Photo Processing for Biomedical Hydrogels Design and Functionality: A Review

Antioxidant and antibacterial hydrogel formed by protocatechualdehyde-ferric iron complex and aminopolysaccharide for infected wound healing

To better treat bacteria-infected wounds and promote healing, new wound dressings must be developed. In this study, we obtained PA@Fe by chelating iron trivalent ions (Fe3+) with protocatechualdehyde (PA), which has a catechol structure. Subsequently, we reacted it with ethylene glycol chitosan (GC) via a Schiff base reaction and loaded vancomycin to obtain an antibacterial Gel@Van hydrogel with a photothermal response. The as-prepared Gel@Van hydrogel exhibited good injectability, self-healing, hemostasis, photothermal stability, biocompatibility, and antioxidant and antibacterial properties. Moreover, Gel@Van hydrogel achieved highly synergistic antibacterial efficacy through photothermal and antibiotic sterilization. In a mouse skin-damaged infection model, Gel@Van hydrogel had a strong ability to promote the healing of methicillin-resistant Staphylococcus aureus (MRSA)-infected wounds, indicating the great potential application value of Gel@Van hydrogel in the field of treating and promoting the healing of infected wounds.

3D Printing of Self-Healing Materials

This review article presents a comprehensive overview of the latest advances in the field of 3D printable structures with self-healing properties. Three-dimensional printing (3DP) is a versatile technology that enables the rapid manufacturing of complex geometric structures with precision and functionality not previously attainable. However, the application of 3DP technology is still limited by the availability of materials with customizable properties specifically designed for additive manufacturing. The addition of self-healing properties within 3D printed objects is of high interest as it can improve the performance and lifespan of structural components, and even enable the mimicking of living tissues for biomedical applications, such as organs printing. The review will discuss and analyze the most relevant results reported in recent years in the development of self-healing polymeric materials that can be processed via 3D printing. After introducing the chemical and physical self-healing mechanism that can be exploited, the literature review here reported will focus in particular on printability and repairing performances. At last, actual perspective and possible development field will be critically discussed.

Biphasic release of betamethasone from an injectable HA hydrogel implant for alleviating lumbar disc herniation induced sciatica

Epidural steroid injection (ESI) is a common therapeutic approach for managing sciatica caused by lumbar disc herniation (LDH). However, the short duration of therapeutic efficacy and the need for repeated injections pose challenges in LDH treatment. The development of a controlled delivery system capable of prolonging the effectiveness of ESI and reducing the frequency of injections, is highly significant in LDH clinical practice. In this study, we utilized a thiol-ene click chemistry to create a series of injectable hyaluronic acid (HA) based release systems loaded with diphasic betamethasone, including betamethasone dipropionate (BD) and betamethasone 21-phosphate disodium (BP) (BD/BP@HA). BD/BP@HA hydrogel implants demonstrated biocompatibility and biodegradability to matched neuronal tissues, avoiding artificial compression following injection. The sustained release of betamethasone from BD/BP@HA hydrogels effectively inhibited both acute and chronic neuroinflammation by suppressing the nuclear factor kappa-B (NF-κB) pathway. In a mouse model of LDH, the epidural administration of BD/BP@HA efficiently alleviated LDH-induced sciatica for at least 10 days by inhibiting the activation of macrophages and microglia in dorsal root ganglion and spinal dorsal horn, respectively. The newly developed HA hydrogels represent a valuable platform for achieving sustained drug release. Additionally, we provide a simple paradigm for fabricating BD/BP@HA for epidural injection, demonstrating greater and sustained efficiency in alleviating LDH-induced sciatica compared to traditional ESI and displaying potentials for clinical translation. This system has the potential to revolutionize drug delivery for co-delivery of both soluble and insoluble drugs, thereby making a significant impact in the pharmaceutical industry. STATEMENT OF SIGNIFICANCE: Lumbar disc herniation (LDH) is a common degenerative disorder leading to sciatica and spine surgery. Although epidural steroid injection (ESI) is routinely used to alleviate sciatica, the efficacy is short and repeated injections are required. There remains challenging to prolong the efficacy of ESI. Herein, an injectable hyaluronic acid (HA) hydrogel implant by crosslinking acrylated-modified HA (HA-A) with thiol-modified HA (HA-SH) was designed to achieve a biphasic release of betamethasone. The hydrogel showed biocompatibility and biodegradability to match neuronal tissues. Notably, compared to traditional ESI, the hydrogel better alleviated sciatica in vivo by synergistically inhibiting the neuroinflammation in central and peripheral nervous systems. We anticipate the injectable HA hydrogel implant has the potential for clinical translation in treating LDH-induced sciatica.

Injectable hydrogels as emerging drug-delivery platforms for tumor therapy

Tumor therapy continues to be a prominent field within biomedical research. The development of various drug carriers has been propelled by concerns surrounding the side effects and targeting efficacy of various chemotherapeutic drugs and other therapeutic agents. These carriers strive to enhance drug concentration at tumor sites, minimize systemic side effects, and improve therapeutic outcomes. Among the reported delivery systems, injectable hydrogels have emerged as an emerging candidate for the in vivo delivery of chemotherapeutic drugs due to their minimal invasive drug delivery properties. This review systematically summarizes the composition and preparation methodologies of injectable hydrogels and further highlights the delivery mechanisms of diverse drugs using these hydrogels for tumor therapy, along with an in-depth discussion on the optimized therapeutic efficiency of drugs encapsulated within the hydrogels. The work concludes by providing a dynamic forward-looking perspective on the potential challenges and possible solutions of the in situ injectable hydrogels for non-surgical and real-time diagnostic applications.

Photo-responsive hydrogels based on a ruthenium complex: synthesis and degradation

We report the synthesis of a photo responsive metallo-hydrogel based on a ruthenium(II) complex as a functional cross-linker. This metal complex contains reactive 4AAMP (= 4-(acrylamidomethyl)pyridine) ligands, which can be cleaved by light-induced ligand substitution. Ru[(bpy)2(4AAMP)2] cross-links 4-arm-PEG-SH macromonomers by thia-Michael-addition to the photocleavable 4AAMP ligand for the preparation of the hydrogel. Irradiation with green light at 529 nm leads to photodegradation of the metallo-hydrogel due to the ligand dissociation, which can be adjusted by adjusting the Ru[(bpy)2(4AAMP)2] concentration. The ligand substitution forming [Ru(bpy)2(L)2]2+ (L = H2O and CH3CN) can be monitored by 1H NMR spectroscopy and UV-visible absorption. The control of degradation by light irradiation plays a significant role in modulating the elasticity and stiffness of the light sensitive metallo-hydrogel network. The photo-responsive hydrogel is a viable substrate for cell cultures.

Injectable smart stimuli-responsive hydrogels: pioneering advancements in biomedical applications

Hydrogels have established their significance as prominent biomaterials within the realm of biomedical research. However, injectable hydrogels have garnered greater attention compared with their conventional counterparts due to their excellent minimally invasive nature and adaptive behavior post-injection. With the rapid advancement of emerging chemistry and deepened understanding of biological processes, contemporary injectable hydrogels have been endowed with an "intelligent" capacity to respond to various endogenous/exogenous stimuli (such as temperature, pH, light and magnetic field). This innovation has spearheaded revolutionary transformations across fields such as tissue engineering repair, controlled drug delivery, disease-responsive therapies, and beyond. In this review, we comprehensively expound upon the raw materials (including natural and synthetic materials) and injectable principles of these advanced hydrogels, concurrently providing a detailed discussion of the prevalent strategies for conferring stimulus responsiveness. Finally, we elucidate the latest applications of these injectable "smart" stimuli-responsive hydrogels in the biomedical domain, offering insights into their prospects.

Recent Progress and Trends in the Development of Electrospun and 3D Printed Polymeric-Based Materials to Overcome Antimicrobial Resistance (AMR)

Antimicrobial resistance (AMR) developed by microorganisms is considered one of the most critical public health issues worldwide. This problem is affecting the lives of millions of people and needs to be addressed promptly. Mainly, antibiotics are the substances that contribute to AMR in various strains of bacteria and other microorganisms, leading to infectious diseases that cannot be effectively treated. To avoid the use of antibiotics and similar drugs, several approaches have gained attention in the fields of materials science and engineering as well as pharmaceutics over the past five years. Our focus lies on the design and manufacture of polymeric-based materials capable of incorporating antimicrobial agents excluding the aforementioned substances. In this sense, two of the emerging techniques for materials fabrication, namely, electrospinning and 3D printing, have gained significant attraction. In this article, we provide a summary of the most important findings that contribute to the development of antimicrobial systems using these technologies to incorporate various types of nanomaterials, organic molecules, or natural compounds with the required property. Furthermore, we discuss and consider the challenges that lie ahead in this research field for the coming years.

Photoresponsive hydrogel-based soft robot: A review

Soft robots have received a lot of attention because of their great human-robot interaction and environmental adaptability. Most soft robots are currently limited in their applications due to wired drives. Photoresponsive soft robotics is one of the most effective ways to promote wireless soft drives. Among the many soft robotics materials, photoresponsive hydrogels have received a lot of attention due to their good biocompatibility, ductility, and excellent photoresponse properties. This paper visualizes and analyzes the research hotspots in the field of hydrogels using the literature analysis tool Citespace, demonstrating that photoresponsive hydrogel technology is currently a key research direction. Therefore, this paper summarizes the current state of research on photoresponsive hydrogels in terms of photochemical and photothermal response mechanisms. The progress of the application of photoresponsive hydrogels in soft robots is highlighted based on bilayer, gradient, orientation, and patterned structures. Finally, the main factors influencing its application at this stage are discussed, including the development directions and insights. Advancement in photoresponsive hydrogel technology is crucial for its application in the field of soft robotics. The advantages and disadvantages of different preparation methods and structures should be considered in different application scenarios to select the best design scheme.

Antibacterial-Based Hydrogel Coatings and Their Application in the Biomedical Field-A Review

Hydrogels exhibit excellent moldability, biodegradability, biocompatibility, and extracellular matrix-like properties, which make them widely used in biomedical fields. Because of their unique three-dimensional crosslinked hydrophilic networks, hydrogels can encapsulate various materials, such as small molecules, polymers, and particles; this has become a hot research topic in the antibacterial field. The surface modification of biomaterials by using antibacterial hydrogels as coatings contributes to the biomaterial activity and offers wide prospects for development. A variety of surface chemical strategies have been developed to bind hydrogels to the substrate surface stably. We first introduce the preparation method for antibacterial coatings in this review, which includes surface-initiated graft crosslinking polymerization, anchoring the hydrogel coating to the substrate surface, and the LbL self-assembly technique to coat crosslinked hydrogels. Then, we summarize the applications of hydrogel coating in the biomedical antibacterial field. Hydrogel itself has certain antibacterial properties, but the antibacterial effect is not sufficient. In recent research, in order to optimize its antibacterial performance, the following three antibacterial strategies are mainly adopted: bacterial repellent and inhibition, contact surface killing of bacteria, and release of antibacterial agents. We systematically introduce the antibacterial mechanism of each strategy. The review aims to provide reference for the further development and application of hydrogel coatings.

3D Bioprinting for Tumor Metastasis Research

Tumor metastasis is a multiple cascade process where tumor cells disseminate from the primary site to distant organs and subsequently adapt to the foreign microenvironment. Simulating the physiology of tumor metastatic events in a realistic and three-dimensional (3D) manner is a challenge for in vitro modeling. 3D bioprinting strategies, which can generate well-customized and bionic structures, enable the exploration of dynamic tumor metastasis process in a species-homologous, high-throughput and reproducible way. In this review, we summarize the recent application of 3D bioprinting in constructing in vitro tumor metastatic models and discuss its advantages and current limitations. Further perspectives on how to harness the potential of accessible 3D bioprinting strategies to better model tumor metastasis and guide anti-cancer therapies are also provided.

Photo-degradable salecan/xanthan gum ionic gel induced by iron (III) coordination for organic dye decontamination

As dye adsorbents with great potential, polysaccharide-based hydrogels are significantly hampered in practical application owing to intricate preparation methods, low absorption, and bad degradability. Salecan is a water-soluble extracellular polysaccharide with excellent physicochemical and biological properties. Here, salecan and xanthan gum were first used as a dual-precursors system, their mixed solution was crosslinked by Fe³⁺ to assemble a photo-degradable ionic gel for malachite green (MG) adsorption. Photo-degradation was done using visible light under very mild conditions, which gave rise to gel network dissolution and homogeneous solution formation. Extensive dynamic coordinate interactions between Fe³⁺ and polysaccharides maintained gel matrix stability and were systematically investigated. The control of water uptake, micro-structure, and rheology can be facilely implemented by tuning salecan/xanthan gum ratios. Furthermore, various parameters such as polysaccharide ratios, pHs, MG concentrations, and contact time affecting adsorption were optimized using batch experiments. Adsorption process accurately adhered to pseudo-second-order kinetic and Langmuir isotherm model, with the maximum adsorption capacity of 463.0 mg/g. Such mechanism implied monolayer chemisorptive characteristics. The gel exhibited satisfactory reusability and was recycled five times without apparent decrease in adsorption capacity. From these results, the photo-degradable Fe³⁺-induced salecan/xanthan gum ionic gel is an alternative and sustainable absorbent for MG removal.

Effects and Progress of Photo-Crosslinking Hydrogels in Wound Healing Improvement

Wound healing is a dynamic physiological process, including three stages: inflammation, tissue formation, and remodeling. The quality of wound healing is affected by many topical and systemic factors, while any small factor may affect the process. Therefore, improving the quality of wound healing is a complex and arduous challenge. Photo-crosslinking reaction using visible light irradiation is a novel method for hydrogel preparation. Photo-crosslinking hydrogels can be controlled in time and space, and are not interfered by temperature conditions, which have been widely used in the fields of medicine and engineering. This review aims to summarize the application of photo-crosslinking hydrogels in improving the quality of wound healing, mainly including the material design, application mechanism, and effect of photo-crosslinking hydrogels applied in wound healing, followed by the applicable animal models for experimental research. Finally, this review analyzes the clinical application prospects of photo-crosslinking hydrogels in the field of wound healing.

Free radical-scavenging composite gelatin methacryloyl hydrogels for cell encapsulation

Gelatin methacryloyl (GelMA) hydrogels have been widely used for cell encapsulation in tissue engineering due to their cell adhesiveness and biocompatibility. However, free radicals generated during gelation decrease the viability of the encapsulated cells by increasing intracellular oxidative stress, so appropriate strategies for scavenging free radicals need to be developed. To meet that need, we developed composite GelMA hydrogels incorporating nanofiber particles (EF) coated with epigallocatechin-gallate (EGCG). The GelMA composite hydrogels were successfully fabricated and had a storage modulus of about 5 kPa, which is similar to that of pristine GelMA hydrogel, and the drastic free radical scavenging activity of EGCG was highly preserved after gelation. In addition, human adipose-derived stem cells encapsulated within our composite hydrogels had better viability (about 1.5 times) and decreased intracellular oxidative stress (about 0.3 times) compared with cells within the pristine GelMA hydrogel. We obtained similar results with human dermal fibroblasts and human umbilical vein endothelial cells, indicating that our composite hydrogels are suitable for various cell types. Furthermore, we found that the ability of the encapsulated cells to spread and migrate increased by 5 times within the composite hydrogels. Collectively, our results demonstrate that incorporating EF into GelMA hydrogels is a promising way to enhance cell viability by reducing free-radical-derived cellular damage when fabricating 3D tissue ex vivo. STATEMENT OF SIGNIFICANCE: Gelatin methacryloyl (GelMA) hydrogels have been widely applied to various tissue engineering applications because of their biocompatibility and cell interactivity. However, free radicals generated during the GelMA hydrogel fabrication decrease the viability of encapsulated cells by elevating intracellular oxidative stress. Here, we demonstrate radical scavenging GelMA hydrogels incorporating epigallocatechin-gallate (EGCG)-coated nanofiber particles (EF). The composite GelMA hydrogels are successfully fabricated, maintaining their mechanical properties, and the viability of encapsulated human adipose-derived stem cells is greatly improved after the gelation, indicating that our composite GelMA hydrogel alleviates damages from free radicals. Collectively, the incorporation of EF within GelMA hydrogels may be a promising way to enhance the viability of encapsulated cells, which could be applied to 3D tissue fabrication.

Surface Functionalities of Polymers for Biomaterial Applications

Changes of a material biointerface allow for specialized cell signaling and diverse biological responses. Biomaterials incorporating immobilized bioactive ligands have been widely introduced and used for tissue engineering and regenerative medicine applications in order to develop biomaterials with improved functionality. Furthermore, a variety of physical and chemical techniques have been utilized to improve biomaterial functionality, particularly at the material interface. At the interface level, the interactions between materials and cells are described. The importance of surface features in cell function is then examined, with new strategies for surface modification being highlighted in detail.

Characteristic and Chondrogenic Differentiation Analysis of Hybrid Hydrogels Comprised of Hyaluronic Acid Methacryloyl (HAMA), Gelatin Methacryloyl (GelMA), and the Acrylate-Functionalized Nano-Silica Crosslinker

Developing a biomaterial suitable for adipose-derived stem cell (ADSCs)-laden scaffolds that can directly bond to cartilage tissue surfaces in tissue engineering has still been a significant challenge. The bioinspired hybrid hydrogel approaches based on hyaluronic acid methacryloyl (HAMA) and gelatin methacryloyl (GelMA) appear to have more promise. Herein, we report the cartilage tissue engineering application of a novel photocured hybrid hydrogel system comprising HAMA, GelMA, and 0~1.0% (w/v) acrylate-functionalized nano-silica (AFnSi) crosslinker, in addition to describing the preparation of related HAMA, GelMA, and AFnSi materials and confirming their related chemical evidence. The study also examines the physicochemical characteristics of these hybrid hydrogels, including swelling behavior, morphological conformation, mechanical properties, and biodegradation. To further investigate cell viability and chondrogenic differentiation, the hADSCs were loaded with a two-to-one ratio of the HAMA-GelMA (HG) hybrid hydrogel with 0~1.0% (w/v) AFnSi crosslinker to examine the process of optimal chondrogenic development. Results showed that the morphological microstructure, mechanical properties, and longer degradation time of the HG+0.5% (w/v) AFnSi hydrogel demonstrated the acellular novel matrix was optimal to support hADSCs differentiation. In other words, the in vitro experimental results showed that hADSCs laden in the photocured hybrid hydrogel of HG+0.5% (w/v) AFnSi not only significantly increased chondrogenic marker gene expressions such as SOX-9, aggrecan, and type II collagen expression compared to the HA and HG groups, but also enhanced the expression of sulfated glycosaminoglycan (sGAG) and type II collagen formation. We have concluded that the photocured hybrid hydrogel of HG+0.5% (w/v) AFnSi will provide a suitable environment for articular cartilage tissue engineering applications.

Novel Application of Photo-Crosslinked Urocanic-Acid-Modified Chitosan in Corneal Wounds

In the last few years, the use of tissue adhesives in corneal perforation has gained immense popularity in clinical practices. The present study aimed to devise a new application of urocanic-acid-modified chitosan (CS) with methylene blue (MB) as a photosensitizer for the development of a photo-crosslinked tissue adhesive. In particular, the curing time was controlled with the aid of a 650 nm red diode. Under the same irradiation condition, the mechanical properties were tuned using the photosensitizer at different concentrations. In vitro tests revealed that the gel was ductile and biocompatible. The application of the gel to a perforated cornea model stopped the leakage of aqueous humor, immediately after the gel was photo-crosslinked. The blue appearance of the gel provided high precision when applied to corneal wounds. Importantly, the crosslinked gel became transparent within 24 h, owing to the dissipation of MB from tears, and the gel spontaneously sloughed off without artificial removal. Altogether, the study reported the development of a novel photo-crosslinkable urocanic-acid-modified CS gel that exhibited significant potential to be utilized in the healing of corneal perforation.

Bioprinting of Chondrocyte Stem Cell Co-Cultures for Auricular Cartilage Regeneration

Advances in 3D bioprinting allows not only controlled deposition of cells or cell-laden hydrogels but also flexibility in creating constructs that match the anatomical features of the patient. This is especially the case for reconstructing the pinna (ear), which is a large feature of the face and made from elastic cartilage that primarily relies on diffusion for nutrient transfer. The selection of cell lines for reconstructing this cartilage becomes a crucial step in clinical translation. Chondrocytes and mesenchymal stem cells are both studied extensively in the area of cartilage regeneration as they are capable of producing cartilage in vitro. However, such monoculture systems involve unfavorable processes and produce cartilage with suboptimal characteristics. Co-cultures of these cell types are known to alleviate these limitations to produce synergically active chondrocytes and cartilage. The current study utilized a 3D bioprinted scaffold made from combined gelatine methacryloyl and methacrylated hyaluronic acid (GelMA/HAMA) to interrogate monocultures and co-cultures of human septal chondrocytes (primary chondrocytes, PCs) and human bone marrow-derived mesenchymal stem cells (BM-hMSCs). This study is also the first to examine co-cultures of healthy human chondrocytes with human BM-hMSCs encapsulated in GelMA/HAMA bioprinted scaffolds. Findings revealed that the combination of MSCs and PCs not only yielded cell proliferation that mimicked MSCs but also produced chondrogenic expressions that mimicked PCs. These findings suggested that co-cultures of BM-hMSCs and healthy septal PCs can be employed to replace monocultures in chondrogenic studies for cartilage regeneration in this model. The opportunity for MSCs used to replace PCs alleviates the requirement of large cartilage biopsies that would otherwise be needed for sufficient cell numbers and therefore can be employed for clinical applications.

Light manipulation for fabrication of hydrogels and their biological applications

The development of biocompatible materials with desired functions is essential for tissue engineering and biomedical applications. Hydrogels prepared from these materials represent an important class of soft matter for mimicking extracellular environments. In particular, dynamic hydrogels with responsiveness to environments are quite appealing because they can match the dynamics of biological processes. Among the external stimuli that can trigger responsive hydrogels, light is considered as a clean stimulus with high spatiotemporal resolution, complete bioorthogonality, and fine tunability regarding its wavelength and intensity. Therefore, photoresponsiveness has been broadly encoded in hydrogels for biological applications. Moreover, light can be used to initiate gelation during the fabrication of biocompatible hydrogels. Here, we present a critical review of light manipulation tools for the fabrication of hydrogels and for the regulation of physicochemical properties and functions of photoresponsive hydrogels. The materials, photo-initiated chemical reactions, and new prospects for light-induced gelation are introduced in the former part, while mechanisms to render hydrogels photoresponsive and their biological applications are discussed in the latter part. Subsequently, the challenges and potential research directions in this area are discussed, followed by a brief conclusion. STATEMENT OF SIGNIFICANCE: Hydrogels play a vital role in the field of biomaterials owing to their water retention ability and biocompatibility. However, static hydrogels cannot meet the dynamic requirements of the biomedical field. As a stimulus with high spatiotemporal resolution, light is an ideal tool for both the fabrication and operation of hydrogels. In this review, light-induced hydrogelation and photoresponsive hydrogels are discussed in detail, and new prospects and emerging biological applications are described. To inspire more research studies in this promising area, the challenges and possible solutions are also presented.

Facile engineering of ECM-mimetic injectable dual crosslinking hydrogels with excellent mechanical resilience, tissue adhesion, and biocompatibility

Injectable hydrogels have aroused ever-increasing interest for their cell/biomaterial delivery ability through minimally invasive procedures. Nevertheless, it is still a challenge to simply fabricate natural biopolymer-based injectable hydrogels possessing satisfactory mechanical properties, bioadhesion, and cell delivery ability. Herein, we describe a facile dual crosslinking (DC) strategy for preparing extracellular matrix (ECM) mimetic hydrogels with desirable comprehensive performance. The chondroitin sulfate (CS)- and gelatin (Gel)-based single crosslinked (SC) hydrogels were first developed via reversible borate ester bonds, and further strengthened through the Michael-addition crosslinking reaction or visible-light initiated photopolymerization with thiol-containing polyethylene glycol (PEG) crosslinkers. The dynamic SC hydrogels showed good injectability, pH-sensitive gel-sol transformation, and self-adhesion ability to various biological tissues such as skin, liver, and intervertebral disc. The mechanically tough DC hydrogels displayed tunable stiffness, and resilience to compression load (up to 90% strain) owing to the effective energy dissipation mechanism. The formed DC hydrogels after subcutaneous injection well integrated with surrounding tissues and exhibited fast self-recovery properties. Moreover, the photoencapsulation of human mesenchymal stem cells (hMSCs) within the developed DC hydrogels was achieved and has been proved to be biocompatible, highlighting the great potential of the photopolymerized DC hydrogels in cell delivery and three-dimensional (3D) cell culture. This biomimetic, mechanically resilient, adhesive, and cytocompatible injectable DC hydrogel could serve as a promising candidate for tissue engineering.

3D Bioprinting Strategies, Challenges, and Opportunities to Model the Lung Tissue Microenvironment and Its Function

Human lungs are organs with an intricate hierarchical structure and complex composition; lungs also present heterogeneous mechanical properties that impose dynamic stress on different tissue components during the process of breathing. These physiological characteristics combined create a system that is challenging to model in vitro. Many efforts have been dedicated to develop reliable models that afford a better understanding of the structure of the lung and to study cell dynamics, disease evolution, and drug pharmacodynamics and pharmacokinetics in the lung. This review presents methodologies used to develop lung tissue models, highlighting their advantages and current limitations, focusing on 3D bioprinting as a promising set of technologies that can address current challenges. 3D bioprinting can be used to create 3D structures that are key to bridging the gap between current cell culture methods and living tissues. Thus, 3D bioprinting can produce lung tissue biomimetics that can be used to develop in vitro models and could eventually produce functional tissue for transplantation. Yet, printing functional synthetic tissues that recreate lung structure and function is still beyond the current capabilities of 3D bioprinting technology. Here, the current state of 3D bioprinting is described with a focus on key strategies that can be used to exploit the potential that this technology has to offer. Despite today's limitations, results show that 3D bioprinting has unexplored potential that may be accessible by optimizing bioink composition and looking at the printing process through a holistic and creative lens.

Advances in Injectable In Situ-Forming Hydrogels for Intratumoral Treatment

Chemotherapy has been linked to a variety of severe side effects, and the bioavailability of current chemotherapeutic agents is generally low, which decreases their effectiveness. Therefore, there is an ongoing effort to develop drug delivery systems to increase the bioavailability of these agents and minimize their side effects. Among these, intratumoral injections using in situ-forming hydrogels can improve drugs' bioavailability and minimize drugs' accumulation in non-target organs or tissues. This review describes different types of injectable in situ-forming hydrogels and their intratumoral injection for cancer treatment, after which we discuss the antitumor effects of intratumoral injection of drug-loaded hydrogels. This review concludes with perspectives on the future applicability of, and challenges for, the adoption of this drug delivery technology.

Surface Creasing-Induced Micropatterned GelMA Using Heating-Hydration Fabrication for Effective Vascularization

BACKGROUND

Surface modification is used to modify the biomaterials for the regulation of cell culture using different approaches, such as chemical graft and mechanical treatment. However, those conventional methodologies often require precise fabrication in a high resolution involving either high cost or laborious steps to remove chemical residues that are toxic to the cells.

METHODS

A novel and simple method was proposed and evaluated to rapidly generate surface ceases on the gelatin methacrylate (gelMA) surface using the heating-hydration process. Human umbilical vein endothelial cells (HUVECs) were cultured on the gelMA surface. The surface binding was characterized using the RGD (Arg-Gly-Asp) antibodies and cell adhesion pattern captured by scanning electron microscopy. The effect of the heating-hydration parameters on the creasing formation was investigated. The morphology of HUVECs cultured on such micropatterned gelMA was characterized and compared.

RESULTS

It is found that the hydration solution, gelMA mixture, and hydration rate are the major factors that influence the cracking sizes in the range from 20 to 120 µm which resulted in capillary-like patterns on the gelMA surface. Low concentration of gelMA, high water concentration of cooling agent, and slow hydration rate result in the long creases, and heating of at least 60 min is required for complete dehydration. Strong fluorescence was around the creases with RGD-staining. Consequently, micropatterned gelMA demonstrated good biocompatibility with endothelial cells with more than 95% cell viability and continuous cell proliferation throughout 2 weeks as well as a good trace of neovascular formation. In comparison, normal gelMA surface did not exhibit RGD-fluorescent signals, and the cultured HUVECs on it were rounded with no spreading for network formation.

CONCLUSION

The heating-hydration approach can successfully and easily produce the micropatterned gelMA that allows rapid and effective vascularization to potentially improve the functionalities of the tissue-engineered construct.

Two-Photon Polymerization: Functionalized Microstructures, Micro-Resonators, and Bio-Scaffolds

The direct laser writing technique based on two-photon polymerization (TPP) has evolved considerably over the past two decades. Its remarkable characteristics, such as 3D capability, sub-diffraction resolution, material flexibility, and gentle processing conditions, have made it suitable for several applications in photonics and biosciences. In this review, we present an overview of the progress of TPP towards the fabrication of functionalized microstructures, whispering gallery mode (WGM) microresonators, and microenvironments for culturing microorganisms. We also describe the key physical-chemical fundamentals underlying the technique, the typical experimental setups, and the different materials employed for TPP.

Combinations of photoinitiator and UV absorber for cell-based digital light processing (DLP) bioprinting

Digital light processing (DLP) bioprinting, which provides predominant speed, resolution, and adaptability for fabricating complex cell-laden three-dimensional (3D) structures, requires a combination of photoinitiator (PI) and UV absorber (UA) that plays critical roles during the photo-polymerization of bioinks. However, the PI and UA combination has not been highlighted for cell-based DLP bioprinting. In this study, the most used PIs and UAs in cell-based bioprinting were compared to optimize a combination that can ensure the maximum DLP printability, while maintaining the cellular activities during the process. The crosslinking time and printability of PIs were assessed, which are critical in minimizing the cell damage by the UV exposure during the fabrication process. On the other hand, the UAs were evaluated based on their ability to prevent the over-curing of layers beyond the focal layer and the scattering of light, which are required for the desirable crosslinking of a hydrogel and high resolution (25-50µms) to create a complex 3D cell-laden construct. Lastly, the cytotoxicity of PIs and UAs was assessed by measuring the cellular activity of 2D cultured and 3D bioprinted cells. The optimized PI and UA combination provided high initial cell viability (>90%) for up to 14 days in culture and could fabricate complex 3D structures like a perfusable heart-shaped construct with open vesicles and atriums. This combination can provide a potential starting condition when preparing the bioink for the cell-based DLP bioprinting in tissue engineering applications.

Photopolymerized Porous Hydrogels

Hydrogels are polymeric networks highly swollen with water. Because of their versatility and properties mimicking biological tissues, they are very interesting for biomedical applications. In this aim, the control of porosity is of crucial importance since it governs the transport properties and influences the fate of cells cultured onto or into the hydrogels. Among the techniques allowing for the elaboration of hydrogels, photopolymerization or photo-cross-linking are probably the most powerful and versatile synthetic routes. This Review aims at giving an overview of the literature dealing with photopolymerized hydrogels for which the generation or characterization of porosity is studied. First, the materials (polymers and photoinitiating systems) used for synthesizing hydrogels are presented. The different ways for generating porosity in the photopolymerized hydrogels are explained, and the characterization techniques allowing adequate study of the porosity are presented. Finally, some applications in the field of controlled release and tissue engineering are reviewed.

Rational Design of Smart Hydrogels for Biomedical Applications

Hydrogels are polymeric three-dimensional network structures with high water content. Due to their superior biocompatibility and low toxicity, hydrogels play a significant role in the biomedical fields. Hydrogels are categorized by the composition from natural polymers to synthetic polymers. To meet the complicated situation in the biomedical applications, suitable host–guest supramolecular interactions are rationally selected. This review will have an introduction of hydrogel classification based on the formulation molecules, and then a discussion over the rational design of the intelligent hydrogel to the environmental stimuli such as temperature, irradiation, pH, and targeted biomolecules. Further, the applications of rationally designed smart hydrogels in the biomedical field will be presented, such as tissue repair, drug delivery, and cancer therapy. Finally, the perspectives and the challenges of smart hydrogels will be outlined.

Tailoring Common Hydrogels into 3D Cell Culture Templates

Physiologically relevant cell-based models require engineered microenvironments which recapitulate the topographical, biochemical, and mechanical properties encountered in vivo. In this context, hydrogels are the materials of choice. Here a light-based toolbox is able to craft such microniches out of common place materials. Extensive use of benzophenone photoinitiators and their interaction with oxygen achieves this. First, the oxygen inhibition of radicals is harnessed to photoprint hydrogel topographies. Then the chemical properties of benzophenone are exploited to crosslink and functionalize native hydrogels lacking photosensitive moieties. At last, photoscission is introduced: an oxygen-driven, benzophenone-enabled reaction that photoliquefies Matrigel and other common gels. Using these tools, soft hydrogel templates are tailored for cells to grow or self-organize into standardized structures. The described workflow emerges as an effective microniche manufacturing toolset for 3D cell culture.

Spatiotemporally Controlled Photoresponsive Hydrogels: Design and Predictive Modeling from Processing through Application

Photoresponsive hydrogels (PRHs) are soft materials whose mechanical and chemical properties can be tuned spatially and temporally with relative ease. Both photo-crosslinkable and photodegradable hydrogels find utility in a range of biomedical applications that require tissue-like properties or programmable responses. Progress in engineering with PRHs is facilitated by the development of theoretical tools that enable optimization of their photochemistry, polymer matrices, nanofillers, and architecture. This review brings together models and design principles that enable key applications of PRHs in tissue engineering, drug delivery, and soft robotics, and highlights ongoing challenges in both modeling and application.

Polymer-Based Scaffolds for Soft-Tissue Engineering

Biomaterials have been used since ancient times. However, it was not until the late 1960s when their development prospered, increasing the research on them. In recent years, the study of biomaterials has focused mainly on tissue regeneration, requiring a biomaterial that can support cells during their growth and fulfill the function of the replaced tissue until its regeneration. These materials, called scaffolds, have been developed with a wide variety of materials and processes, with the polymer ones being the most advanced. For this reason, the need arises for a review that compiles the techniques most used in the development of polymer-based scaffolds. This review has focused on three of the most used techniques: freeze-drying, electrospinning and 3D printing, focusing on current and future trends. In addition, the advantages and disadvantages of each of them have been compared.

Materials engineering, processing, and device application of hydrogel nanocomposites

Hydrogels are widely implemented as key materials in various biomedical applications owing to their soft, flexible, hydrophilic, and quasi-solid nature. Recently, however, new material properties over those of bare hydrogels have been sought for novel applications. Accordingly, hydrogel nanocomposites, i.e., hydrogels converged with nanomaterials, have been proposed for the functional transformation of conventional hydrogels. The incorporation of suitable nanomaterials into the hydrogel matrix allows the hydrogel nanocomposite to exhibit multi-functionality in addition to the biocompatible feature of the original hydrogel. Therefore, various hydrogel composites with nanomaterials, including nanoparticles, nanowires, and nanosheets, have been developed for diverse purposes, such as catalysis, environmental purification, bio-imaging, sensing, and controlled drug delivery. Furthermore, novel technologies for the patterning of such hydrogel nanocomposites into desired shapes have been developed. The combination of such material engineering and processing technologies has enabled the hydrogel nanocomposite to become a key soft component of electronic, electrochemical, and biomedical devices. We herein review the recent research trend in the field of hydrogel nanocomposites, particularly focusing on materials engineering, processing, and device applications. Furthermore, the conclusions are presented with the scope of future research outlook, which also includes the current technical limitations.

Fabrication of Patterned Hydrogel Interfaces: Exploiting the Maleimide Group as a Dual Purpose Handle for Cross-Linking and Bioconjugation

Functional hydrogels that can be obtained through facile fabrication procedures and subsequently modified using straightforward reagent-free methods are indispensable materials for biomedical applications such as sensing and diagnostics. Herein a novel hydrogel platform is obtained using polymeric precursors containing the maleimide functional group as a side chain. The maleimide groups play a dual role in fabrication of functional hydrogels. They enable photochemical cross-linking of the polymers to yield bulk and patterned hydrogels. Moreover, the maleimide group can be used as a handle for efficient functionalization using the thiol-maleimide conjugation and Diels-Alder cycloaddition click reactions. Obtained hydrogels are characterized in terms of their morphology, water uptake capacity, and functionalization. Micropatterned hydrogels are obtained under UV-irradiation using a photomask to obtain reactive micropatterns, which undergo facile functionalization upon treatment with thiol-containing functional molecules such as fluorescent dyes and bioactive ligands. The maleimide group also undergoes conjugation through the Diels-Alder reaction, where the attached molecule can be released through thermal treatment via the retro Diels-Alder reaction. The antibiofouling nature of these hydrogel micropatterns enables efficient ligand-directed biomolecular immobilization, as demonstrated by attachment of streptavidin-coated quantum dots.

Functionalized Enzyme-Responsive Biomaterials to Model Tissue Stiffening in vitro

The mechanical properties of the cellular microenvironment play a crucial role in modulating cell function, and many pathophysiological processes are accompanied by variations in extracellular matrix (ECM) stiffness. Lysyl oxidase (LOx) is one of the enzymes involved in several ECM-stiffening processes. Here, we engineered poly(ethylene glycol) (PEG)-based hydrogels with controlled mechanical properties in the range typical of soft tissues. These hydrogels were functionalized featuring free primary amines, which allows an additional chemical LOx-responsive behavior with increase in crosslinks and hydrogel elastic modulus, mimicking biological ECM-stiffening mechanisms. Hydrogels with elastic moduli in the range of 0.5-4 kPa were obtained after a first photopolymerization step. The increase in elastic modulus of the functionalized and enzyme-responsive hydrogels was also characterized after the second-step enzymatic reaction, recording an increase in hydrogel stiffness up to 0.5 kPa after incubation with LOx. Finally, hydrogel precursors containing HepG2 (bioinks) were used to form three-dimensional (3D) in vitro models to mimic hepatic tissue and test PEG-based hydrogel biocompatibility. Hepatic functional markers were measured up to 7 days of culture, suggesting further use of such 3D models to study cell mechanobiology and response to dynamic variation of hydrogels stiffness. The results show that the functionalized hydrogels presented in this work match the mechanical properties of soft tissues, allow dynamic variations of hydrogel stiffness, and can be used to mimic changes in the microenvironment properties of soft tissues typical of inflammation and pathological changes at early stages (e.g., fibrosis, cancer).

3D Printed Neural Regeneration Devices

Neural regeneration devices interface with the nervous system and can provide flexibility in material choice, implantation without the need for additional surgeries, and the ability to serve as guides augmented with physical, biological (e.g., cellular), and biochemical functionalities. Given the complexity and challenges associated with neural regeneration, a 3D printing approach to the design and manufacturing of neural devices could provide next-generation opportunities for advanced neural regeneration via the production of anatomically accurate geometries, spatial distributions of cellular components, and incorporation of therapeutic biomolecules. A 3D printing-based approach offers compatibility with 3D scanning, computer modeling, choice of input material, and increasing control over hierarchical integration. Therefore, a 3D printed implantable platform could ultimately be used to prepare novel biomimetic scaffolds and model complex tissue architectures for clinical implants in order to treat neurological diseases and injuries. Further, the flexibility and specificity offered by 3D printed in vitro platforms have the potential to be a significant foundational breakthrough with broad research implications in cell signaling and drug screening for personalized healthcare. This progress report examines recent advances in 3D printing strategies for neural regeneration as well as insight into how these approaches can be improved in future studies.

Alginate Hydrogels: A Tool for 3D Cell Encapsulation, Tissue Engineering, and Biofabrication

A wide variety of hydrogels have been proposed for tissue engineering applications, cell encapsulation, and bioinks for bioprinting applications. Cell-laden hydrogel constructs rely on natural hydrogels such as alginate, agarose, chitosan, collagen, gelatin, fibroin, and hyaluronic acid (HA), as well as on synthetic hydrogels such as poloxamers (Pluronics^®) and polyethylene glycol (PEG). Alginate has become more and more important in the last years, thanks to the possibility to prepare alginate hydrogels suitable for cell encapsulation mainly because of the mild and reversible cross-linking conditions. In this paper alginate will be described in detail with respect to its chemistry, cross-linking behavior, biocompatibility, manufacturing capacity, and possible modifications.

Thiol-Ene Click Reaction Initiated Rapid Gelation of PEGDA/Silk Fibroin Hydrogels

In this work, poly(ethylene glycol) diacrylate (PEGDA) molecules were grafted to silk fibroin (SF) molecules via a thiol-ene click reaction under 405 nm UV illumination for the fabrication of a PEGDA/SF composite hydrogel. The composite hydrogels could be prepared in a short and controllable gelation time without the use of a photoinitiator. Features relevant to the drug delivery of the PEGDA/SF hydrogels were assessed, and the hydrogels were characterized by various techniques. The results showed that the prepared PEGDA/SF hydrogels demonstrated a good sustained-release performance with limited swelling behavior. It was found that a prior cooling step can improve the compressive strength of the hydrogels effectively. Additionally, the MTT assay indicated the prepared PEGDA/SF hydrogel is non-cytotoxic. Subcutaneous implantation of the PEGDA/SF hydrogel in Kunming mice did not induce an obvious inflammation, which revealed that the prepared PEGDA/SF hydrogel possessed good biocompatibility. Furthermore, the mechanism of the gelation process was discussed.

Ionic Hydrogel for Accelerated Dopamine Delivery via Retrodialysis

Local drug delivery directly to the source of a given pathology using retrodialysis is a promising approach to treating otherwise untreatable diseases. As the primary material component in retrodialysis, the semipermeable membrane represents a critical point for innovation. This work presents a new ionic hydrogel based on polyethylene glycol and acrylate with dopamine counterions. The ionic hydrogel membrane is shown to be a promising material for controlled diffusive delivery of dopamine. The ionic nature of the membrane accelerates uptake of cationic species compared to a nonionic membrane of otherwise similar composition. It is demonstrated that the increased uptake of cations can be exploited to confer an accelerated transport of cationic species between reservoirs as is desired in retrodialysis applications. This effect is shown to enable nearly 10-fold increases in drug delivery rates from low concentration solutions. The processability of the membrane is found to allow for integration with microfabricated devices which will in turn accelerate adaptation into both existing and emerging device modalities. It is anticipated that a similar materials design approach may be broadly applied to a variety of cationic and anionic compounds for drug delivery applications ranging from neurological disorders to cancer.

Design and Applications of Photoresponsive Hydrogels

Hydrogels are the most relevant biochemical scaffold due to their tunable properties, inherent biocompatibility, and similarity with tissue and cell environments. Over the past decade, hydrogels have developed from static materials to "smart" responsive materials adapting to various stimuli, such as pH, temperature, chemical, electrical, or light. Light stimulation is particularly interesting for many applications because of the capability of contact-free remote manipulation of biomaterial properties and inherent spatial and temporal control. Moreover, light can be finely adjusted in its intrinsic properties, such as wavelength and intensity (i.e., the energy of an individual photon as well as the number of photons over time). Water is almost transparent for light in the photochemically relevant range (NIR-UV), thus hydrogels are well-suited scaffolds for light-responsive functionality. Hydrogels' chemical and physical variety combined with light responsiveness makes photoresponsive hydrogels ideal candidates for applications in several fields, ranging from biomaterials, medicine to soft robotics. Herein, the progress and new developments in the field of light-responsive hydrogels are elaborated by first introducing the relevant photochemistries before discussing selected applications in detail.

Pharmaceutical Product Development Exploiting 3D Printing Technology: Conventional to Novel Drug Delivery System

Background:

3D printed pharmaceutical products are revolutionizing the pharmaceutical industry as a prospective mean to achieve a personalized method of treatments acquired to the specially designed need of each patient. It will depend upon age, weight, concomitants, pharmacogenetics and pharmacokinetic profile of the patient and thus transforming the current pharmaceutical market as a potential alternative to conventional medicine. 3D printing technology is getting more consideration in new medicine formulation development as a modern and better alternative to control many challenges associated with conventional medicinal products. There are many advantages of 3D printed medicines which create tremendous opportunities for improving the acceptance, accuracy and effectiveness of these medicines. In 2015, United State Food and Drug Administration has approved the first 3D printed tablet (Spritam®) and had shown the emerging importance of this technology.

Methods:

This review article summarizes as how in-depth knowledge of drugs and their manufacturing processes can assist to manage different strategies for various 3D printing methods. The principal goal of this review is to provide a brief introduction about the present techniques employed in tech -medicine evolution from conventional to a novel drug delivery system.

Results:

It is evidenced that through its unparalleled advantages of high-throughput, versatility, automation, precise spatial control and fabrication of hierarchical structures, the implementation of 3D printing for the expansion and delivery of controlled drugs acts as a pivotal role.

Conclusion:

3D printing technology has an extraordinary ability to provide elasticity in the manufacturing and designing of composite products that can be utilized in programmable and personalized medicine. Personalized medicine helps in improving drug safety and minimizes side effects such as toxicity to individual human being which is associated with unsuitable drug dose.

The relationship between thiol-acrylate photopolymerization kinetics and hydrogel mechanics: An improved model incorporating photobleaching and thiol-Michael addition

Biocompatible hydrogels with defined mechanical properties are critical to tissue engineering and regenerative medicine. Thiol-acrylate photopolymerized hydrogels have attracted special interest for their degradability and cytocompatibility, and for their tunable mechanical properties through controlling factors that affect reaction kinetics (e.g., photopolymerization, stoichiometry, temperature, and solvent choice). In this study, we hypothesized that the mechanical property of these hydrogels can be tuned by photoinitiators via photobleaching and by thiol-Michael addition reactions. To test this hypothesis, a multiscale mathematical model incorporating both photobleaching and thiol-Michael addition reactions was developed and validated. After validating the model, the effects of thiol concentration, light intensity, and pH values on hydrogel mechanics were investigated. Results revealed that hydrogel stiffness (i) was maximized at a light intensity-specific optimal concentration of thiol groups; (ii) increased with decreasing pH when synthesis occurred at low light intensity; and (iii) increased with decreasing light intensity when synthesis occurred at fixed precursor composition. The multiscale model revealed that the latter was due to higher initiation efficiency at lower light intensity. More broadly, the model provides a framework for predicting mechanical properties of hydrogels based upon the controllable kinetics of thiol-acrylate photopolymerization.

Nanogels for Pharmaceutical and Biomedical Applications and Their Fabrication Using 3D Printing Technologies

Nanogels are hydrogels formed by connecting nanoscopic micelles dispersed in an aqueous medium, which give an opportunity for incorporating hydrophilic payloads to the exterior of the micellar networks and hydrophobic payloads in the core of the micelles. Biomedical and pharmaceutical applications of nanogels have been explored for tissue regeneration, wound healing, surgical device, implantation, and peroral, rectal, vaginal, ocular, and transdermal drug delivery. Although it is still in the early stages of development, due to the increasing demands of precise nanogel production to be utilized for personalized medicine, biomedical applications, and specialized drug delivery, 3D printing has been explored in the past few years and is believed to be one of the most precise, efficient, inexpensive, customizable, and convenient manufacturing techniques for nanogel production.