Dual crosslinking strategy to generate mechanically viable cell-laden printable constructs using methacrylated collagen bioinks

Comprehensive review on collagen extraction from food by-products and waste as a value-added material

The consumption of animal products has witnessed a significant increase over the years, leading to a growing need for industries to adopt strict waste control measures to mitigate environmental impacts. The disposal of animal waste in landfill can result in diverse and potentially hazardous decomposition by-products. Animal by-products, derived from meat, poultry, seafood and fish industries, offer a substantial raw material source for collagen and gelatin production due to their high protein content. Collagen, being a major protein component of animal tissues, represents an abundant resource that finds application in various chemical and material industries. The demand for collagen-based products continues to grow, yet the availability of primary material remains limited and insufficient to meet projected needs. Consequently, repurposing waste materials that contain collagen provides an opportunity to meet this need while at the same time minimizing the amount of waste that is dumped. This review examines the potential to extract value from the collagen content present in animal-derived waste and by-products. It provides a systematic evaluation of different species groups and discusses various approaches for processing and fabricating repurposed collagen. This review specifically focuses on collagen-based research, encompassing an examination of its physical and chemical properties, as well as the potential for chemical modifications. We have detailed how the research and knowledge built on collagen structure and function will drive the new initiatives that will lead to the development of new products and opportunities in the future. Additionally, it highlights emerging approaches for extracting high-quality protein from waste and discusses efforts to fabricate collagen-based materials leading to the development of new and original products within the chemical, biomedical and physical science-based industries.

Application of In Situ Mucoadhesive Hydrogel with Anti-Inflammatory and Pro-Repairing Dual Properties for the Treatment of Chemotherapy-Induced Oral Mucositis

Chemotherapy-induced oral mucositis (CIOM) is a prevalent complication of chemotherapy and significantly affects the treatment process. However, effective treatment for CIOM is lacking due to the unique environment of the oral cavity and the single effect of current drug delivery systems. In this present study, we propose an innovative approach by combining a methacrylate-modified human recombinant collagen III (rhCol3MA) hydrogel system with hyaluronic acid-epigallocatechin gallate (HA-E) and dopamine-modified methacrylate-alginate (AlgDA-MA). HA-E is used as an antioxidant and anti-inflammatory agent and synergizes with AlgDA-MA to improve the wet adhesion of hydrogel. The results of rhCol3MA/HA-E/AlgDA-MA (Col/HA-E/Alg) hydrogel demonstrate suitable physicochemical properties, excellent wet adhesive capacity, and biocompatibility. Notably, the hydrogel could promote macrophage polarization from M1 to M2 and redress human oral keratinocyte (HOK) inflammation by inhibiting NF-κB activation. Wound healing evaluations in vivo demonstrate that the Col/HA-E/Alg hydrogel exhibits a pro-repair effect by mitigating inflammatory imbalances, fostering early angiogenesis, and facilitating collagen repair. In summary, the Col/HA-E/Alg hydrogel could serve as a promising multifunctional dressing for the treatment of CIOM.

Clay Sculpture-Inspired 3D Printed Microcage Module Using Bioadhesion Assembly for Specific-Shaped Tissue Vascularization and Regeneration

3D bioprinting techniques have enabled the fabrication of irregular large-sized tissue engineering scaffolds. However, complicated customized designs increase the medical burden. Meanwhile, the integrated printing process hinders the cellular uniform distribution and local angiogenesis. A novel approach is introduced to the construction of sizable tissue engineering grafts by employing hydrogel 3D printing for modular bioadhesion assembly, and a poly (ethylene glycol) diacrylate (PEGDA)-gelatin-dopamine (PGD) hydrogel, photosensitive and adhesive, enabling fine microcage module fabrication via DLP 3D printing is developed. The PGD hydrogel printed micocages are flexible, allowing various shapes and cell/tissue fillings for repairing diverse irregular tissue defects. In vivo experiments demonstrate robust vascularization and superior graft survival in nude mice. This assembly strategy based on scalable 3D printed hydrogel microcage module could simplify the construction of tissue with large volume and complex components, offering promise for diverse large tissue defect repairs.

Enhancing the Repair of Substantial Volumetric Muscle Loss by Creating Different Levels of Blood Vessel Networks Using Pre-Vascularized Nerve Hydrogel Implants

Volumetric muscle loss (VML), a severe muscle tissue loss from trauma or surgery, results in scarring, limited regeneration, and significant fibrosis, leading to lasting reductions in muscle mass and function. A promising approach for VML recovery involves restoring vascular and neural networks at the injury site, a process not extensively studied yet. Collagen hydrogels have been investigated as scaffolds for blood vessel formation due to their biocompatibility, but reconstructing blood vessels and guiding innervation at the injury site is still difficult. In this study, collagen hydrogels with varied densities of vessel-forming cells are implanted subcutaneously in mice, generating pre-vascularized hydrogels with diverse vessel densities (0-145 numbers/mm2) within a week. These hydrogels, after being transplanted into muscle injury sites, are assessed for muscle repair capabilities. Results showed that hydrogels with high microvessel densities, filling the wound area, effectively reconnected with host vasculature and neural networks, promoting neovascularization and muscle integration, and addressing about 63% of the VML.

MicroRNA-200c Release from Gelatin-Coated 3D-Printed PCL Scaffolds Enhances Bone Regeneration

The fabrication of clinically relevant synthetic bone grafts relies on combining multiple biodegradable biomaterials to create a structure that supports the regeneration of defects while delivering osteogenic biomolecules that enhance regeneration. MicroRNA-200c (miR-200c) functions as a potent osteoinductive biomolecule to enhance osteogenic differentiation and bone formation; however, synthetic tissue-engineered bone grafts that sustain the delivery of miR-200c for bone regeneration have not yet been evaluated. In this study, we created novel, multimaterial, synthetic bone grafts from gelatin-coated 3D-printed polycaprolactone (PCL) scaffolds. We attempted to optimize the release of pDNA encoding miR-200c by varying gelatin types, concentrations, and polymer crosslinking materials to improve its functions for bone regeneration. We revealed that by modulating gelatin type, coating material concentration, and polymer crosslinking, we effectively altered the release rates of pDNA encoding miR-200c, which promoted osteogenic differentiation in vitro and bone regeneration in a critical-sized calvarial bone defect animal model. We also demonstrated that crosslinking the gelatin coatings on the PCL scaffolds with low-concentration glutaraldehyde was biocompatible and increased cell attachment. These results strongly indicate the potential use of gelatin-based systems for pDNA encoding microRNA delivery in gene therapy and further demonstrate the effectiveness of miR-200c for enhancing bone regeneration from synthetic bone grafts.

Advances and Trends of Photoresponsive Hydrogels for Bone Tissue Engineering

The development of static hydrogels as an optimal choice for bone tissue engineering (BTE) remains a difficult challenge primarily due to the intricate nature of bone healing processes, continuous physiological functions, and pathological changes. Hence, there is an urgent need to exploit smart hydrogels with programmable properties that can effectively enhance bone regeneration. Increasing evidence suggests that photoresponsive hydrogels are promising bioscaffolds for BTE due to their advantages such as controlled drug release, cell fate modulation, and the photothermal effect. Here, we review the current advances in photoresponsive hydrogels. The mechanism of photoresponsiveness and its advanced applications in bone repair are also elucidated. Future research would focus on the development of more efficient, safer, and smarter photoresponsive hydrogels for BTE. This review is aimed at offering comprehensive guidance on the trends of photoresponsive hydrogels and shedding light on their potential clinical application in BTE.

Advantages of photo-curable collagen-based cell-laden bioinks compared to methacrylated gelatin (GelMA) in digital light processing (DLP) and extrusion bioprinting

The development of cell-laden bioinks that possess high biocompatibility and printability is crucial in the field of bioprinting for the creation of cell-embedded tissue engineering scaffolds. As widely known, methacrylated gelatin (GelMA) is one of the most commonly used photo-crosslinkable bioink for cell-laden bioprinting with different printing methods, but GelMA is the derivative of gelatin, so it loses the unique triple-helix molecular structure of collagen and may not be able to successfully activate the cellular pathways or facilitate cell-matrix interaction as effectively as collagen. Recently, methacrylated collagen (CMA) was developed to be an alternative photocrosslinkable bioink with a good bioactivity, but its low printability and biocompatibility limited that application in tissue engineering. In this study, the synthetic process for CMA was improved by synthesizing under 4 °C and using acidic aqueous solution as solvent. Our CMA bioinks were demonstrated a similar printability as GelMA in extrusion bioprinting, while a better formability in digital light processing (DLP). To further analyze the bioactive properties, CMA bioinks were encapsulated with Schwann cells (SCs) and bone mesenchymal stem cells (BMSCs) for printing. SCs-laden CMA bioinks had a significantly higher proliferation rate and expression of neural stem cell-associated genes than GelMA in DLP bioprinting. While, BMSCs-laden CMA bioinks demonstrated >95% cellular viability, better cell spreading and higher expression of osteogenesis-related genes than that of GelMA. Overall, we speculate that the CMA-based bioink developed in this study could be potential bioinks for 3D cell-laden bioprinting in the future.

An injectable and 3D printable pro-chondrogenic hyaluronic acid and collagen type II composite hydrogel for the repair of articular cartilage defects

Current treatments for repairing articular cartilage defects are limited. However, pro-chondrogenic hydrogels formulated using articular cartilage matrix components (such as hyaluronic acid (HA) and collagen type II (Col II)), offer a potential solution if they could be injected into the defect via minimally invasive arthroscopic procedures, or used as bioinks to 3D print patient-specific customised regenerative scaffolds-potentially combined with cells. However, HA and Col II are difficult to incorporate into injectable/3D printable hydrogels due to poor physicochemical properties. This study aimed to overcome this by developing an articular cartilage matrix-inspired pro-chondrogenic hydrogel with improved physicochemical properties for both injectable and 3D printing (3DP) applications. To achieve this, HA was methacrylated to improve mechanical properties and mixed in a 1:1 ratio with Col I, a Col I/Col II blend or Col II. Col I possesses superior mechanical properties to Col II and so was hypothesised to enhance hydrogel mechanical properties. Rheological analysis showed that the pre-gels had viscoelastic and shear thinning properties. Subsequent physicochemical analysis of the crosslinked hydrogels showed that Col II inclusion resulted in a more swollen and softer polymer network, without affecting degradation time. While all hydrogels exhibited exemplary injectability, only the Col I-containing hydrogels had sufficient mechanical stability for 3DP applications. To facilitate 3DP of multi-layered scaffolds using methacrylated HA (MeHA)-Col I and MeHA-Col I/Col II, additional mechanical support in the form of a gelatin slurry support bath freeform reversible embedding of suspended hydrogels was utilised. Biological analysis revealed that Col II inclusion enhanced hydrogel-embedded MSC chondrogenesis, thus MeHA-Col II was selected as the optimal injectable hydrogel, and MeHA-Col I/Col II as the preferred bioink. In summary, this study demonstrates how tailoring biomaterial composition and physicochemical properties enables development of pro-chondrogenic hydrogels with potential for minimally invasive delivery to injured articular joints or 3DP of customised regenerative implants for cartilage repair.

Bioprinting: a focus on improving bioink printability and cell performance based on different process parameters

Three dimensional (3D) bioprinting is an emerging biofabrication technique that shows great potential in the field of tissue engineering, regenerative medicine and advanced drug delivery. Despite the current advancement of bioprinting technology, it faces several obstacles such as the challenge of optimizing the printing resolution of 3D constructs while retaining cell viability before, during, and after bioprinting. Therefore, it is of great significance to fully understand factors that influence the shape fidelity of printed structures and the performance of cells encapsulated in bioinks. This review presents a comprehensive analysis of bioprinting process parameters that influence bioink printability and cell performance, including bioink properties (composition, concentration, and component ratio), printing speed and pressure, nozzle charateristics (size, length, and geometry), and crosslinking parameters (crosslinker types, concentration, and crosslinking time). Key examples are provided to analyze how these parameters could be tailored to achieve the optimal printing resolution as well as cell performance. Finally, future prospects of bioprinting technology, including correlating process parameters to particular cell types with predefined applications, applying statistical analysis and artificial intelligence (AI)/machine learning (ML) technique in parameter screening, and optimizing 4D bioprinting process parameters, are highlighted.

3D bioprinted functional skeletal muscle models have potential applications for studies of muscle wasting in cancer cachexia

Acquired muscle diseases such as cancer cachexia are responsible for the poor prognosis of many patients suffering from cancer. In vitro models are needed to study the underlying mechanisms of those pathologies. Extrusion bioprinting is an emerging tool to emulate the aligned architecture of fibers while implementing additive manufacturing techniques in tissue engineering. However, designing bioinks that reconcile the rheological needs of bioprinting and the biological requirements of muscle tissue is a challenging matter. Here we formulate a biomaterial with dual crosslinking to modulate the physical properties of bioprinted models. We design 3D bioprinted muscle models that resemble the mechanical properties of native tissue and show improved proliferation and high maturation of differentiated myotubes suggesting that the GelMA-AlgMA-Fibrin biomaterial possesses myogenic properties. The electrical stimulation of the 3D model confirmed the contractile capability of the tissue and enhanced the formation of sarcomeres. Regarding the functionality of the models, they served as platforms to recapitulate skeletal muscle diseases such as muscle wasting produced by cancer cachexia. The genetic expression of 3D models demonstrated a better resemblance to the muscular biopsies of cachectic mouse models. Altogether, this biomaterial is aimed to fabricate manipulable skeletal muscle in vitro models in a non-costly, fast and feasible manner.

Characterization of a Bioink Combining Extracellular Matrix-like Hydrogel with Osteosarcoma Cells: Preliminary Results

Three-dimensional (3D) bioprinting allows the production of artificial 3D cellular microenvironments thanks to the controlled spatial deposition of bioinks. Proper bioink characterization is required to achieve the essential characteristics of printability and biocompatibility for 3D bioprinting. In this work, a protocol to standardize the experimental characterization of a new bioink is proposed. A functionalized hydrogel based on gelatin and chitosan was used. The protocol was divided into three steps: pre-printing, 3D bioprinting, and post-printing. For the pre-printing step, the hydrogel formulation and its repeatability were evaluated. For the 3D-bioprinting step, the hydrogel-printability performance was assessed through qualitative and quantitative tests. Finally, for the post-printing step, the hydrogel biocompatibility was investigated using UMR-106 cells. The hydrogel was suitable for printing grids with good resolution from 4 h after the cross-linker addition. To guarantee a constant printing pressure, it was necessary to set the extruder to 37 °C. Furthermore, the hydrogel was shown to be a valid biomaterial for the UMR-106 cells' growth. However, fragmentation of the constructs appeared after 14 days, probably due to the negative osteosarcoma-cell interference. The protocol that we describe here denotes a strong approach to bioink characterization to improve standardization for future biomaterial screening and development.

Species-Based Differences in Mechanical Properties, Cytocompatibility, and Printability of Methacrylated Collagen Hydrogels

Collagen methacrylation is a promising approach to generate photo-cross-linkable cell-laden hydrogels with improved mechanical properties. However, the impact of species-based variations in amino acid composition and collagen isolation method on methacrylation degree (MD) and its subsequent effects on the physical properties of methacrylated collagen (CMA) hydrogels and cell response are unknown. Herein, we compared the effects of three collagen species (bovine, human, and rat), two collagen extraction methods (pepsin digestion and acid extraction), and two photoinitiators (lithium phenyl-2,4,6-trimethylbenzoylphosphinate (LAP) and Irgacure-2959 (I-2959)) on the physical properties of CMA hydrogels, printability and mesenchymal stem cell (MSC) response. Human collagen showed the highest MD. LAP was more cytocompatible than I-2959. The compressive modulus and cell viability of rat CMA were significantly higher (p < 0.05) than bovine CMA. Human CMA yielded constructs with superior print fidelity. Together, these results suggest that careful selection of collagen source and cross-linking conditions is essential for biomimetic design of CMA hydrogels for tissue engineering applications.

Articulation inspired by nature: a review of biomimetic and biologically active 3D printed scaffolds for cartilage tissue engineering

In the human body, articular cartilage facilitates the frictionless movement of synovial joints. However, due to its avascular and aneural nature, it has a limited ability to self-repair when damaged due to injury or wear and tear over time. Current surgical treatment options for cartilage defects often lead to the formation of fibrous, non-durable tissue and thus a new solution is required. Nature is the best innovator and so recent advances in the field of tissue engineering have aimed to recreate the microenvironment of native articular cartilage using biomaterial scaffolds. However, the inability to mirror the complexity of native tissue has hindered the clinical translation of many products thus far. Fortunately, the advent of 3D printing has provided a potential solution. 3D printed scaffolds, fabricated using biomimetic biomaterials, can be designed to mimic the complex zonal architecture and composition of articular cartilage. The bioinks used to fabricate these scaffolds can also be further functionalised with cells and/or bioactive factors or gene therapeutics to mirror the cellular composition of the native tissue. Thus, this review investigates how the architecture and composition of native articular cartilage is inspiring the design of biomimetic bioinks for 3D printing of scaffolds for cartilage repair. Subsequently, we discuss how these 3D printed scaffolds can be further functionalised with cells and bioactive factors, as well as looking at future prospects in this field.

Characterization of Alginate–Gelatin–Cholesteryl Ester Liquid Crystals Bioinks for Extrusion Bioprinting of Tissue Engineering Scaffolds

Tissue engineering (TE) is an innovative approach to tackling many diseases and body parts that need to be replaced by developing artificial tissues and organs. Bioinks play an important role in the success of various TE applications. A bioink refers to a combination of a living cell, biomaterials, and bioactive molecules deposited in a layer-by-layer form to fabricate tissue-like structures. The research on bioink attempts to offer a 3D complex architecture and control cellular behavior that improve cell physical properties and viability. This research proposed a new multi-material bioink based on alginate (A), gelatin (G), and cholesteryl ester liquid crystals (CELC) biomaterials, namely (AGLC) bioinks. The development of AGLC was initiated with the optimization of different concentrations of A and G gels to obtain a printable formulation of AG gels. Subsequently, the influences of different concentrations of CELC with AG gels were investigated by using a microextrusion-based 3D bioprinting system to obtain a printed structure with high shape fidelity and minimum width. The AGLC bioinks were formulated using AG gel with 10% weight/volume (w/v) of A and 50% w/v G (AG10:50) and 1%, 5%, 10%, 20%, and 40% of CELC, respectively. The AGLC bioinks yield a high printability and resolution blend. The printed filament has a minimum width of 1.3 mm at a 1 mL/min extrusion rate when the A equals 10% w/v, G equals 50% w/v, and CELC equals 40% v/v (AGLC40). Polymerization of the AGLC bioinks with calcium (Ca²⁺) ions shows well-defined and more stable structures in the post-printing process. The physicochemical and viability properties of the AGLC bioinks were examined by FTIR, DSC, contact angle, FESEM, MTT assay, and cell interaction evaluation methods. The FTIR spectra of the AGLC bioinks exhibit a combination of characteristics vibrations of AG10:50 and CELC. The DSC analysis indicates the high thermal stability of the bioinks. Wettability analysis shows a reduction in the water absorption ability of the AGLC bioinks. FESEM analysis indicates that the surface morphologies of the bioinks exhibit varying microstructures. In vitro cytotoxicity by MTT assay shows the ability of the bioinks to support the biological activity of HeLa cells. The AGLC bioinks show average cell viability of 82.36% compared to the control (90%). Furthermore, cultured cells on the surface of AGLC bioinks showed that bioinks provide favorable interfaces for cell attachment.

Cross-Linked Gelatine by Modified Dextran as a Potential Bioink Prepared by a Simple and Non-Toxic Process

Essential features of well-designed materials intended for 3D bioprinting via microextrusion are the appropriate rheological behavior and cell-friendly environment. Despite the rapid development, few materials are utilizable as bioinks. The aim of our work was to design a novel cytocompatible material facilitating extrusion-based 3D printing while maintaining a relatively simple and straightforward preparation process without the need for harsh chemicals or radiation. Specifically, hydrogels were prepared from gelatines coming from three sources—bovine, rabbit, and chicken—cross-linked by dextran polyaldehyde. The influence of dextran concentration on the properties of hydrogels was studied. Rheological measurements not only confirmed the strong shear-thinning behavior of prepared inks but were also used for capturing cross-linking reaction kinetics and demonstrated quick achievement of gelation point (in most cases < 3 min). Their viscoelastic properties allowed satisfactory extrusion, forming a self-supported multi-layered uniformly porous structure. All gelatin-based hydrogels were non-cytototoxic. Homogeneous cells distribution within the printed scaffold was confirmed by fluorescence confocal microscopy. In addition, no disruption of cells structure was observed. The results demonstrate the great potential of the presented hydrogels for applications related to 3D bioprinting.

Cellulose Nanocrystal-Enhanced Thermal-Sensitive Hydrogels of Block Copolymers for 3D Bioprinting

The hydrogel formed by polyethylene glycol-aliphatic polyester block copolymers is an ideal bioink and biomaterial ink for three-dimensional (3D) bioprinting because of its unique temperature sensitivity, mild gelation process, good biocompatibility, and biodegradability. However, the gel forming mechanism based only on hydrophilic-hydrophobic interaction renders the stability and mechanical strength of the formed hydrogels insufficient, and cannot meet the requirements of extrusion 3D printing. In this study, cellulose nanocrystals (CNC), which is a kind of rigid, hydrophilic, and biocompatible nanomaterial, were introduced to enhance the hydrogels so as to meet the requirements of extrusion 3D printing. First, a series of poly(ε-caprolactone/lactide)-b-poly(ethylene glycol)-b-poly(ε-caprolactone/lactide) (PCLA-PEG-PCLA) triblock copolymers with different molecular weights were prepared. The thermodynamic and rheological properties of CNC-enhanced hydrogels were investigated. The results showed that the addition of CNC significantly improved the thermal stability and mechanical properties of the hydrogels, and within a certain range, the enhancement effect was directly proportional to the concentration of CNC. More importantly, the PCLA-PEG-PCLA hydrogels enhanced by CNC could be extruded and printed through temperature regulation. The printed objects had high resolution and fidelity with effectively maintained structure. Moreover, the hydrogels have good biocompatibility with a high cell viability. Therefore, this is a simple and effective strategy. The addition of the hydrophilic rigid nanoparticles such as CNC improves the mechanical properties of the soft hydrogels which made it able to meet the requirements of 3D bioprinting.

A Feasibility Study on 3D Bioprinting of Microfat Constructs Towards Wound Healing Applications

Chronic wounds affect over 400,000 people in the United States alone, with up to 60,000 deaths each year from non-healing ulcerations. Tissue grafting (e.g., autografts, allografts, and xenografts) and synthetic skin substitutes are common treatment methods, but most solutions are limited to symptomatic treatment and do not address the underlying causes of the chronic wound. Use of fat grafts for wound healing applications has demonstrated promise but these grafts suffer from low cell viability and poor retention at the wound site resulting in suboptimal healing of chronic wounds. Herein, we report on an innovative closed-loop fat processing system (MiniTC^TM) that can efficiently process lipoaspirates into microfat clusters comprising of highly viable regenerative cell population (i.e., adipose stromal cells, endothelial progenitors) preserved in their native niche. Cryopreservation of MiniTC^TM isolated microfat retained cell count and viability. To improve microfat retention and engraftment at the wound site, microfat was mixed with methacrylated collagen (CMA) bioink and 3D printed to generate microfat-laden collagen constructs. Modulating the concentration of microfat in CMA constructs had no effect on print fidelity or stability of the printed constructs. Results from the Alamar blue assay showed that the cells remain viable and metabolically active in microfat-laden collagen constructs for up to 10 days in vitro. Further, quantitative assessment of cell culture medium over time using ELISA revealed a temporal expression of proinflammatory and anti-inflammatory cytokines indicative of wound healing microenvironment progression. Together, these results demonstrate that 3D bioprinting of microfat-laden collagen constructs is a promising approach to generate viable microfat grafts for potential use in treatment of non-healing chronic wounds.

Bioprinting of human nasoseptal chondrocytes-laden collagen hydrogel for cartilage tissue engineering

Skin cancer patients often have tumorigenic lesions on their noses. Surgical resection of the lesions often results in nasal cartilage removal. Cartilage grafts taken from other anatomical sites are used for the surgical reconstruction of the nasal cartilage, but donor-site morbidity is a common problem. Autologous tissue-engineered nasal cartilage grafts can mitigate the problem, but commercially available scaffolds define the shape and sizes of the engineered grafts during tissue fabrication. Moreover, the engineered grafts suffer from the inhomogeneous distribution of the functional matrix of cartilage. Advances in 3D bioprinting technology offer the opportunity to engineer cartilages with customizable dimensions and anatomically shaped configurations without the inhomogeneous distribution of cartilage matrix. Here, we report the fidelity of Freeform Reversible Embedding of Suspended Hydrogel (FRESH) bioprinting as a strategy to generate customizable and homogenously distributed functional cartilage matrix engineered nasal cartilage. Using FRESH and in vitro chondrogenesis, we have fabricated tissue-engineered nasal cartilage from combining bovine type I collagen hydrogel and human nasoseptal chondrocytes. The engineered nasal cartilage constructs displayed molecular, biochemical and histological characteristics akin to native human nasal cartilage.

The Technique of Thyroid Cartilage Scaffold Support Formation for Extrusion-Based Bioprinting

During biofabrication, a tissue scaffold may require temporary support. The aim of this study was to develop an approach of human thyroid cartilage scaffold temporal support formation. The scaffold 3D-model was based on DICOM images. XY plane projections were used to form scaffold supporting part. To verify the technique, collagen hydrogel was chosen as the main scaffold component. Gelatin was applied for the supporting part. To test the applicability of the approach, a model of thyroid cartilage scaffold with the support was printed. The scaffold corresponded to a given model, although some discrepancy in geometry was observed during verification by computed tomography.

Engineering bioinks for 3D bioprinting

In recent years, three-dimensional (3D) bioprinting has attracted wide research interest in biomedical engineering and clinical applications. This technology allows for unparalleled architecture control, adaptability and repeatability that can overcome the limits of conventional biofabrication techniques. Along with the emergence of a variety of 3D bioprinting methods, bioinks have also come a long way. From their first developments to support bioprinting requirements, they are now engineered to specific injury sites requirements to mimic native tissue characteristics and to support biofunctionality. Current strategies involve the use of bioinks loaded with cells and biomolecules of interest, without altering their functions, to deliverin situthe elements required to enhance healing/regeneration. The current research and trends in bioink development for 3D bioprinting purposes is overviewed herein.

Photopolymerization of Bio-Based Polymers in a Biomedical Engineering Perspective

Photopolymerization is an effective method to covalently cross-link polymer chains that can be shaped into several biomedical products and devices. Additionally, polymerization reaction may induce a fluid-solid phase transformation under physiological conditions and is ideal for in vivo cross-linking of injectable polymers. The photoinitiator is a key ingredient able to absorb the energy at a specific light wavelength and create radicals that convert the liquid monomer solution into polymers. The combination of photopolymerizable polymers, containing appropriate photoinitiators, and effective curing based on dedicated light sources offers the possibility to implement photopolymerization technology in 3D bioprinting systems. Hence, cell-laden structures with high cell viability and proliferation, high accuracy in production, and good control of scaffold geometry can be biofabricated. In this review, we provide an overview of photopolymerization technology, focusing our efforts on natural polymers, the chemistry involved, and their combination with appropriate photoinitiators to be used within 3D bioprinting and manufacturing of biomedical devices. The reviewed articles showed the impact of different factors that influence the success of the photopolymerization process and the final properties of the cross-linked materials.

In situ and non-cytotoxic cross-linking strategy for 3D printable biomaterials

3D bioprinting allows the production of patient-specific tissue constructs with desired structural characteristics such as high resolution, controlled swelling degree, and controlled degradation behavior by mostly using hydrogels. Crosslinking of hydrogels is an essential parameter in bioprinting applications, which is beneficial for tuning structural specifications. In this study, gelatin-alginate-whey protein isolate based hydrogels have been used for 3D printing structures in a layer-by-layer fashion. These structures were cross-linked by the Amino Acid (monomer) Decorated and Light Underpinning Conjugation Approach (ANADOLUCA) method, which is a unique, non-invasive photosensitive cross-linking technique for protein-based mixtures. In that aim, hydrogel properties (e.g., printability, biocompatibility, rheologic and mechanical behavior) and cross-linking properties (e.g., swelling and degradation behavior) were studied. Results were compared with UV and ionic cross-linking techniques, which are the abundantly used techniques in such studies. The results showed that the ANADOLUCA method can be used for in situ cross-linking under mild conditions for the printing of bio-inks, and the proposed method can be used as an alternative for UV-based and chemical cross-linking techniques.

Bioglass incorporated methacrylated collagen bioactive ink for 3D printing of bone tissue

Bioactive 3D printed scaffolds are promising candidates for bone tissue engineering (BTE) applications. Here, we introduce a bioactive ink composed of Bioglass 45S5 (BG) and methacrylated collagen (CMA) for 3D printing of biomimetic constructs that resemble the organic and inorganic composition of native bone tissue. A uniform dispersion of BG particles within the collagen network improved stability and reduced swelling of collagen hydrogels. Rheological testing showed significant improvement in the yield stress and percent recovery of 3D printed constructs upon BG incorporation. Further, addition of BG improved the bone bioactivity of 3D printed constructs in stimulated body fluid. BG incorporated CMA (BG-CMA) constructs maintained high cell viability and enhanced alkaline phosphatase activity of human mesenchymal stem cells. In addition, cell-mediated calcium deposition was significantly higher on BG-CMA constructs, compared to CMA alone. In conclusion, 3D printed BG-CMA constructs have significant potential for use in BTE applications.

Sodium alginate/collagen composite multiscale porous scaffolds containing poly(ε-caprolactone) microspheres fabricated based on additive manufacturing technology

Biocompatible porous scaffolds with adjustable pore structures, appropriate mechanical properties and drug loading properties are important components of bone tissue engineering. In this work, biocompatible sodium alginate (SA)/collagen (Col) multiscale porous scaffolds containing poly(ε-caprolactone) microspheres (Ms-PCL) have been facilely fabricated based on 3D extrusion printing of the pre-crosslinked composite hydrogels. The prepared composite hydrogels can be 3D extrusion printed into porous scaffolds with different designed shapes and adjustable pore structures. The hydroxyapatite (HAP) nanoparticles have been added into the SA/Col hydrogels to achieve stress dispersion and form double crosslinking networks. SA-Ca²⁺ crosslinking networks and Col–genipin (GP) crosslinking networks have been constructed to improve the mechanical properties of the scaffolds (about 2557 kPa of compressive stress at 70% strain), and reduce the swelling rate and degradation rate of SA/Col scaffolds. Moreover, the SA/Col hydrogels contain hydrophobic antibacterial drug enrofloxacin loaded Ms-PCL, and in vitro drug release research shows a sustained-release function of porous scaffolds, indicating the potential application of SA/Col porous scaffolds as drug carriers. In addition, the antibacterial experiments show that the composite scaffolds display a distinguished and long-term antibacterial activity against Escherichia coli and Staphylococcus aureus. Furthermore, mouse bone mesenchymal stem cells (mBMSCs) are seeded on the SA/Col composite scaffolds, and an in vitro biocompatibility experiment shows that the mBMSCs can adhere well on the composite scaffolds, which indicate that the fabricated composite scaffolds are biocompatible. In short, all of the above results suggest that the biocompatible SA/Col composite porous scaffolds have enormous application and potential in bone tissue engineering.

Biocompatible porous scaffolds with adjustable pore structures, appropriate mechanical properties and drug loading properties are important components of bone tissue engineering.

Crystallization enhanced thermal-sensitive hydrogels of PCL-PEG-PCL triblock copolymer for 3D printing

Temperature-sensitive hydrogels with mild gel-forming process, good biocompatibility and biodegradability have been widely studied as bioinks and biomaterial inks for 3D bioprinting. However, the hydrogels synthesized via copolymerization of aliphatic polyesters and polyethylene glycols have low mechanical strength and cannot meet the needs of 3D printing. In this paper, we propose a strategy of enhancing the strength of hydrogels by introducing crystallization between blocks to meet the requirements of 3D bioprinting inks. A series of polycaprolactone-polyethylene glycol-polycaprolactone (PCL-PEG-PCL) triblock polymers were prepared by ring-opening polymerization, of which the strong crystallinity of polycaprolactone blocks improved the printability and enhanced the mechanical properties of the ink. It was found that the resulted hydrogels were temperature-responsive, and the PCL blocks could form a crystalline phase in the state of the hydrogel, thereby significantly increasing the modulus of the hydrogel. Moreover, the mechanical strength of the hydrogel could be adjusted by changing the composition ratio of each block of the copolymer. The 3D printing results showed that the PCL-PEG-PCL hydrogel with crystallinity can not only be extruded and printed via temperature adjustment, but also the three-dimensional structure can be effectively maintained after 3D printing. The gels demonstrated good cell compatibility, and the cell survival rate was maintained at a high level.

In vitro characterization of xeno-free clinically relevant human collagen and its applicability in cell-laden 3D bioprinting

Collagen type I, commonly derived from xenogenic sources, is extensively used as a biomaterial for tissue engineering applications. However, the use of xenogenic collagen is typically associated with species specific variation in mechanical, structural, and biological properties that are known to influence cellular response and remodeling. In addition, immunological complications and risks of disease transmission are also major concerns. The goal of this study is to characterize a new xeno-free human skin-derived collagen and assess its applicability as a bioink for cell-laden 3 D bioprinting. Four different concentrations of human collagen (i.e., 0.5 mg/mL, 1 mg/mL, 3 mg/mL and 6 mg/mL) were employed for the synthesis of collagen hydrogels. In addition, bovine collagen was used as a xenogenic control. Results from SDS-PAGE analysis showed the presence of α1, α2, and β chains, confirming that the integrity of type I human collagen is maintained post isolation. Polymerization rate and compressive modulus increased significantly with increase in the concentration of human collagen. When comparing two different sources of collagen, the polymerization rate of xenogenic collagen was significantly faster (p < 0.05) than human collagen while the compressive modulus was comparable. Raman spectroscopy showed a large peak in the Amide I band around 1600 cm^-1, indicating a dense and supraorganized fibrillar structure in human collagen hydrogels. Conversely, Amide I band intensity for xenogenic collagen was comparable to that of Amide II and Amide III bands. Further, the use of 6 mg/mL human collagen as a bioink yielded 3 D printed constructs with high shape fidelity and cell viability. On the other hand, xenogenic collagen failed to yield stable 3 D printed constructs. Together, the results from this study provides an impetus for using human-derived collagen as a viable alternative to xenogenic sources for 3 D bioprinting of clinically relevant scaffolds for tissue engineering applications.

Overview of Current Advances in Extrusion Bioprinting for Skin Applications

Bioprinting technologies, which have the ability to combine various human cell phenotypes, signaling proteins, extracellular matrix components, and other scaffold-like biomaterials, are currently being exploited for the fabrication of human skin in regenerative medicine. We performed a systematic review to appraise the latest advances in 3D bioprinting for skin applications, describing the main cell phenotypes, signaling proteins, and bioinks used in extrusion platforms. To understand the current limitations of this technology for skin bioprinting, we briefly address the relevant aspects of skin biology. This field is in the early stage of development, and reported research on extrusion bioprinting for skin applications has shown moderate progress. We have identified two major trends. First, the biomimetic approach uses cell-laden natural polymers, including fibrinogen, decellularized extracellular matrix, and collagen. Second, the material engineering line of research, which is focused on the optimization of printable biomaterials that expedite the manufacturing process, mainly involves chemically functionalized polymers and reinforcement strategies through molecular blending and postprinting interventions, i.e., ionic, covalent, or light entanglement, to enhance the mechanical properties of the construct and facilitate layer-by-layer deposition. Skin constructs manufactured using the biomimetic approach have reached a higher level of complexity in biological terms, including up to five different cell phenotypes and mirroring the epidermis, dermis and hypodermis. The confluence of the two perspectives, representing interdisciplinary inputs, is required for further advancement toward the future translation of extrusion bioprinting and to meet the urgent clinical demand for skin equivalents.

Effects of Gelatin Methacrylate Bio-ink Concentration on Mechano-Physical Properties and Human Dermal Fibroblast Behavior

Gelatin-methacryloyl (GelMa) is a very versatile biomaterial widely used in various biomedical applications. The addition of methacryloyl makes it possible to have hydrogels with varying mechanical properties due to its photocuring characteristics. In addition, gelatin is obtained and derived from natural material; thus, it retains various cell-friendly motifs, such as arginine-glycine-aspartic acid, which then provides implanted cells with a friendly environment for proliferation and differentiation. In this study, we fabricated human dermal fibroblast cell (hDF)-laden photocurable GelMa hydrogels with varying physical properties (5%, 10%, and 15%) and assessed them for cellular responses and behavior, including cell spreading, proliferation, and the degree of extracellular matrix remodeling. Under similar photocuring conditions, lower concentrations of GelMa hydrogels had lower mechanical properties than higher concentrations. Furthermore, other properties, such as swelling and degradation, were compared in this study. In addition, our findings revealed that there were increased remodeling and proliferation markers in the 5% GelMa group, which had lower mechanical properties. However, it was important to note that cellular viabilities were not affected by the stiffness of the hydrogels. With this result in mind, we attempted to fabricate 5-15% GelMa scaffolds (20 × 20 × 3 mm3) to assess their feasibility for use in skin regeneration applications. The results showed that both 10% and 15% GelMa scaffolds could be fabricated easily at room temperature by adjusting several parameters, such as printing speed and extrusion pressure. However, since the sol-gel temperature of 5% GelMa was noted to be lower than its counterparts, 5% GelMa scaffolds had to be printed at low temperatures. In conclusion, GelMa once again was shown to be an ideal biomaterial for various tissue engineering applications due to its versatile mechanical and biological properties. This study showed the feasibility of GelMa in skin tissue engineering and its potential as an alternative for skin transplants.