-filled vat for photoinitiation and cross-linking. These Deep Vat printing (DVP) approaches alleviate the need for layer-wise printing and enable unprecedented fabrication speeds (within a few seconds) with high resolution (>10 μm). Here, we elucidate the physics and chemistry of these processes, their commonalities and differences, as well as their emerging applications in biomedical and non-biomedical fields. Importantly, we highlight their limitations, and future scope of research that will improve the scalability and applicability of these DVP techniques in a wide variety of engineering and regenerative medicine applications.

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.

Delivering drugs with microrobots

Biomedical microrobots could overcome current challenges in targeted therapies.

Intelligent Vascularized 3D/4D/5D/6D-Printed Tissue Scaffolds

Blood vessels are essential for nutrient and oxygen delivery and waste removal. Scaffold-repairing materials with functional vascular networks are widely used in bone tissue engineering. Additive manufacturing is a manufacturing technology that creates three-dimensional solids by stacking substances layer by layer, mainly including but not limited to 3D printing, but also 4D printing, 5D printing and 6D printing. It can be effectively combined with vascularization to meet the needs of vascularized tissue scaffolds by precisely tuning the mechanical structure and biological properties of smart vascular scaffolds. Herein, the development of neovascularization to vascularization to bone tissue engineering is systematically discussed in terms of the importance of vascularization to the tissue. Additionally, the research progress and future prospects of vascularized 3D printed scaffold materials are highlighted and presented in four categories: functional vascularized 3D printed scaffolds, cell-based vascularized 3D printed scaffolds, vascularized 3D printed scaffolds loaded with specific carriers and bionic vascularized 3D printed scaffolds. Finally, a brief review of vascularized additive manufacturing-tissue scaffolds in related tissues such as the vascular tissue engineering, cardiovascular system, skeletal muscle, soft tissue and a discussion of the challenges and development efforts leading to significant advances in intelligent vascularized tissue regeneration is presented.

Four-Dimensional Micro/Nanorobots via Laser Photochemical Synthesis towards the Molecular Scale

Miniaturized four-dimensional (4D) micro/nanorobots denote a forerunning technique associated with interdisciplinary applications, such as in embeddable labs-on-chip, metamaterials, tissue engineering, cell manipulation, and tiny robotics. With emerging smart interactive materials, static micro/nanoscale architectures have upgraded to the fourth dimension, evincing time-dependent shape/property mutation. Molecular-level 4D robotics promises complex sensing, self-adaption, transformation, and responsiveness to stimuli for highly valued functionalities. To precisely control 4D behaviors, current-laser-induced photochemical additive manufacturing, such as digital light projection, stereolithography, and two-photon polymerization, is pursuing high-freeform shape-reconfigurable capacities and high-resolution spatiotemporal programming strategies, which challenge multi-field sciences while offering new opportunities. Herein, this review summarizes the recent development of micro/nano 4D laser photochemical manufacturing, incorporating active materials and shape-programming strategies to provide an envisioning of these miniaturized 4D micro/nanorobots. A comparison with other chemical/physical fabricated micro/nanorobots further explains the advantages and potential usage of laser-synthesized micro/nanorobots.