A highly elastic tissue sealant based on photopolymerised gelatin

Robust tissue adhesion in biomedical applications: enhancing polymer stability in an injectable protein-based hydrogel

Protein-based hydrogels are appealing materials for a variety of therapeutic uses because they are compatible, biodegradable, and adaptable to biological and chemical changes. Therefore, adherent varieties of hydrogels have received significant study; nevertheless, the majority of them show weak mechanical characteristics, transient adherence, poor biocompatibility activity, and low tensile strength. Here we are reporting, a two-component (BSA-gelatin) protein solution crosslinked with Tetrakis (hydroxymethyl) phosphonium chloride (THPC) to form a novel hydrogel. Compared with classical adhesive hydrogels, this hydrogel showed enhanced mechanical properties, was biocompatible with L929 cells, and had minimal invasive injectability. A considerable, high tensile strength of 73.33 ± 11.54 KPa and faultless compressive mechanical properties of 173 KPa at 75% strain were both demonstrated by this adhesive hydrogel. Moreover, this maximum tissue adhesion strength could reach 18.29 ± 2.22 kPa, significantly higher than fibrin glue. Cell viability was 97.09 ± 6.07%, which indicated that these hydrogels were non-toxic to L929. The fastest gelation time of the BSA-gelatin hydrogel was 1.25 ± 0.17 min at physiological pH and 37 °C. Therefore, the obtained novel work can potentially serve as a tissue adhesive hydrogel in the field of biomedical industries.

A New Approach for On-Chip Production of Biological Microgels Using Photochemical Cross-Linking

Photochemical cross-linking is a key step for manufacturing microgels in numerous applications, including drug delivery, tissue engineering, material production, and wound healing. Existing photochemical cross-linking techniques in microfluidic devices rely on UV curing, which can cause cell and DNA damage. We address this challenge by developing a microfluidic workflow for producing microgels using visible light-driven photochemical cross-linking of aqueous droplets dispersed in a continuous oil phase. We report a proof-of-concept to construct microgels from the protein Bovine Serum Albumin (BSA) with [Ru(bpy)3]2+ mediated cross-linking. By controlling the capillary number of the continuous and dispersed phases, the volumetric flow rate, and the photochemical reaction time within the microfluidic tubing, we demonstrate the construction of protein microgels with controllable and uniform dimensions. Our technique can, in principle, be applied to a wide range of different proteins with biological and responsive properties. This work therefore bridges the gap between hydrogel manufacturing using visible light and microfluidic microgel templating, facilitating numerous biomedical applications.

Photocurable extracellular matrix sealant for cessation of venous hemorrhage

Hemorrhage is the second leading cause of death in patients under 46 years of age in the United States. Cessation of hemorrhage prevents hemorrhagic shock and tissue hypoxia. Controlling the bleed via direct pressure or tourniquet is often the first line of defense, but long-term care requires staples, hemostatic agents, or sealants that seal the vessel and restore blood flow. Here, we compare a new photocurable extracellular matrix sealant (pcECM) with low, medium, and high crosslink density formulations to a commercially available fibrin-based sealant, TISSEEL®. pcECM has potential uses in surgical and remote settings due to room temperature storage conditions and fast preparation time. Here, we determine if pcECM sealant can stop venous hemorrhage in a murine model, adhere to the wound site in vivo throughout the wound-healing process, and has the mechanical properties necessary for stopping hemorrhage. Adjusting pcECM crosslinking density significantly affected viscosity, swelling, burst strength, tensile strength, and elasticity of the sealant. 3-Dimensional ultrasound volume segmentations showed pcECM degrades to 17 ± 8% of its initial implant volume by day 28. Initially, local hemodynamic changes were observed, but returned close to baseline levels by day 28. Acute inflammation was observed near the puncture site in pcECM implanted mice, and we observed inflammatory markers at the 14-day explant for both sealants. pcECM and fibrin sealant successfully sealed the vessel in all cases, and consistently degraded over 14-28 days. pcECM is a durable sealant with tunable mechanical properties and possible uses in hemorrhage control and other surgical procedures.

Natural polymer-based bioadhesives as hemostatic platforms for wound healing

Medical adhesives are advanced but challenging alternatives to wound closure and repair, especially in mitigating uncontrolled hemorrhage. Ideal hemostatic adhesives need to meet good biocompatibility and biodegradability, adequate mechanical strength, and strong tissue adhesion functionality under wet and dynamic conditions. Considering these requirements, natural polymers such as polysaccharide, protein and DNA, attract great attention as candidates for making bioadhesives because of their distinctive physicochemical performances and biological properties. This review systematically summarizes the advances of bioadhesives based on natural polysaccharide, protein and DNA. Various physical and chemical cross-linking strategies have been introduced for adhesive synthesis and their hemostatic applications are introduced from the aspect of versatility. Furthermore, the possible challenges and future opportunities of bioadhesives are discussed, providing insights into the development of high-performance hemostatic materials.

Photo-Cross-Linkable, Injectable, and Highly Adhesive GelMA-Glycol Chitosan Hydrogels for Cartilage Repair

Hydrogels provide a promising platform for cartilage repair and regeneration. Although hydrogels have shown some efficacy, they still have shortcomings including poor mechanical properties and suboptimal integration with surrounding cartilage. Herein, hydrogels that are injectable, cytocompatible, mechanically robust, and highly adhesive to cartilage are developed. This approach uses GelMA-glycol chitosan (GelMA-GC) that is crosslinkable with visible light and photoinitiators (lithium acylphosphinate and tris (2,2'-bipyridyl) dichlororuthenium (II) hexahydrate ([RuII(bpy)3 ]2+ and sodium persulfate (Ru/SPS)). Ru/SPS-cross-linked hydrogels have higher compressive and tensile modulus, and most prominently higher adhesive strength with cartilage, which also depends on inclusion of GC. Tensile and push-out tests of the Ru/SPS-cross-linked GelMA-GC hydrogels demonstrate adhesive strength of ≈100 and 46 kPa, respectively. Hydrogel precursor solutions behave in a Newtonian manner and are injectable. After injection in focal bovine cartilage defects and in situ cross-linking, this hydrogel system remains intact and integrated with cartilage following joint manipulation ex vivo. Cells remain viable (>85%) in the hydrogel system and further show tissue regeneration potential after three weeks of in vitro culture. These preliminary results provide further motivation for future research on bioadhesive hydrogels for cartilage repair and regeneration.

Bioadhesives with Antimicrobial Properties

Bioadhesives with antimicrobial properties enable easier and safer treatment of wounds as compared to the traditional methods such as suturing and stapling. Composed of natural or synthetic polymers, these bioadhesives seal wounds and facilitate healing while preventing infections through the activity of locally released antimicrobial drugs, nanocomponents, or inherently antimicrobial polers. Although many different materials and strategies are employed to develop antimicrobial bioadhesives, the design of these biomaterials necessitates a prudent approach as achieving all the required properties including optimal adhesive and cohesive properties, biocompatibility, and antimicrobial activity can be challenging. Designing antimicrobial bioadhesives with tunable physical, chemical, and biological properties will shed light on the path for future advancement of bioadhesives with antimicrobial properties. In this review, the requirements and commonly used strategies for developing bioadhesives with antimicrobial properties are discussed. In particular, different methods for their synthesis and their experimental and clinical applications on a variety of organs are reviewed. Advances in the design of bioadhesives with antimicrobial properties will pave the way for a better management of wounds to increase positive clinical outcomes.

Evaluating and developing sealants for the prevention of pulmonary air leakage: A systematic review of animal models

Sealants may provide a solution for pulmonary air leakage (PAL), but their clinical application is debatable. For sealant comparison, standardized animal models are lacking. This systematic review aims to assess methodology and quality of animal models for PAL and sealant evaluation. All animal models investigating lung sealing devices (e.g., staplers, glues, energy devices) to prevent or treat PAL were retrieved systematically from Embase, Pubmed and Web of science. Methodological study characteristics, risk of bias, reporting quality and publication bias were assessed. A total of 71 studies were included (N  =  75 experiments, N  =  1659 animals). Six different species and 18 strains were described; 92% of experiments used healthy animals, disease models were used in only six studies. Lesions to produce PAL were heterogenous, and only 11 studies used a previously reported technique, encompassing N  =  5 unique lesions. Clinically relevant outcomes were used in the minority of studies (imaging 16%, air leak 10.7%, air leak duration 4%). Reporting quality was poor, but revealed an upward trend per decade. Overall, high risk of bias was present, and only 18.7% used a negative control group. All but one study without control groups claimed positive outcomes (95.8%), in contrast to 84.3% using positive or negative control groups, which also concluded equivocal, adverse or inconclusive outcomes. In conclusion, animal studies evaluating sealants for prevention of PAL are heterogenous and of poor reporting quality. Using negative control groups, disease models and quantifiable outcomes seem important to increase validity and relevance. Further research is needed to reach consensus for model development and standardization.

Clinical Applicability of Visible Light-Mediated Cross-linking for Structural Soft Tissue Reconstruction

Visible light-mediated cross-linking has utility for enhancing the structural capacity and shape fidelity of laboratory-based polymers. With increased light penetration and cross-linking speed, there is opportunity to extend future applications into clinical spheres. This study evaluated the utility of a ruthenium/sodium persulfate photocross-linking system for increasing structural control in heterogeneous living tissues as an example, focusing on unmodified patient-derived lipoaspirate for soft tissue reconstruction. Freshly-isolated tissue is photocross-linked, then the molar abundance of dityrosine bonds is measured using liquid chromatography tandem mass spectrometry and the resulting structural integrity assessed. The cell function and tissue survival of photocross-linked grafts is evaluated ex vivo and in vivo, with tissue integration and vascularization assessed using histology and microcomputed tomography. The photocross-linking strategy is tailorable, allowing progressive increases in the structural fidelity of lipoaspirate, as measured by a stepwise reduction in fiber diameter, increased graft porosity and reduced variation in graft resorption. There is an increase in dityrosine bond formation with increasing photoinitiator concentration, and tissue homeostasis is achieved ex vivo, with vascular cell infiltration and vessel formation in vivo. These data demonstrate the capability and applicability of photocrosslinking strategies for improving structural control in clinically-relevant settings, potentially achieving more desirable patient outcomes using minimal manipulation in surgical procedures.

Progress of tissue adhesives based on proteins and synthetic polymers

In recent years, polymer-based tissue adhesives (TAs) have been developed as an alternative to sutures to close and seal incisions or wounds owing to their ease of use, rapid application time, low cost, and minimal tissue damage. Although significant research is being conducted to develop new TAs with improved performances using different strategies, the applications of TAs are limited by several factors, such as weak adhesion strength and poor mechanical properties. Therefore, the next-generation advanced TAs with biomimetic and multifunctional properties should be developed. Herein, we review the requirements, adhesive performances, characteristics, adhesive mechanisms, applications, commercial products, and advantages and disadvantages of proteins- and synthetic polymer-based TAs. Furthermore, future perspectives in the field of TA-based research have been discussed.

Mechanical Properties of Protein-Based Hydrogels Derived from Binary Protein Mixtures-A Feasibility Study

Hydrogels based on natural polymers such as proteins are considered biocompatible and, therefore, represent an interesting class of materials for application in the field of biomedicine and high-performance materials. However, there is a lack of understanding of the proteins which are able to form hydrogel networks by photoinduced dityrosine crosslinking as well as a profound knowledge of the formed network itself and the mechanisms which are responsible for the resulting mechanical properties of such protein-based hydrogels. In this study, casein, bovine serum albumin, α-amylase, and a hydrophobic elastin-like protein were used to prepare binary protein mixtures with defined concentration ratios. After polymerization, the mechanical properties of the resulting homopolymeric and copolymeric hydrogels were determined using rheological methods depending on the protein shares used. In additional uniaxial compression tests, the fracture strain was shown to be independent of the protein shares, while hydrogel toughness and compressive strength were increased for protein-based hydrogels containing casein.

Vat Photopolymerization 3D-Printing of Dynamic Thiol-Acrylate Photopolymers Using Bio-Derived Building Blocks

As an energy-efficient additive manufacturing process, vat photopolymerization 3D-printing has become a convenient technology to fabricate functional devices with high resolution and freedom in design. However, due to their permanently crosslinked network structure, photopolymers are not easily reprocessed or repaired. To improve the environmental footprint of 3D-printed objects, herein, we combine the dynamic nature of hydroxyl ester links, undergoing a catalyzed transesterification at elevated temperature, with an acrylate monomer derived from renewable resources. As a sustainable building block, we synthesized an acrylated linseed oil and mixed it with selected thiol crosslinkers. By careful selection of the transesterification catalyst, we obtained dynamic thiol-acrylate resins with a high cure rate and decent storage stability, which enabled the digital light processing (DLP) 3D-printing of objects with a structure size of 550 µm. Owing to their dynamic covalent bonds, the thiol-acrylate networks were able to relax 63% of their initial stress within 22 min at 180 °C and showed enhanced toughness after thermal annealing. We exploited the thermo-activated reflow of the dynamic networks to heal and re-shape the 3D-printed objects. The dynamic thiol-acrylate photopolymers also demonstrated promising healing, shape memory, and re-shaping properties, thus offering great potential for various industrial fields such as soft robotics and electronics.

A Simple Preparation Method of Gelatin Hydrogels Incorporating Cisplatin for Sustained Release

The objective of this study was to develop a new preparation method for cisplatin (CDDP)-incorporated gelatin hydrogels without using chemical crosslinking nor a vacuum heating instrument for dehydrothermal crosslinking. By simply mixing CDDP and gelatin, CDDP-crosslinked gelatin hydrogels (CCGH) were prepared. CDDP functions as a crosslinking agent of gelatin to form the gelatin hydrogel. Simultaneously, CDDP is incorporated into the gelatin hydrogel as a controlled release carrier. CDDP's in vitro and in vivo anticancer efficacy after incorporation into CCGH was evaluated. In the in vitro system, the CDDP was released gradually due to CCGH degradation with an initial burst release of approximately 16%. CDDP metal-coordinated with the degraded fragment of gelatin was released from CCGH with maintaining the anticancer activity. After intraperitoneal administration of CCGH, CDDP was detected in the blood circulation while its toxicity was low. Following intraperitoneal administration of CCGH in a murine peritoneal dissemination model of human gastric cancer MKN45-Luc cell line, the survival time was significantly prolonged compared with free CDDP solution. It is concluded that CCGH prepared by the CDDP-based crosslinking of gelatin is an excellent sustained release system of CDDP to achieve superior anticancer effects with minimal side effects compared with free CDDP solution.

Acetic Acid Enables Precise Tailoring of the Mechanical Behavior of Protein-Based Hydrogels

Engineering viscoelastic and biocompatible materials with tailored mechanical and microstructure properties capable of mimicking the biological stiffness (<17 kPa) or serving as bioimplants will bring protein-based hydrogels to the forefront in the biomaterials field. Here, we introduce a method that uses different concentrations of acetic acid (AA) to control the covalent tyrosine-tyrosine cross-linking interactions at the nanoscale level during protein-based hydrogel synthesis and manipulates their mechanical and microstructure properties without affecting protein concentration and (un)folding nanomechanics. We demonstrated this approach by adding AA as a precursor to the preparation buffer of a photoactivated protein-based hydrogel mixture. This strategy allowed us to synthesize hydrogels made from bovine serum albumin (BSA) and eight repeats protein L structure, with a fine-tailored wide range of stiffness (2-35 kPa). Together with protein engineering technologies, this method will open new routes in developing and investigating tunable protein-based hydrogels and extend their application toward new horizons.

Changing mechanical properties of photopolymerized, dityrosine-crosslinked protein-based hydrogels

Hydrogels based on renewable resources are a promising class of materials for future applications in pharmaceutics, drug delivery and personalized medicine. Thus, optional adjustments of mechanical properties such as swelling behavior, elasticity and network strength are desired. In this context, hydrogels based on the biological raw materials bovine serum albumin and casein were prepared by dityrosine-crosslinking of their tyrosine residues through visible light-induced photopolymerization. Changing the tyrosine accessibility by urea addition before photopolymerization increased the storage modulus of the hydrogels by 650% while simultaneously being more elastic. Furthermore, contributions of the buffer system composition, variation of protein concentration and storage medium towards mechanical properties of the hydrogel such as storage moduli, elasticity, fracture strain, compressive strength and relative weight swelling ratio are discussed. It could be shown, that changes in precursor solution and storage medium characteristics are crucial parameters towards tuning the mechanical properties of protein-based hydrogels.

Medical Adhesives and Their Role in Laparoscopic Surgery—A Review of Literature

Laparoscopic surgery is undergoing rapid development. Replacing the traditional method of joining cut tissues with sutures or staples could greatly simplify and speed up laparoscopic procedures. This alternative could undoubtedly be adhesives. For decades, scientists have been working on a material to bond tissues together to create the best possible conditions for tissue regeneration. The results of research on tissue adhesives achieved over the past years show comparable treatment effects to traditional methods. Tissue adhesives are a good alternative to surgical sutures in wound closure. This article is a review of the most important groups of tissue adhesives including their properties and possible applications. Recent reports on the development of biological adhesives are also discussed.

Natural Deep Eutectic Solvents Based on Choline Chloride and Phenolic Compounds as Efficient Bioadhesives and Corrosion Protectors

Natural deep eutectics solvents (NADES), owing to their high solvation capacity and nontoxicity, are actively being sought for many technological applications. Herein, we report a series of novel NADES based on choline chloride and plant-derived polyphenols. Most of the obtained phenolic NADES have a wide liquid range and high thermal stability above 150 °C. Among them, small-sized polyphenols, like pyrogallol, vanillyl alcohol, or gentisic acid, lead to low-viscosity liquids with ionic conductivities in the order of 10^-3 S cm^-1 at room temperature. Interestingly, polyphenols possess valuable properties as therapeutic agents, antioxidants, adhesives, or redox-active compounds, among others. Thus, we evaluated the potential of these novel NADES for two applications: bioadhesives and corrosion protection. The mixture of choline chloride-vanillyl alcohol (2:3 mol ratio) and gelatin resulted in a highly adhesive viscoelastic liquid (adhesive stress ≈ 135 kPa), affording shear thinning behavior. Furthermore, choline chloride-tannic acid (20:1) showed an extraordinary ability to coordinate iron ions, reaching excellent corrosion inhibitive efficiencies in mild steel protection.

A Review of Multi-Material 3D Printing of Functional Materials via Vat Photopolymerization

Additive manufacturing or 3D printing of materials is a prominent process technology which involves the fabrication of materials layer-by-layer or point-by-point in a subsequent manner. With recent advancements in additive manufacturing, the technology has excited a great potential for extension of simple designs to complex multi-material geometries. Vat photopolymerization is a subdivision of additive manufacturing which possesses many attractive features, including excellent printing resolution, high dimensional accuracy, low-cost manufacturing, and the ability to spatially control the material properties. However, the technology is currently limited by design strategies, material chemistries, and equipment limitations. This review aims to provide readers with a comprehensive comparison of different additive manufacturing technologies along with detailed knowledge on advances in multi-material vat photopolymerization technologies. Furthermore, we describe popular material chemistries both from the past and more recently, along with future prospects to address the material-related limitations of vat photopolymerization. Examples of the impressive multi-material capabilities inspired by nature which are applicable today in multiple areas of life are briefly presented in the applications section. Finally, we describe our point of view on the future prospects of 3D printed multi-material structures as well as on the way forward towards promising further advancements in vat photopolymerization.

3D Fiber Reinforced Hydrogel Scaffolds by Melt Electrowriting and Gel Casting as a Hybrid Design for Wound Healing

Emerging biomanufacturing technologies have revolutionized the field of tissue engineering by offering unprecedented possibilities. Over the past few years, new opportunities arose by combining traditional and novel fabrication techniques, shaping the hybrid designs in biofabrication. One of the potential application fields is skin tissue engineering, in which a combination of traditional principles of wound dressing with advanced biofabrication methods could yield more efficient therapies. In this study, a hybrid design of fiber-reinforced scaffolds combined with gel casting is developed and the efficiency for in vivo wound healing applications is assessed. For this purpose, 3D fiber meshes produced by melt electrowriting are selectively filled with photocrosslinkable gelatin hydrogel matrices loaded with different growth factor carrier microspheres. Additionally, the influence of the inclusion of inorganic bioactive glass particles within the composite fibrous mesh is evaluated. Qualitative evaluation of secondary wound healing criteria and histological analysis shows that hybrid scaffolds containing growth factors and bioactive glass enhances the healing process significantly, compared to the designs merely providing a fiber-reinforced bioactive hydrogel matrix as the wound dressing. This study aims to explore a new application area for melt electrowriting as a powerful tool in fabricating hybrid therapeutic designs for skin tissue engineering.

Bioinspired Super-Strong Aqueous Synthetic Tissue Adhesives

Existing tissue adhesives and sealants are far from satisfactory when applied on wet and dynamic tissues. Herein, we report a strategy for designing biodegradable super-strong aqueous glue (B-Seal) for surgical uses inspired by an English ivy adhesion strategy and a cement particle packing theory. B-Seal is a fast-gelling, super-strong, and elastic adhesive sealant composed of injectable water-borne biodegradable polyurethane (WPU) nanodispersions with mismatched particle sizes and counterions in its A-B formulation. B-Seal showed 24-fold greater burst pressure than DuraSeal®, 138-fold greater T-pull adhesive strength than fibrin glue, and 16-fold greater lap shear strength than fibrin glue. In vivo evaluation on a rat cerebrospinal fluid (CSF) rhinorrhea model and a porcine craniotomy model validated the safety and efficacy of B-Seal for effective CSF leak prevention and dura repair. The plant-inspired adhesion strategy combined with particle packing theory represents a new direction of designing the next-generation wet tissue adhesives for surgeries.

Stereolithographic Visible-Light Printing of Poly(l-glutamic acid) Hydrogel Scaffolds

Bioprinting is a promising fabrication technique aimed at developing biologically functional, tissue-like constructs for various biomedical applications. Among the different bioprinting approaches, vat polymerization-based techniques offer the highest feature resolution compared to more commonly used extrusion-based methods and therefore have greater potential to be utilized for printing complex hierarchical tissue architectures. Although significant efforts have been directed toward harnessing digital light processing techniques for high-resolution bioprinting, the use of stereolithography (SLA) setups for producing distinct hydrogel filaments smaller than 20 μm has received less attention. Improving the bioprinting resolution is still a technical challenge that must consider both the practical limitations of the bioprinter apparatus and the formulation of the cytocompatible bioresin. In this study, we developed a novel bioresin compatible with SLA and capable of printing high-resolution features. This resin, composed of a biosynthetic polypeptide poly(l-glutamic acid) functionalized with tyramine moieties (PLGA-Tyr), was crosslinked using a visible-light photoinitiator system. Varying concentrations of PLGA-Tyr and the co-photoinitiator were evaluated for the hydrogel system's gelation ability, swelling characteristics, degradation profiles, mechanical properties, and cell viability post-encapsulation. This study introduces a custom-built, cost-effective, visible-light SLA bioprinting system named the "MicroNC". Using the newly developed visible-light bioresin, we demonstrated for the first time the ability to fabricate hydrogel scaffolds with well-resolved filaments (less than 8 μm in width) capable of supporting cell viability and proliferation and directing cellular morphology at the single-cell level for up to 14 days. Overall, these experiments have underscored the exciting potential of using the visible-light-photoinitiated PLGA-Tyr material system for developing physiologically relevant in vitro hydrogel scaffolds with feature resolutions comparable to the dimensions of individual human cells for a wide range of biomedical applications.

Elucidating the degradation mechanism of a self-degradable dextran-based medical adhesive

We developed a self-degradable medical adhesive, LYDEX, consisting of periodate-oxidized aldehyde-functionalized dextran (AD) and succinic anhydride-treated ε-poly-l-lysine (SAPL). After gelation and adhesion of LYDEX by Schiff base bond formation between the AD aldehyde groups and SAPL amino groups, molecular degradation associated with the Maillard reaction is initiated, but the detailed degradation mechanism remains unknown. Herein, we elucidated the degradation mechanism of LYDEX by analyzing the main degradation products under typical solution conditions in vitro. The degradation of the LYDEX gel with a sodium periodate/dextran content of 2.5/20 was observed using gel permeation chromatography and infrared and ¹H NMR spectroscopy. The AD ratio in the AD-SAPL mixture increased as the molecular weight decreased with the degradation time. This discovery of LYDEX self-degradability is useful for clarifying other polysaccharide hydrogel degradation mechanisms, and valuable for the use of LYDEX in medical applications, such as hemostatic or sealant materials.

Multifunctional GelMA platforms with nanomaterials for advanced tissue therapeutics

Polymeric hydrogels are fascinating platforms as 3D scaffolds for tissue repair and delivery systems of therapeutic molecules and cells. Among others, methacrylated gelatin (GelMA) has become a representative hydrogel formulation, finding various biomedical applications. Recent efforts on GelMA-based hydrogels have been devoted to combining them with bioactive and functional nanomaterials, aiming to provide enhanced physicochemical and biological properties to GelMA. The benefits of this approach are multiple: i) reinforcing mechanical properties, ii) modulating viscoelastic property to allow 3D printability of bio-inks, iii) rendering electrical/magnetic property to produce electro-/magneto-active hydrogels for the repair of specific tissues (e.g., muscle, nerve), iv) providing stimuli-responsiveness to actively deliver therapeutic molecules, and v) endowing therapeutic capacity in tissue repair process (e.g., antioxidant effects). The nanomaterial-combined GelMA systems have shown significantly enhanced and extraordinary behaviors in various tissues (bone, skin, cardiac, and nerve) that are rarely observable with GelMA. Here we systematically review these recent efforts in nanomaterials-combined GelMA hydrogels that are considered as next-generation multifunctional platforms for tissue therapeutics. The approaches used in GelMA can also apply to other existing polymeric hydrogel systems.

Photocurable GelMA Adhesives for Corneal Perforations

The current treatments for the management of corneal and scleral perforations include sutures and adhesives. While sutures are invasive, induce astigmatism and carry a risk of infection, cyanoacrylate glues are toxic, proinflammatory and form an opaque and rough surface that precludes vision. Consequently, the clinical need for a fast curing and strong tissue adhesive with minimised cytotoxicity and host inflammation remains unmet. In this paper, we engineer a gelatine methacryloyl (GelMA) adhesive that can be crosslinked in situ within 2 min using UV or visible light and a riboflavin (RF)/sodium persulfate (SPS) system. Optical coherence tomography (OCT) images demonstrated that the flowable GelMA adhesive could completely fill corneal wounds and restore the ocular curvature by forming a smooth contour on the ocular surface. Further, ex vivo studies in porcine eyes showed that GelMA bioadhesives exhibited burst pressures that were comparable to cyanoacrylates (49 ± 9 kPa), with the hydrogels exhibiting a transmittance (90%), water content (85%) and storage modulus (5 kPa) similar to the human cornea. Finally, using human dermal fibroblasts, we showed that our GelMA adhesive was non-toxic and could effectively support cell adhesion and proliferation. Taken together, the adhesive’s performance, injectability and ease of administration, together with gelatin’s availability and cost-effectiveness, make it a potential stromal filler or sealant for corneal and conjunctival applications.

Engineering a naturally derived hemostatic sealant for sealing internal organs

Controlling bleeding from a raptured tissue, especially during the surgeries, is essentially important. Particularly for soft and dynamic internal organs where use of sutures, staples, or wires is limited, treatments with hemostatic adhesives have proven to be beneficial. However, major drawbacks with clinically used hemostats include lack of adhesion to wet tissue and poor mechanics. In view of these, herein, we engineered a double-crosslinked sealant which showed excellent hemostasis (comparable to existing commercial hemostat) without compromising its wet tissue adhesion. Mechanistically, the engineered hydrogel controlled the bleeding through its wound-sealing capability and inherent chemical activity. This mussel-inspired hemostatic adhesive hydrogel, named gelatin methacryloyl-catechol (GelMAC), contained covalently functionalized catechol and methacrylate moieties and showed excellent biocompatibility both in vitro and in vivo. Hemostatic property of GelMAC hydrogel was initially demonstrated with an in vitro blood clotting assay, which showed significantly reduced clotting time compared to the clinically used hemostat, Surgicel®. This was further assessed with an in vivo liver bleeding test in rats where GelMAC hydrogel closed the incision rapidly and initiated blood coagulation even faster than Surgicel®. The engineered GelMAC hydrogel-based seaalant with excellent hemostatic property and tissue adhesion can be utilized for controlling bleeding and sealing of soft internal organs.

Emerging bioadhesives: from traditional bioactive and bioinert to a new biomimetic protein-based approach

Bioadhesives have reached significant milestones over the past two decades. Research has shown not only to produce adhesives capable of adhering to dry tissue but recently wet tissue as well. However, most bioadhesives developed have exhibited high adhesion strength yet lack other properties required for versatility in application, such as elasticity, biocompatibility and biodegradability. Adapting from limitations met from early bioadhesives and meeting the current demand allows novel bioadhesives to reach new milestones for the future. In this review, we overview the progression and variations of bioadhesives, current trends, characterisation techniques and conclude with future perspectives for bioadhesives for tissue engineering applications.

Applications of Bioadhesives: A Mini Review

Bioadhesives have demonstrated their superiority in clinical applications as tissue adhesives, hemostats, and tissue sealants. Because of the intrinsic stickiness, the applications have been expanded to various areas, such as functional wound dressing, factor delivery vehicles, and even medical device fixation. While many literature works discussed the mechanism of bioadhesives, few of them specifically summarized the applications of bioadhesives. To fill in the blanks, this review covers recent research articles and focuses precisely on the applications of bioadhesives which can be generally classified as follows: 1) wound closure, 2) sealing leakage, and 3) immobilization, including those already in the clinic and those showing great potential in the clinic. It is expected that this article will provide a whole picture on bioadhesives' applications and lead to innovations in the application of bioadhesives in new fields.

Rational engineering and applications of functional bioadhesives in biomedical engineering

For the past decades, several bioadhesives have been developed to replace conventional wound closure medical tools such as sutures, staples, and clips. The bioadhesives are easy to use and can minimize tissue damage. They are designed to provide strong adhesion with stable mechanical support on tissue surfaces. However, this monofunctionality of the bioadhesives hinders their practical applications. In particular, a bioadhesive can lose its intended function under harsh tissue environments or delay tissue regeneration during wound healing. Based on several natural and synthetic biomaterials, functional bioadhesives have been developed to overcome the aforementioned limitations. The functional bioadhesives are designed to have specific characteristics such as antimicrobial, cell infiltrative, stimuli-responsive, electrically conductive, and self-healing to ensure stability under harsh tissue conditions, facilitate tissue regeneration, and effectively monitor biosignals. Herein, we thoroughly review the functional bioadhesives from their fundamental background to recent progress with their practical applications for the enhancement of tissue healing and effective biosignal sensing. Furthermore, the future perspectives on the applications of functional bioadhesives and current challenges in their commercialization are also discussed.

Bidirectional differentiation of BMSCs induced by a biomimetic procallus based on a gelatin-reduced graphene oxide reinforced hydrogel for rapid bone regeneration

Developmental engineering strategy needs the biomimetic composites that can integrate the progenitor cells, biomaterial matrices and bioactive signals to mimic the natural bone healing process for faster healing and reconstruction of segmental bone defects. We prepared the gelatin-reduced graphene oxide (GOG) and constructed the composites that mimicked the procallus by combining the GOG with the photo-crosslinked gelatin hydrogel. The biological effects of the GOG-reinforced composites could induce the bi-differentiation of bone marrow stromal cells (BMSCs) for rapid bone repair. The proper ratio of GOG in the composites regulated the composites' mechanical properties to a suitable range for the adhesion and proliferation of BMSCs. Besides, the GOG-mediated bidirectional differentiation of BMSCs, including osteogenesis and angiogenesis, could be activated through Erk1/2 and AKT pathway. The methyl vanillate (MV) delivered by GOG also contributed to the bioactive signals of the biomimetic procallus through priming the osteogenesis of BMSCs. During the repair of the calvarial defect in vivo, the initial hypoxic condition due to GOG in the composites gradually transformed into a well-vasculature robust situation with the bi-differentiation of BMSCs, which mimicked the process of bone healing resulting in the rapid bone regeneration. As an inorganic constituent, GOG reinforced the organic photo-crosslinked gelatin hydrogel to form a double-phase biomimetic procallus, which provided the porous extracellular matrix microenvironment and bioactive signals for the bi-directional differentiation of BMSCs. These show a promised application of the bio-reduced graphene oxide in biomedicine with a developmental engineering strategy.

Sutureless Transplantation of Amniotic Membrane Using a Visible Light-Curable Protein Bioadhesive for Ocular Surface Reconstruction

The conjunctiva is a thin mucous membrane of the eye. Pterygium, a commonly appearing disease on the ocular surface, requires surgery to excise the conjunctiva to prevent visual deterioration. Recently, transplantation of the amniotic membrane (AM), which is the innermost membrane of the placenta, has been highlighted as an efficient method to cure conjunctiva defects because of its advantages of no side effects compared to mitomycin C treatment and not leaving additional scars on donor site compared to conjunctival autografting. However, to minimize additional damage to the ocular surface by suturing, AM transplantation (AMT) needs to be simplified by using a less invasive, time-saving method. In this work, a visible light-curable protein bioadhesive (named FixLight) for efficient sutureless AMT is applied. FixLight, which is based on bioengineered mussel adhesive protein (MAP), is easily applied between damaged ocular surfaces and transplanted AM, and rapidly cured by harmless blue light activation. Through in vivo evaluation using a rabbit model, the authors demonstrated that FixLight enabled facile, fast, and strong attachment of AM on sclera and promoted ocular surface reconstruction with good biocompatibility. Thus, FixLight can be successfully used as a promising clinical bioadhesive in opthalmological surgeries that require sutureless and rapid operation.

Multifaceted Design and Emerging Applications of Tissue Adhesives

Tissue adhesives can form appreciable adhesion with tissues and have found clinical use in a variety of medical settings such as wound closure, surgical sealants, regenerative medicine, and device attachment. The advantages of tissue adhesives include ease of implementation, rapid application, mitigation of tissue damage, and compatibility with minimally invasive procedures. The field of tissue adhesives is rapidly evolving, leading to tissue adhesives with superior mechanical properties and advanced functionality. Such adhesives enable new applications ranging from mobile health to cancer treatment. To provide guidelines for the rational design of tissue adhesives, here, existing strategies for tissue adhesives are synthesized into a multifaceted design, which comprises three design elements: the tissue, the adhesive surface, and the adhesive matrix. The mechanical, chemical, and biological considerations associated with each design element are reviewed. Throughout the report, the limitations of existing tissue adhesives and immediate opportunities for improvement are discussed. The recent progress of tissue adhesives in topical and implantable applications is highlighted, and then future directions toward next-generation tissue adhesives are outlined. The development of tissue adhesives will fuse disciplines and make broad impacts in engineering and medicine.

A highly efficient microwave-assisted synthesis of an LED-curable methacrylated gelatin for bio applications

This study deals with the development of an LED-curable methacrylated gelatin (GelMA) synthesis via microwave (MW) irradiation with a reaction and purification time-, energy-, and methacrylation reagent-saving approach. To investigate the efficiency of MW irradiation in GelMA synthesis, characteristics of the GelMAs prepared by using glycidyl methacrylate (GMA) or methacrylic anhydride (MA) via the MW-assisted (MWA) method were compared comprehensively with those synthesized via the conventional heating method. Moreover, MWA reaction conditions were optimized in terms of methacrylation reagent concentrations (C), reaction time (t), and MW power (P). Characterization and assessment of the GelMAs were conducted with 1H NMR, FT-IR, and Raman spectroscopy along with physical-mechanical, thermal, and hydrophilicity analysis. The results demonstrated that the MWA synthesized GMA-GelMA hydrogels were possessed of increased methacrylation degree (MD), gel fraction (GF), tensile strength (TS), elongation at break (EB), glass transition temperature (T g), and water contact angle (WCA) as well as decreased swelling degree (SD) values in comparison to those of MA-GelMA and GMA-GelMA hydrogels prepared via the MWA and conventional method, respectively. Enhanced properties of the MWA synthesized GMA-hydrogels suggested an effective methacryloyl conjugation leading to a greater amount of covalent crosslinking density justified by the dipolar moment calculations. Optimal GMA C, t, P, and purification time for a highly crosslinked GelMA hydrogel (MD: 96.1%, GF: 98.3%, SD: 10.11%, TS: 6.7 MPa, EB: 175.2%, T g: 75.34 °C, and WCA: 72.22°) were found to be a 5 times molar excess over the primary amine groups of gelatin, 5 min, 500 W, and 24 h, respectively. Thus, the optimized MW conditions offer a promising green method to prepare GelMAs for bio applications.

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.

Recent Advances in 3D Printing with Protein-Based Inks

Three-dimensional (3D) printing is a transformative manufacturing strategy, allowing rapid prototyping, customization, and flexible manipulation of structure-property relationships. Proteins are particularly appealing to formulate inks for 3D printing as they serve as essential structural components of living systems, provide a support presence in and around cells and for tissue functions, and also provide the basis for many essential ex vivo secreted structures in nature. Protein-based inks are beneficial in vivo due to their mechanics, chemical and physical match to the specific tissue, and full degradability, while also to promoting implant-host integration and serving as an interface between technology and biology. Exploiting the biological, chemical, and physical features of protein-based inks can provide key opportunities to meet the needs of tissue engineering and regenerative medicine. Despite these benefits, protein-based inks impose nontrivial challenges to 3D printing such as concentration and rheological features and reconstitution of the structural hierarchy observed in nature that is a source of the robust mechanics and functions of these materials. This review introduces photo-crosslinking mechanisms and rheological principles that underpins a variety of 3D printing techniques. The review also highlights recent advances in the design, development, and biomedical utility of monolithic and composite inks from a range of proteins, including collagen, silk, fibrinogen, and others. One particular focus throughout the review is to introduce unique material characteristics of proteins, including amino acid sequences, molecular assembly, and secondary conformations, which are useful for designing printing inks and for controlling the printed structures. Future perspectives of 3D printing with protein-based inks are also provided to support the promising spectrum of biomedical research accessible to these materials.

Rapid and cytocompatible cell-laden silk hydrogel formation via riboflavin-mediated crosslinking

Bioactive hydrogels based on naturally-derived polymers are of great interest for regenerative medicine applications. Among naturally-derived polymers, silk fibroin has been extensively explored as a biomaterial for tissue engineering due to its unique mechanical properties. Here, we demonstrate the rapid gelation of cell-laden silk fibroin hydrogels by visible light-induced crosslinking using riboflavin as a photo-initiator, in presence of an electron acceptor. The gelation kinetics were monitored by in situ photo-rheometry. Gelation was achieved in minutes and could be tuned owing to its direct proportionality to the electron acceptor concentration. The concentration of the electron acceptor did not affect the elastic modulus of the hydrogels, which could be altered by varying the polymer content. Further, the biocompatible riboflavin photo-initiator combined with sodium persulfate allowed for the encapsulation of cells within silk fibroin hydrogels. To confirm the cytocompatibility of the silk fibroin formulations, three cell types (articular cartilage-derived progenitor cells, mesenchymal stem cells and dental-pulp-derived stem cells) were encapsulated within the hydrogels, which associated with a viability >80% for all cell types. These results demonstrated that fast gelation of silk fibroin can be achieved by combining it with riboflavin and electron acceptors, which results in a hydrogel that can be used in tissue engineering and cell delivery applications.

An injectable, self-healing phenol-functionalized chitosan hydrogel with fast gelling property and visible light-crosslinking capability for 3D printing

Self-healing hydrogels attract broad attention as cell/drug carriers for direct injection into damaged tissues or as bioinks for three-dimensional (3D) printing of tissue-like constructs. For application in 3D printing, the self-healing hydrogels should maintain the steady rheological properties during printing process, and be further stabilized by secondary post-printing crosslinking. Here, a chitosan self-healing hydrogel is developed for injectable hydrogel and printable ink using phenol-functionalized chitosan and dibenzaldehyde-terminated telechelic poly(ethylene glycol). Phenol functionalization of chitosan can introduce unique interaction that allows the hydrogel to possess fast gelling rate, good self-healing ability, and long-range critical gel behavior, as well as secondary visible light-crosslinking capability. The hydrogel is easily pre-formed in a syringe and extruded through a 26-gauge needle to produce a continuous and stackable filament. The cell-laden hydrogel is successfully printed into a 3D construct. Moreover, the hydrogel is developed for modular 3D printing, where hydrogel modules (LEGO-like building blocks) are individually printed and assembled into an integrated construct followed by secondary visible light-crosslinking. The versatile phenol-functionalized chitosan self-healing hydrogel will open up numerous potential applications, particularly in 3D bioprinting and modular 3D bioprinting.

A simple, green chemistry technology for fabrication of tissue-engineered scaffolds based on mussel-inspired 3D centrifugal spun

The fabrication of 3D fibrous scaffolds with highly interconnected pores has been crucial in the development of tissue regeneration techniques. The present study describes the fabrication of 3D fibrous scaffolds by freeze-drying of polydopamine (PDA) coated centrifugal spun gelatin fibers. We wanted to combine the mussel-inspired chemistry, Maillard reaction, and the 3D microstructural advantages of centrifugal spun fibers to develop the green fibrous scaffolds at low cost, high speed, and desired mold shape. The resultant PDA-gelatin fibers exhibited a smooth 3D microstructure with a uniform formation of PDA thin ad-layer that enhanced the mechanical properties and stability of the scaffolds, and thereby decreased the degradation rate. All scaffolds showed promising properties including good dimensional and mechanical stability under wet state, optimal porosity over 94%, and high water uptake of approximately 1500%. The results of cell culture studies, further confirmed that all scaffolds exhibited appropriate biocompatibility, cell proliferation, migration, and infiltration. Particularly, the PDA-coated scaffolds showed a significant enhancement in proliferation, migration, and infiltration of HDF-GFP⁺ cells. These results show that a 3D porous fibrous scaffold with simplifying tunable density and desirable shape on a large scale can be readily prepared for different fields of tissue engineering applications.

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.

Enlisting a Traditional Chinese Medicine to tune the gelation kinetics of a bioactive tissue adhesive for fast hemostasis or minimally invasive therapy

Gelation kinetics is important in tailoring chemically crosslinked hydrogel-based injectable adhesives for different applications. However, the regulation of gelation rate is usually limited to varying the gel precursor and/or crosslinker concentration, which cannot reach a fine level and inevitably alters the physical properties of hydrogels. Amidation reactions are widely used to synthesize hydrogel adhesives. In this work, we propose a traditional Chinese medicine (Borax)-input strategy to tune the gelation rate of amidation reaction triggered systems. Borax provides an initial basic buffer environment to promote the deprotonation process of amino groups and accelerate this reaction. By using a tissue adhesive model PEG-lysozyme (PEG-LZM), the gelation time can be modulated from seconds to minutes with varying Borax concentrations, while the physical properties remain constant. Moreover, the antibacterial ability can be improved due to the bioactivity of Borax. The hydrogel precursors can be regulated to solidify instantly to close the bleeding wound at emergency. Meanwhile, they can also be customized to match the flowing time in the catheter, thereby facilitating minimally invasive tissue sealing. Because this method is easily operated, we envision Borax adjusted amidation-type hydrogel has a promising prospect in clinical application.

One-Step Photoactivation of a Dual-Functionalized Bioink as Cell Carrier and Cartilage-Binding Glue for Chondral Regeneration

Cartilage defects can result in pain, disability, and osteoarthritis. Hydrogels providing a chondroregeneration-permissive environment are often mechanically weak and display poor lateral integration into the surrounding cartilage. This study develops a visible-light responsive gelatin ink with enhanced interactions with the native tissue, and potential for intraoperative bioprinting. A dual-functionalized tyramine and methacryloyl gelatin (GelMA-Tyr) is synthesized. Photo-crosslinking of both groups is triggered in a single photoexposure by cell-compatible visible light in presence of tris(2,2'-bipyridyl)dichlororuthenium(II) and sodium persulfate as initiators. Neo-cartilage formation from embedded chondroprogenitor cells is demonstrated in vitro, and the hydrogel is successfully applied as bioink for extrusion-printing. Visible light in situ crosslinking in cartilage defects results in no damage to the surrounding tissue, in contrast to the native chondrocyte death caused by UV light (365-400 nm range), commonly used in biofabrication. Tyramine-binding to proteins in native cartilage leads to a 15-fold increment in the adhesive strength of the bioglue compared to pristine GelMA. Enhanced adhesion is observed also when the ink is extruded as printable filaments into the defect. Visible-light reactive GelMA-Tyr bioinks can act as orthobiologic carriers for in situ cartilage repair, providing a permissive environment for chondrogenesis, and establishing safe lateral integration into chondral defects.

Gelatin Methacryloyl Bioadhesive Improves Survival and Reduces Scar Burden in a Mouse Model of Myocardial Infarction

Background Delivery of hydrogels to the heart is a promising strategy for mitigating the detrimental impact of myocardial infarction (MI). Challenges associated with the in vivo delivery of currently available hydrogels have limited clinical translation of this technology. Gelatin methacryloyl (GelMA) bioadhesive hydrogel could address many of the limitations of available hydrogels. The goal of this proof-of-concept study was to evaluate the cardioprotective potential of GelMA in a mouse model of MI. Methods and Results The physical properties of GelMA bioadhesive hydrogel were optimized in vitro. Impact of GelMA bioadhesive hydrogel on post-MI recovery was then assessed in vivo. In 20 mice, GelMA bioadhesive hydrogel was applied to the epicardial surface of the heart at the time of experimental MI. An additional 20 mice underwent MI but received no GelMA bioadhesive hydrogel. Survival rates were compared for GelMA-treated and untreated mice. Left ventricular function was assessed 3 weeks after experimental MI with transthoracic echocardiography. Left ventricular scar burden was measured with postmortem morphometric analysis. Survival rates at 3 weeks post-MI were 89% for GelMA-treated mice and 50% for untreated mice (P=0.011). Left ventricular contractile function was better in GelMA-treated than untreated mice (fractional shortening 37% versus 26%, P<0.001). Average scar burden in GelMA-treated mice was lower than in untreated mice (6% versus 22%, P=0.017). Conclusions Epicardial GelMA bioadhesive application at the time of experimental MI was performed safely and was associated with significantly improved post-MI survival compared with control animals. In addition, GelMA treatment was associated with significantly better preservation of left ventricular function and reduced scar burden.

Gelatin Hydrogels Reinforced by Absorbable Nanoparticles and Fibrils Cured In Situ by Visible Light for Tissue Adhesive Applications

Most gelatin hydrogels used in regenerative medicine applications today are fabricated by photocrosslinking due to the convenience and speed of this method. However, in most cases photoinitiators are used, which require UV light, which, in turn, can cause cell and tissue damage, or using functionalized gelatin. Recently, ruthenium (II) tris-bipyridyl chloride has been studied as an initiator that can induce dityrosine bond formation using visible light. In addition, continuous fibrils and small particles are often used to reinforce composite materials. Therefore, this study investigated the visible-light-induced photocrosslinking of native gelatin molecules via dityrosine bonds formation as well as gel reinforcement by collagen fibrils and mesoporous bioactive glass (MBG) particles. The results show that collagen and MBG exerted a synergistic effect on maintaining gel integrity with a dental LED curing light when the irradiation time was shortened to 30 s. Without the two reinforcing components, the gel could not form a geometric shape stable gel even when the exposure time was 120 s. The shear strength increased by 62% with the collagen and MBG compared with the blank control. Furthermore, our results demonstrate that the addition of collagen and MBG enhanced gel stability in an artificial saliva solution. These results demonstrate the considerable advantages of using tyrosine-containing biomolecules, and using a dental LED curing light for the crosslinking of hydrogels in terms of their suitability and feasibility for use as bioadhesives in confined clinical working space, such as the oral cavity, and in application as in situ-crosslinked injectable hydrogels.

Advances in Photoreactive Tissue Adhesives Derived from Natural Polymers

To stop blood loss and accelerate wound healing, conventional wound closure techniques such as sutures and staples are currently used in the clinic. These tissue-piercing wound closure techniques have several disadvantages such as the potential for causing inflammation, infections, and scar formation. Surgical sealants and tissue adhesives can address some of the disadvantages of current sutures and staples. An ideal tissue adhesive will demonstrate strong interfacial adhesion and cohesive strength to wet tissue surfaces. Most reported studies rely on the liquid-to-solid transition of organic molecules by taking advantage of polymerization and crosslinking reactions for improving the cohesive strength of the adhesives. Crosslinking reactions triggered using light are commonly used for increasing tissue adhesive strength since the reactions can be controlled spatially and temporally, providing the on-demand curing of the adhesives with minimum misplacements. In this review, we describe the recent advances in the field of naturally derived tissue adhesives and sealants in which the adhesive and cohesive strengths are modulated using photochemical reactions.

Recent Advances in Tissue Adhesives for Clinical Medicine

Tissue adhesives have attracted more attention to the applications of non-invasive wound closure. The purpose of this review article is to summarize the recent progress of developing tissue adhesives, which may inspire researchers to develop more outstanding tissue adhesives. It begins with a brief introduction to the emerging potential use of tissue adhesives in the clinic. Next, several critical mechanisms for adhesion are discussed, including van der Waals forces, capillary forces, hydrogen bonding, static electric forces, and chemical bonds. This article further details the measurement methods of adhesion and highlights the different types of adhesive, including natural or biological, synthetic and semisynthetic, and biomimetic adhesives. Finally, this review article concludes with remarks on the challenges and future directions for design, fabrication, and application of tissue adhesives in the clinic. This review article has promising potential to provide novel creative design principles for the generation of future tissue adhesives.

Non-cytotoxic Dityrosine Photocrosslinked Polymeric Materials With Targeted Elastic Moduli

Controlling mechanical properties of polymeric biomaterials, including the elastic modulus, is critical to direct cell behavior, such as proliferation and differentiation. Dityrosine photocrosslinking is an attractive and simple method to prepare materials that exhibit a wide range of elastic moduli by rapidly crosslinking tyrosyl-containing polymers. However, high concentrations of commonly used oxidative crosslinking reagents, such as ruthenium-based photoinitiators and persulfates, present cytotoxicity concerns. We found the elastic moduli of materials prepared by crosslinking an artificial protein with tightly controlled tyrosine molarity can be modulated up to 40 kPa by adjusting photoinitiator and persulfate concentrations. Formulations with various concentrations of the crosslinking reagents were able to target a similar material elastic modulus, but excess unreacted persulfate resulted in cytotoxic materials. Therefore, we identified a systematic method to prepare non-cytotoxic photocrosslinked polymeric materials with targeted elastic moduli for potential biomaterials applications in diverse fields, including tissue engineering and 3D bioprinting.

Thermal control of SZ2080 photopolymerization in four-beam interference lithography

Photopolymerization by four-beam interference lithography on a preheated SZ2080 sample was explored at different initial temperatures of the sample: 20 °C, 50 °C, 75 °C, 100 °C, 125 °C, and 150 °C, and at exposure times ranging from 0.5 s to 5 s. The average laser power selected was ∼100 mW for the 300 ps duration pulses at a 1 kHz repetition rate. The experimental results demonstrate that the higher initial temperature of the sample positively influences the crosslinking of the patterns. These findings will improve polymerization protocols for multi-beam interference lithography.