Effect of gelatin on dimensional stability of silk fibroin hydrogel structures fabricated by digital light processing 3D printing

Design considerations for digital light processing bioprinters

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.

Natural bioink of interpenetrating network hydrogels mimicking extracellular polymeric substances for microbial immobilization in water pollution control

Artificial biomanufacturing has been developed as a promising biotechnology for water pollution control. Effective bioimmobilization techniques are limited in application because of low productivity and the difficulty in achieving both mechanical strength and biocompatibility. Bioprinting technology, using biomaterials as bioink to enable the rapid on-demand production of bioactive structures, opens a new path for bioimmobilization. In this study, mimicking extracellular polysaccharide and protein of aerobic granular sludge (AGS), sodium alginate (SA) and silk fibroin methacryloyl (SilMA) were developed as the dual-component bioink with a suitable viscosity for bioprinting hydrogel. Interpenetrating network (IPN) hydrogel beads were manufactured using 1.5% (w/v) SA combined with 20% (w/v) SilMA through physical and covalent crosslinking, which exhibited excellent structural stability and bioactivity. The addition of SilMA provided a solution to the poor mechanical stability of SA-Ca hydrogels limited by Ca2+-Na+ ionic exchange. The unique structure of SilMA contributed to the reduction of hydrogel swelling as well as the prevention of SA loss. IPN hydrogels showed a swelling rate of less than 20% compared to the high swelling rate of more than 60% for SA hydrogels. On the other hand, SA controlled the hardening induced by excessive self-assembly of SilMA and improved mass transport in SilMA hydrogels. Compared to IPN hydrogels, SilMA hydrogels experienced a 15% volumetric shrinkage and exhibited a low water content of 92%. Sonication pretreatment of the dual-component bioink not only increased the intermolecular chain entanglement to form IPN, but also led to β-sheet content in SiMA reaching 46%-48%, which resulted in the formation of stable IPN hydrogels dominated entirely by physical crosslinking. Satisfactory proliferation and viability were achieved for the encapsulated bacteria in IPN hydrogels (μmax 1.49-2.18 d-1). Further, the IPN biohydrogels could maintain structural stability as well as achieve pollutant removal for treating synthetic wastewater with high Na+ concentration of 300 mg/L. The novel SA/SilMA hydrogel bioprinting strategy established in this study offers a new direction for bioimmobilization in water pollution control and other environmental applications.

Silk fibroin and hydroxypropyl cellulose composite injectable hydrogel-containing extracellular vesicles for myocardial infarction repair

Extracellular vesicles (EVs) have been recognized as one of the promising specific drugs for myocardial infarction (MI) prognosis. Nevertheless, low intramyocardial retention of EVs remains a major impediment to their clinical application. In this study, we developed a silk fibroin/hydroxypropyl cellulose (SF/HPC) composite hydrogel combined with AC16 cell-derived EVs targeted modification by folic acid for the treatment of acute myocardial infarction repair. EVs were functionalized by distearoylphosphatidyl ethanolamine-polyethylene glycol (DSPE-PEG-FA) via noncovalent interaction for targeting and accelerating myocardial infarction repair.In vitro, cytocompatibility analyses revealed that the as-prepared hydrogels had excellent cell viability by MTT assay and the functionalized EVs had higher cell migration by scratch assay.In vivo, the composite hydrogels can promote myocardial tissue repair effects by delaying the process of myocardial fibrosis and promoting angiogenesis of infarct area in MI rat model.

Advancements in gelatin-based hydrogel systems for biomedical applications: A state-of-the-art review

A gelatin-based hydrogel system is a stimulus-responsive, biocompatible, and biodegradable polymeric system with solid-like rheology that entangles moisture in its porous network that gradually protrudes to assemble a hierarchical crosslinked arrangement. The hydrolysis of collagen directs gelatin construction, which retains arginyl glycyl aspartic acid and matrix metalloproteinase-sensitive degeneration sites, further confining access to chemicals entangled within the gel (e.g., cell encapsulation), modulating the release of encapsulated payloads and providing mechanical signals to the adjoining cells. The utilization of various types of functional tunable biopolymers as scaffold materials in hydrogels has become highly attractive due to their higher porosity and mechanical ability; thus, higher loading of proteins, peptides, therapeutic molecules, etc., can be further modulated. Furthermore, a stimulus-mediated gelatin-based hydrogel with an impaired concentration of gellan demonstrated great shear thinning and self-recovering characteristics in biomedical and tissue engineering applications. Therefore, this contemporary review presents a concise version of the gelatin-based hydrogel as a conceivable biomaterial for various biomedical applications. In addition, the article has recapped the multiple sources of gelatin and their structural characteristics concerning stimulating hydrogel development and delivery approaches of therapeutic molecules (e.g., proteins, peptides, genes, drugs, etc.), existing challenges, and overcoming designs, particularly from drug delivery perspectives.

Tailoring the Swelling-Shrinkable Behavior of Hydrogels for Biomedical Applications

Hydrogels with tailor-made swelling-shrinkable properties have aroused considerable interest in numerous biomedical domains. For example, as swelling is a key issue for blood and wound extrudates absorption, the transference of nutrients and metabolites, as well as drug diffusion and release, hydrogels with high swelling capacity have been widely applicated in full-thickness skin wound healing and tissue regeneration, and drug delivery. Nevertheless, in the fields of tissue adhesives and internal soft-tissue wound healing, and bioelectronics, non-swelling hydrogels play very important functions owing to their stable macroscopic dimension and physical performance in physiological environment. Moreover, the negative swelling behavior (i.e., shrinkage) of hydrogels can be exploited to drive noninvasive wound closure, and achieve resolution enhancement of hydrogel scaffolds. In addition, it can help push out the entrapped drugs, thus promote drug release. However, there still has not been a general review of the constructions and biomedical applications of hydrogels from the viewpoint of swelling-shrinkable properties. Therefore, this review summarizes the tactics employed so far in tailoring the swelling-shrinkable properties of hydrogels and their biomedical applications. And a relatively comprehensive understanding of the current progress and future challenge of the hydrogels with different swelling-shrinkable features is provided for potential clinical translations.

Drug-loaded mucoadhesive microneedle patch for the treatment of oral submucous fibrosis

Oral submucous fibrosis is a chronic, inflammatory and potentially malignant oral disease. Local delivery of triamcinolone to lesion site is a commonly used therapy. The existing methods for local drug delivery include topical administration and submucosal injection. However, in the wet and dynamic oral microenvironment, these methods have drawbacks such as limited drug delivery efficiency and injection pain. Therefore, it is urgently needed to develop an alternative local drug delivery system with high efficiency and painlessness. Inspired by the structure of band-aid, this study proposed a novel double-layered mucoadhesive microneedle patch for transmucosal drug delivery. The patch consisted of a mucoadhesive silk fibroin/tannic acid top-layer and a silk fibroin microneedle under-layer. When applying the annealing condition for the medium content of β-sheets of silk fibroin, the microneedles in under-layer displayed both superior morphology and mechanical property. The mechanical strength of per needle (0.071N) was sufficient to penetrate the oral mucosa. Sequentially, the gelation efficiency of silk fibroin and tannic acid in top-layer was maximized as the weight ratio of tannic acid to silk fibroin reached 5:1. Moreover, in vitro results demonstrated the double-layered patch possessed undetectable cytotoxicity. The sustained release of triamcinolone was observed from the double-layered patch for at least 7 days. Furthermore, compared with other commercial buccal patches, the double-layered patch exhibited an enhanced wet adhesion strength of 37.74 kPa. In addition, ex vivo mucosal tissue penetration experiment confirmed that the double-layered patch could reach the lamina propria, ensuring effective drug delivery to the lesion site of oral submucous fibrosis. These results illustrate the promising potential of the drug-loaded mucoadhesive microneedle patch for the treatment of oral submucous fibrosis.

3D Embedded Printing of Complex Biological Structures with Supporting Bath of Pluronic F-127

Biofabrication is crucial in contemporary tissue engineering. The primary challenge in biofabrication lies in achieving simultaneous replication of both external organ geometries and internal structures. Particularly for organs with high oxygen demand, the incorporation of a vascular network, which is usually intricate, is crucial to enhance tissue viability, which is still a difficulty in current biofabrication technology. In this study, we address this problem by introducing an innovative three-dimensional (3D) printing strategy using a thermo-reversible supporting bath which can be easily removed by decreasing the temperature. This technology is capable of printing hydrated materials with diverse crosslinked mechanisms, encompassing gelatin, hyaluronate, Pluronic F-127, and alginate. Furthermore, the technology can replicate the external geometry of native tissues and organs from computed tomography data. The work also demonstrates the capability to print lines around 10 μm with a nozzle with a diameter of 60 μm due to the extra force exerted by the supporting bath, by which the line size was largely reduced, and this technique can be used to fabricate intricate capillary networks.

Recent advances in bioprinting using silk protein-based bioinks

3D printing has experienced swift growth for biological applications in the field of regenerative medicine and tissue engineering. Essential features of bioprinting include determining the appropriate bioink, printing speed mechanics, and print resolution while also maintaining cytocompatibility. However, the scarcity of bioinks that provide printing and print properties and cell support remains a limitation. Silk Fibroin (SF) displays exceptional features and versatility for inks and shows the potential to print complex structures with tunable mechanical properties, degradation rates, and cytocompatibility. Here we summarize recent advances and needs with the use of SF protein from Bombyx mori silkworm as a bioink, including crosslinking methods for extrusion bioprinting using SF and the maintenance of cell viability during and post bioprinting. Additionally, we discuss how encapsulated cells within these SF-based 3D bioprinted constructs are differentiated into various lineages such as skin, cartilage, and bone to expedite tissue regeneration. We then shift the focus towards SF-based 3D printing applications, including magnetically decorated hydrogels, in situ bioprinting, and a next-generation 4D bioprinting approach. Future perspectives on improvements in printing strategies and the use of multicomponent bioinks to improve print fidelity are also discussed.

High-resolution 3D printing of xanthan gum/nanocellulose bio-inks

The current study provides a comprehensive rheology study and a survey on direct ink writing of xanthan gum/cellulose nanocrystal (XG/CNC) bio-inks for developing 3D geometries that mimic soft tissue engineering scaffolds' physical and mechanical properties. The presence of CNC was found to be a critical prerequisite for the printability of XG bio-inks; accordingly, the hybrid XG/CNC bio-inks revealed the excellent viscoelastic properties that enabled precise control of hydrogel shaping and printing of lattice structures composed of up to eleven layers with high fidelity and fair resolution without any deformation after printing. The lyophilized 3D scaffolds presented a porous structure with open and interconnected pores and a porosity higher than 70%, vital features for tissue engineering scaffolds. Moreover, they showed a relatively high swelling of approximately 11 g/g, facilitating oxygen and nutrient exchange. Furthermore, the elastic and compressive moduli of the scaffolds that enhanced significantly upon increasing CNC content were in the range of a few kPa, similar to soft tissues. Finally, no significant cell cytotoxicity was observed against human liver cancer cells (HepG2), highlighting the potential of these developed 3D printed scaffolds for soft tissue engineering applications.

Next Evolution in Organ-Scale Biofabrication: Bioresin Design for Rapid High-Resolution Vat Polymerization

The field of bioprinting has made significant advancements in recent years and allowed for the precise deposition of biomaterials and cells. However, within this field lies a major challenge, which is developing high resolution constructs, with complex architectures. In an effort to overcome these challenges a biofabrication technique known as vat polymerization is being increasingly investigated due to its high fabrication accuracy and control of resolution (µm scale). Despite the progress made in developing hydrogel precursors for bioprinting techniques, such as extrusion-based bioprinting, there is a major lack in developing hydrogel precursor bioresins for vat polymerization. This is due to the specific unique properties and characteristics required for vat polymerization, from lithography to the latest volumetric printing. This is of major concern as the shortage of bioresins available has a significant impact on progressing this technology and exploring its full potential, including speed, resolution, and scale. Therefore, this review discusses the key requirements that need to be addressed in successfully developing a bioresin. The influence of monomer architecture and bioresin composition on printability is described, along with key fundamental parameters that can be altered to increase printing accuracy. Finally, recent advancements in bioresins are discussed together with future directions.

New Insights of Scaffolds Based on Hydrogels in Tissue Engineering

In recent years, biomaterials development and characterization for new applications in regenerative medicine or controlled release represent one of the biggest challenges. Tissue engineering is one of the most intensively studied domain where hydrogels are considered optimum applications in the biomedical field. The delicate nature of hydrogels and their low mechanical strength limit their exploitation in tissue engineering. Hence, developing new, stronger, and more stable hydrogels with increased biocompatibility, is essential. However, both natural and synthetic polymers possess many limitations. Hydrogels based on natural polymers offer particularly high biocompatibility and biodegradability, low immunogenicity, excellent cytocompatibility, variable, and controllable solubility. At the same time, they have poor mechanical properties, high production costs, and low reproducibility. Synthetic polymers come to their aid through superior mechanical strength, high reproducibility, reduced costs, and the ability to regulate their composition to improve processes such as hydrolysis or biodegradation over variable periods. The development of hydrogels based on mixtures of synthetic and natural polymers can lead to the optimization of their properties to obtain ideal scaffolds. Also, incorporating different nanoparticles can improve the hydrogel's stability and obtain several biological effects. In this regard, essential oils and drug molecules facilitate the desired biological effect or even produce a synergistic effect. This study's main purpose is to establish the main properties needed to develop sustainable polymeric scaffolds. These scaffolds can be applied in tissue engineering to improve the tissue regeneration process without producing other side effects to the environment.

Histidine-based hydrogels via singlet-oxygen photooxidation

The formation of hydrogels by photosensitized oxidation and crosslinking of histidine-derived polymers is demonstrated for the first time. The photooxidation of pendant His mediated by singlet oxygen was used to promote covalent coupling by its dimerization. As a proof-of-concept, two systems were studied: (i) chondroitin sulfate (CS) functionalized with His, and (ii) an elastin-like peptide (ELP) containing His produced by recombinant techniques. Both materials were crosslinked by irradiation at 425 nm in the presence of Zn-porphyrin derivatives yielding His-based hydrogels. The molecular structure and physicochemical properties of ELP-His and other 5 ELPs with photooxidizable amino acids were studied in silica by computer simulation. A correlation between the protein conformation and its elastic properties is discussed. CS-His hydrogels demonstrate larger storage moduli than ELPs with other amino acids. The obtained results show the potential use of photooxidation to create a new type of His-based hydrogels.

3D bioprinted silk fibroin hydrogels for tissue engineering

The development of biocompatible and precisely printable bioink addresses the growing demand for three-dimensional (3D) bioprinting applications in the field of tissue engineering. We developed a methacrylated photocurable silk fibroin (SF) bioink for digital light processing 3D bioprinting to generate structures with high mechanical stability and biocompatibility for tissue engineering applications. Procedure 1 describes the synthesis of photocurable methacrylated SF bioink, which takes 2 weeks to complete. Digital light processing is used to fabricate 3D hydrogels using the bioink (1.5 h), which are characterized in terms of methacrylation, printability, mechanical and rheological properties, and biocompatibility. The physicochemical properties of the bioink can be modulated by varying photopolymerization conditions such as the degree of methacrylation, light intensity, and concentration of the photoinitiator and bioink. The versatile bioink can be used broadly in a range of applications, including nerve tissue engineering through co-polymerization of the bioink with graphene oxide, and for wound healing as a sealant. Procedure 2 outlines how to apply 3D-printed SF hydrogels embedded with chondrocytes and turbinate-derived mesenchymal stem cells in one specific in vivo application, trachea tissue engineering, which takes 2-9 weeks.

3D printing and properties of cellulose nanofibrils-reinforced quince seed mucilage bio-inks

Plant-based hydrogels have attracted great attention in biomedical fields since they are biocompatible and based on natural, sustainable, cost-effective, and widely accessible sources. Here, we introduced new viscoelastic bio-inks composed of quince seed mucilage and cellulose nanofibrils (QSM/CNF) easily extruded into 3D lattice structures through direct ink writing in ambient conditions. The QSM/CNF inks enabled precise control on printing fidelity where CNF endowed objects with shape stability after freeze-drying and with suitable porosity, water uptake capacity, and mechanical strength. The compressive and elastic moduli of samples produced at the highest CNF content were both increased by ~100% (from 5.1 ± 0.2 kPa and 32 ± 1 kPa to 10.7 ± 0.5 and 64 ± 2 kPa, respectively). These values ideally matched those reported for soft tissues; accordingly, the cell compatibility of the printed samples was evaluated against HepG2 cells (human liver cancer). The results confirmed the 3D hydrogels as being non-cytotoxic and suitable to support attachment, survival, and proliferation of the cells. All in all, the newly developed inks allowed sustainable 3D bio-hydrogels fitting the requirements as scaffolds for soft tissue engineering.

In Situ 3D Printing: Opportunities with Silk Inks

In situ 3D printing is an emerging technique designed for patient-specific needs and performed directly in the patient's tissues in the operating room. While this technology has progressed rapidly, several improvements are needed to push it forward for widespread utility, including ink formulations and optimization for in situ context. Silk fibroin inks emerge as a viable option due to the diverse range of formulations, aqueous processability, robust and tunable mechanical properties, and self-assembly via biophysical adsorption to avoid exogenous chemical or photochemical sensitizer additives, among other features. In this review, we focus on this new frontier of 3D in situ printing for tissue regeneration, where silk is proposed as candidate biomaterial ink due to the unique and useful properties of this protein polymer.

Photo-Crosslinked Silk Fibroin for 3D Printing

Silk fibroin in material formats provides robust mechanical properties, and thus is a promising protein for 3D printing inks for a range of applications, including tissue engineering, bioelectronics, and bio-optics. Among the various crosslinking mechanisms, photo-crosslinking is particularly useful for 3D printing with silk fibroin inks due to the rapid kinetics, tunable crosslinking dynamics, light-assisted shape control, and the option to use visible light as a biocompatible processing condition. Multiple photo-crosslinking approaches have been applied to native or chemically modified silk fibroin, including photo-oxidation and free radical methacrylate polymerization. The molecular characteristics of silk fibroin, i.e., conformational polymorphism, provide a unique method for crosslinking and microfabrication via light. The molecular design features of silk fibroin inks and the exploitation of photo-crosslinking mechanisms suggest the exciting potential for meeting many biomedical needs in the future.