Advances in light-based process for cell-based 3D bioprinting applications

Cell-based bioprinting has emerged as a transformative technology within biomaterials and biofabrication, offering unparalleled capabilities for constructing complex, biomimetic tissues and organs. This revolutionary approach enables the precise placement of living cells and biomaterials, facilitating the creation of functional constructs essential for both fundamental biological research and advanced regenerative medicine. This special issue aims to comprehensively present the most recent advancements in light-based bioprinting.

A primary challenge in tissue engineering remains vascularization, the creation of a functional blood supply. Light-based 3D bioprinting directly addresses this by enabling the fabrication of complex engineered vascular networks. Methods such as stereolithography, digital light projection (DLP), volumetric bioprinting, and two-photon polymerization (2PP) are being employed to achieve this [1]. These techniques are crucial for developing implantable blood vessel grafts and sophisticated models of diseased vessels, along with the multi-scale integration of capillaries needed for larger tissue constructs. Among these, DLP printing stands out for its accessibility and high resolution (down to ∼10 μm), making it instrumental in building complex organ models. Current research is intensely focused on optimizing DLP hardware, bioinks, and photoinitiators to further advance personalized medicine applications [2]. For creating micro-scaffolds that precisely guide cell growth, 2PP's ability to print sub-cellular features (nanometer resolution) is unparalleled. These scaffolds can be functionally enhanced to bind and release crucial growth factors like VEGF, effectively steering cell behavior. Beyond static models, the direct translation of these technologies to clinical settings is opened with the development of handheld biopens. One such device uses controlled light to precisely deposit and crosslink a hydrogel, successfully repairing corneal defects in porcine eyes [3]. This approach powerfully demonstrates a viable path for direct surgical application of bioprinting.

The development of advanced photo-crosslinkable bioink systems is a critical factor for the widespread adoption of light-based bioprinting. While gelatin methacryloyl (GelMA)-based hydrogels are extensively utilized in cell-based bioprinting, they are constrained by reactive oxygen species (ROS)-induced cytotoxicity generated during the photo-crosslinking process. To address this limitation, Kim et al. devised tannic acid-mineral nanoparticles (TMNs) to augment ROS scavenging and, at the same time, enhance osteoinductive potential [4]. By incorporating TMNs into GelMA hydrogels, stem cell viability and osteogenic differentiation were substantially improved, showing great promise for bone tissue bioprinting applications. Beyond synthetic polymers, decellularized extracellular matrix (dECM)-based bioinks offer highly physiomimetic properties that closely support native tissue environments for tissue engineering and regenerative medicine [5]. Photo-crosslinking techniques have been developed to overcome intrinsic challenges of dECM-based bioinks, such as their weak mechanical properties and poor printability, by significantly enhancing gelation kinetics and structural integrity. For instance, Rezaei et al. developed a photo-crosslinkable liver dECM-based bioink using a methacrylate reaction [6]. This innovative bioink provides not only structural stability but also crucial liver-specific effects, thereby enabling the development of more accurate hepatic tissue models.

Developing advanced liver models is particularly crucial due to the liver's central role in drug metabolism and detoxification. Watanabe et al. proposed an advanced liver-on-a-chip model that replicates the hepatic lobule with a continuous microvascular network, precisely fabricated by a femtosecond near-infrared (NIR) laser patterning technique [7]. This liver model enables the exploration of drug-induced hepatotoxicity, primarily based on the production of albumin and urea. Further demonstrating progress, Moon et al. successfully developed vascularized, thick human hepatic tissue constructs using DLP-based process [8]. Particularly, with perfusion integration, these hepatic tissue constructs produced albumin and bilirubin at levels comparable to those found in human blood, highlighting their physiological relevance.

Light-based processes are also proving highly effective in neuroregeneration. Aligned microarchitecture, crucial for guiding neurite outgrowth, can be precisely achieved through light-based methods. Sharaf et al. utilized 2PP to fabricate 2.5D microgroove structures and 3D microstructures, enabling investigations into the effect of mechanical confinement on neural cell morphology and protein expression [9]. Moreover, Liu et al. employed filamented light (FLight) biofabrication technology to create complex filamented hydrogels specifically designed to facilitate neurite formation within a 3D matrix [10]. This technology allows for precise modulation of the 3D internal structure with tunable properties, including matrix stiffness, pore size, protein incorporation, and matrix composition. This light-based approach presents substantial potential as a fabrication technique for nerve conduits characterized by complex microstructures, thereby significantly facilitating nerve repair.

The capabilities of light-based bioprinting are also extending into cancer research through the development of sophisticated in vitro cancer models. Breideband et al. investigated the synergistic effects of ECM stiffness and microgravity on breast cancer progression [11]. By employing hydrogels with varying stiffness, composed of GelMA and poly(ethylene glycol) diacrylate (PEGDA), researchers demonstrated that microgravity can modulate and, in certain instances, reverse stiffness-induced aggressive cellular behavior. In a separate study, Mei et al. developed a 3D bioprinted, cancer biopsy-derived in vitro breast cancer model using direct laser printing for drug screening purposes [12]. This model exhibited good cytocompatibility, maintained cell phenotypes, and especially incorporated vascular-like structures that mitigated drug chemoresistance. Preliminary findings indicated a strong correlation with in vivo outcomes, highlighting its potential for predictive drug testing. Furthermore, Tang et al. introduced a 3D bioprinted glioblastoma model with tunable ECM stiffness to assess the efficacy of CAR-T cell therapy in solid tumors [13]. This study demonstrated that increased stiffness hindered CAR-T cell infiltration and tumor killing. To overcome these stiffness-related barriers, a novel heat-inducible CAR-T strategy was developed, presenting a promising therapeutic approach.

Overall, as exemplified by the collection of studies composing this special issue, light-based 3D bioprinting offers significant potential for creating increasingly clinically relevant tissue and organ constructs, promising to extremely enhance patient care and advance personalized medicine. Its inherent high resolution is invaluable for developing complex biological structures, ensuring its continued utility in regenerative medicine applications and the creation of sophisticated in vitro tissue and disease models. Further progress in this dynamic field will undoubtedly come from developing even more tissue-specific bioprinting processes, precisely tailored to the unique physiological and anatomical requirements of different tissues and organs. This ongoing specialization will greatly expand the versatility and applicability of these powerful technologies, moving us closer to truly personalized and regenerative therapeutic solutions.