Photopolymerizable hydrogels for tissue engineering applications

Photopolymerized Zwitterionic Hydrogels with a Sustained Delivery of Cerium Oxide Nanoparticle-miR146a Conjugate Accelerate Diabetic Wound Healing

In the United States, $87 billion per year is spent on the care of diabetic ulcers alone. Although the pathophysiology of diabetic wound healing is multifaceted, high systemic levels of inflammation and increased reactive oxygen species are often implicated in the wound healing impairment. Zwitterionic materials have been demonstrated to reduce inflammation and increase extracellular matrix deposition in wound beds, and here, we demonstrate a fabrication method for photopolymerized zwitterionic hydrogels that also enables sustained drug delivery over time. A therapeutic molecule of interest that is examined in this work is cerium oxide nanoparticle tagged with microRNA-146a (CNP-miR146a) to combat both oxidative stress and inflammation. The hydrogels are composed of zwitterionic and nonzwitterionic monomers, and the hydrogel formation occurs in the absence of a crosslinker. The hydrogels exhibit a wide range of stiffness and mechanical properties depending on their monomer content. Additionally, these hydrogels exhibit sustained release of nanoparticles and proteins. Finally, when employed in an in vivo diabetic mouse wound healing model, the zwitterionic hydrogels alone and laden with the CNP-miR146a conjugate significantly improved the rate of diabetic wound healing. Overall, these materials have excellent potential to be used as a topical treatment for chronic diabetic wounds.

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.

An Overview on Recent Progress of the Hydrogels: From Material Resources, Properties to Functional Applications

Hydrogels, as the most typical elastomer materials with three-dimensional network structures, have attracted wide attention owing to their outstanding features in fields of sensitive stimulus response, low surface friction coefficient, good flexibility and bio-compatibility. Because of numerous fresh polymer materials (or polymerization monomers), hydrogels with various structure diversities and excellent properties are emerging, and the development of hydrogels is very vigorous over the past decade. This review focuses on state-of-the-art advances, systematically reviews the recent progress on construction of novel hydrogels utilized several kinds of typical polymerization monomers, and explores the main chemical and physical cross-linking methods to develop the diversity of hydrogels. Following the aspects mentioned above, the classification and emerging applications of hydrogels, such as pH response, ionic response, electrical response, thermal response, biomolecular response, and gas response, are extensively summarized. Finally, we have done this review with the promises and challenges for the future evolution of hydrogels and their biological applications. cross-linking methods; functional applications; hydrogels; material resources This article is protected by copyright. All rights reserved.

Gamma Radiation- and Ultraviolet-Induced Polymerization of Bis(amino acid)fumaramide Gel Assemblies

Controlling the polymerization of supramolecular self-assembly through external stimuli holds great potential for the development of responsive soft materials and manipulation at the nanoscale. Vinyl esters of bis(leu or val)fumaramide (1a and 2a) have been found to be gelators of various organic solvents and were applied in this investigation of the influence of organogelators’ self-assembly on solid-state polymerization induced by gamma and ultraviolet irradiation. Here, we report our investigation into the influences of self-assemblies of bis(amino acid vinyl ester)fumaramides on gamma-ray- and ultraviolet-induced polymerization. The gelator molecules self-assembled by non-covalent interactions, mainly through hydrogen bonds between the amide group (CONH) and the carboxyl group (COO), thus forming a gel network. NMR and FTIR spectroscopy were used to investigate and characterize supramolecular gels. TEM and SEM microscopy were used to investigate the morphology of gels and polymers. Morphology studies showed that the gels contained a filamentous structure of nanometer dimensions that was exhaustive in a three-dimensional network. The prepared derivatives contained reactive alkyl groups suitable for carrying out the polymerization reaction initiated by gamma or ultraviolet radiation in the supramolecular aggregates of selected gels. It was found that the polymerization reaction occurred only in the network of the gel and was dependent on the structure of aggregates or the proximity and orientation of double bonds in the gel network. Polymers were formed by the gels exposure to gamma and ultraviolet radiation in toluene, and water/DMF gels with transcripts of their gel structure into polymers. The polymeric material was able to immobilize various solvents by swelling. Furthermore, methyl esters of bis(leu and val)fumaramide (1b and 2b) were synthesized; these compounds showed no gelling properties, and the crystal structure of the valine derivative 2b was determined.

PEG-based Cleavable Hydrogel Microparticles with controlled porosity for permiselective trafficking of biomolecular complexes in biosensing applications

In the last decade, PEG-based hydrogels have been extensively used for the production of microparticles for biosensing applications. The biomolecule accessibility and mass transport rate represent key parameters for the...

Photoinduced synthesis of fluorescent hydrogels without fluorescent monomers

A fluorescent monomer-free one-step strategy is developed for the synthesis of fluorescent acrylamide gels, using inexpensive and commercially available rhodamine B as the hydrogen donor in type II photoinitiation system....

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.

Towards organoid culture without Matrigel

Organoids-cellular aggregates derived from stem or progenitor cells that recapitulate organ function in miniature-are of growing interest in developmental biology and medicine. Organoids have been developed for organs and tissues such as the liver, gut, brain, and pancreas; they are used as organ surrogates to study a wide range of questions in basic and developmental biology, genetic disorders, and therapies. However, many organoids reported to date have been cultured in Matrigel, which is prepared from the secretion of Engelbreth-Holm-Swarm mouse sarcoma cells; Matrigel is complex and poorly defined. This complexity makes it difficult to elucidate Matrigel-specific factors governing organoid development. In this review, we discuss promising Matrigel-free methods for the generation and maintenance of organoids that use decellularized extracellular matrix (ECM), synthetic hydrogels, or gel-forming recombinant proteins.

Injectable hydrogel mediated delivery of gene-engineered adipose-derived stem cells for enhanced osteoarthritis treatment

Osteoarthritis (OA), a chronic and degenerative joint disease, remains a challenge in treatment due to the lack of disease-modifying therapies. As a promising therapeutic agent, adipose-derived stem cells (ADSCs) have an effective anti-inflammatory and chondroprotective paracrine effect that can be enhanced by genetic modification. Unfortunately, direct cell delivery without matrix support often results in poor viability of therapeutic cells. Herein, a hydrogel implant approach that enabled intra-articular delivery of gene-engineered ADSCs was developed for improved therapeutic outcomes in a surgically induced rat OA model. An injectable extracellular matrix (ECM)-mimicking hydrogel was prepared as the carrier for cell delivery, providing a favorable microenvironment for ADSC spreading and proliferation. The ECM-mimicking hydrogel could reduce cell death during and post injection. Additionally, ADSCs were genetically modified to overexpress transforming growth factor-β1 (TGF-β1), one of the paracrine factors that exert an anti-inflammatory and pro-anabolic effect. The gene-engineered ADSCs overexpressing TGF-β1 (T-ADSCs) had an enhanced paracrine effect on OA-like chondrocytes, which effectively decreased the expression of tumor necrosis factor-alpha and increased the expression of collagen II and aggrecan. In a surgically induced rat OA model, intra-articular injection of the T-ADSC-loaded hydrogel markedly reduced cartilage degeneration, joint inflammation, and the loss of the subchondral bone. Taken together, this study provides a potential biomaterial strategy for enhanced OA treatment by delivering the gene-engineered ADSCs within an ECM-mimicking hydrogel.

Microfluidic droplet-based functional materials for cell manipulation

Functional materials from the microfluidic-based droplet community are emerging as enabling tools for various applications in tissue engineering and cell biology. The innovative micro- and nano-scale materials with diverse sizes, shapes and components can be fabricated without the use of complicated devices, allowing unprecedented control over the cells that interact with them. Here, we review the current development of microfluidic-based droplet techniques for creation of functional materials (i.e., liquid droplet, microcapsule, and microparticle). We also describe their various applications for manipulating cell fate and function.

Safety and Biocompatibility of Mupirocin Nanoparticle-Loaded Hydrogel on Burn Wound in Rat Model

Mupirocin nanoparticle-loaded hydrogel (MLH) was successfully developed. This study focused on the safety of cell lines and the biocompatibility of MLH for wound healing in rat models. MLH was assessed by an analysis of cytotoxicity and the secretion of inflammatory cytokines in cell lines. The cytocompatibility of MLH was compared with mupirocin ointment on full-thickness burn wounds in rats. The results indicated that MLH and blank hydrogel had no toxicity to human epidermal keratinocytes and human fibroblast cells. MLH inhibited lipopolysaccharide (LPS) activity in macrophage-like cells resulting in low nitric oxide production and reduced inflammatory cytokine production (interleukin (IL)-1β) compared with a positive control (LPS only). In burn wounds, MLH and hydrogel healed the wound better than the other groups determined by wound contraction, reduced secretion, and the generation of new blood vessels, as well as promotion of hair follicle cells. Better granulation tissue proliferation, less necrosis, and a lower degree of inflammation were found in the MLH and blank hydrogel than in the mupirocin ointment. The hydrogel group reduced the macrophages (CD68) on day 14 at the edge of the wound. On day 28, T cells (CD3), B cells (CD20), and CD68+ cells were concentrated in the deeper subcutaneous tissue. Additionally, the transforming growth factor β1 (TGF-β1) concentration and matrix prometalloproteinase-2/tissue inhibitor of metalloproteinases-2 ratio in the MLH and hydrogel groups were less than those in the other groups. The MLH formulation was safe and effective in burn wound healing. Therefore, MLH formulations are promising candidates for further evaluation in clinical trials.

Smart and Biomimetic 3D and 4D Printed Composite Hydrogels: Opportunities for Different Biomedical Applications

In recent years, smart/stimuli-responsive hydrogels have drawn tremendous attention for their varied applications, mainly in the biomedical field. These hydrogels are derived from different natural and synthetic polymers but are also composite with various organic and nano-organic fillers. The basic functions of smart hydrogels rely on their ability to change behavior; functions include mechanical, swelling, shaping, hydrophilicity, and bioactivity in response to external stimuli such as temperature, pH, magnetic field, electromagnetic radiation, and biological molecules. Depending on the final applications, smart hydrogels can be processed in different geometries and modalities to meet the complicated situations in biological media, namely, injectable hydrogels (following the sol-gel transition), colloidal nano and microgels, and three dimensional (3D) printed gel constructs. In recent decades smart hydrogels have opened a new horizon for scientists to fabricate biomimetic customized biomaterials for tissue engineering, cancer therapy, wound dressing, soft robotic actuators, and controlled release of bioactive substances/drugs. Remarkably, 4D bioprinting, a newly emerged technology/concept, aims to rationally design 3D patterned biological matrices from synthesized hydrogel-based inks with the ability to change structure under stimuli. This technology has enlarged the applicability of engineered smart hydrogels and hydrogel composites in biomedical fields. This paper aims to review stimuli-responsive hydrogels according to the kinds of external changes and t recent applications in biomedical and 4D bioprinting.

Extracellular Matrix-Based Biomaterials for Cardiovascular Tissue Engineering

Regenerative medicine and tissue engineering strategies have made remarkable progress in remodeling, replacing, and regenerating damaged cardiovascular tissues. The design of three-dimensional (3D) scaffolds with appropriate biochemical and mechanical characteristics is critical for engineering tissue-engineered replacements. The extracellular matrix (ECM) is a dynamic scaffolding structure characterized by tissue-specific biochemical, biophysical, and mechanical properties that modulates cellular behavior and activates highly regulated signaling pathways. In light of technological advancements, biomaterial-based scaffolds have been developed that better mimic physiological ECM properties, provide signaling cues that modulate cellular behavior, and form functional tissues and organs. In this review, we summarize the in vitro, pre-clinical, and clinical research models that have been employed in the design of ECM-based biomaterials for cardiovascular regenerative medicine. We highlight the research advancements in the incorporation of ECM components into biomaterial-based scaffolds, the engineering of increasingly complex structures using biofabrication and spatial patterning techniques, the regulation of ECMs on vascular differentiation and function, and the translation of ECM-based scaffolds for vascular graft applications. Finally, we discuss the challenges, future perspectives, and directions in the design of next-generation ECM-based biomaterials for cardiovascular tissue engineering and clinical translation.

Modulation of hydrogel stiffness by external stimuli: soft materials for mechanotransduction studies

Mechanotransduction is an important process in determining cell survival, proliferation, migration and differentiation. The extracellular matrix (ECM) is the component of natural tissue that provides structural support and biochemical signals to adhering cells. The ECM is dynamic and undergoes physical and biochemical changes in response to various stimuli and there is an interest in understanding the effect of dynamic changes in stiffness on cell behaviour and fate. Therefore, stimuli-responsive hydrogels have been developed to mimic the cells' microenvironment in a controlled fashion. Herein, we review strategies for dynamic modulation of stiffness using various stimuli, such as light, temperature and pH. Special emphasis is placed on conducting polymer (CP) hydrogels and their fabrication procedures. We believe that the redox properties of CPs and hydrogels' biological properties make CPs hydrogels a promising substrate to investigate the effect of dynamic stiffness changes and mechanical actuation on cell fate in future studies.

Photo-crosslinking polymers by dynamic covalent disulfide bonds

A simple and general strategy to construct photo-crosslinkable polymers by introducing sidechain 1,2-dithiolanes based on natural thioctic acid is presented. The disulfide five-membered rings act both as light-absorbing and dynamic covalent crosslinking units, enabling efficient photo-crosslinking and reversible chemical decrosslinking of polydimethylsiloxane polymers.

Synthesis and characterization of diacylgermanes: persistent derivatives with superior photoreactivity

Acylgermanes are known as highly efficient photoinitiators. In this contribution, we present the synthesis of new diacylgermanes 4a-evia a multiple silyl abstraction methodology. The method outperforms the state-of-the-art approach (Corey-Seebach reaction) towards diacylgermanes in terms of group tolerance and toxicity of reagents. Moreover, these compounds are decorated with bulky mesityl groups in order to improve their storage stability. The isolated diacylgermanes were characterized by multinuclear NMR-, UV-Vis spectroscopy and X-ray crystallography, as well as photolysis experiments (photobleaching) and photo-DSC measurements (photopolymerization behavior). Upon irradiation with an LED emitting at 385 nm, all compounds except for 4a and 4c bleach efficiently with quantum yields above 0.6. Due to their broad absorption bands, the compounds can be also bleached with blue light (470 nm), where especially 4e bleaches more efficiently than Ivocerin®.

Application of self-healing, swellable and biodegradable polymers for wound treatment

AIM

Self-healing, swellable and biodegradable polymers are vital materials that may facilitate the different stages of wound healing. The aim of this research was to prepare wound healing films using self-healing polyvinyl alcohol (PVA), swellable hydroxypropyl methylcellulose (HPMC), biodegradable polyglycolic acid (PGA) sutures and ciprofloxacin antibiotic for improved treatment outcome.

METHODS

Films were formulated through aqueous-based mixing of varying amounts of polyvinyl alcohol (10-20% weight/weight (w/w)) and hydroxypropyl methylcellulose (0.5, 1% w/w) with fixed quantities of ciprofloxacin. PGA sutures were placed as grids within the wet mixtures of the polymers and ciprofloxacin, and thereafter products were air dried. The formulated films were evaluated for swelling ratio, breaking elongation, folding endurance, moisture uptake and loss, compatibility and in vitro antibiotic release. Furthermore, in vivo wound healing was studied using excision model and histopathological examinations.

RESULTS

Swelling ratios were above 1.0 and the films were minimally stretchable, with folding endurance greater than 500. Films were stable while moisture uptake and loss were observed to be less than 30%. Among the optimised hydrogel batches, those containing 10% w/w PVA and 1% w/w HPMC with no PGA showed the highest drug release of 73%, whereas the batches with higher PGA content showed higher percentage wound size reduction with minimal scar. The completeness of wound healing with batches containing PVA, HPMC, ciprofloxacin and PGA, along with the standard, is evident considering the massive cornification, regeneration of the epithelial front and stratum spinosum.

CONCLUSION

The findings show that polymer-based multifunctional composite films are suitable for use as dressings for improved wound healing.

Photoinitiated Polymerization of Hydrogels by Graphene Quantum Dots

As a smart stimulus-responsive material, hydrogel has been investigated extensively in many research fields. However, its mechanical brittleness and low strength have mattered, and conventional photoinitiators used during the polymerization steps exhibit high toxicity, which limits the use of hydrogels in the field of biomedical applications. Here, we address the dual functions of graphene quantum dots (GQDs), one to trigger the synthesis of hydrogel as photoinitiators and the other to improve the mechanical strength of the as-synthesized hydrogel. GQDs embedded in the network effectively generated radicals when exposed to sunlight, leading to the initiation of polymerization, and also played a significant role in improving the mechanical strength of the crosslinked chains. Thus, we expect that the resulting hydrogel incorporated with GQDs would enable a wide range of applications that require biocompatibility as well as higher mechanical strength, including novel hydrogel contact lenses and bioscaffolds for tissue engineering.

Cellular therapy for myocardial ischemia using a temperature-responsive biodegradable injectable polymer system with adipose-derived stem cells

Adipose-derived stem cell (AdSC) has been attracting attention as a convenient stem cell source. Not only AdSC can differentiate into various tissue cells, but it can also accelerate cell proliferation, anti-inflammation, and angiogenesis by secreting paracrine factors. Studies have demonstrated AdSC treatment of ischemic heart. However, an improvement in the remaining live AdSCs administered at the injected site while maintaining paracrine factor secretion is desired to achieve effective regenerative medicine. We previously reported the ABA-type tri-block copolymer of poly(ɛ-caprolactone-co-glycolic acid) and poly(ethylene glycol) (tri-PCG), exhibiting temperature-responsive sol-to-gel transition as biodegradable injectable polymer (IP) systems. Moreover, we recently reported that the biodegradable temperature-triggered chemically cross-linked gelation systems exhibited longer gel state durations using tri-PCG attaching acryloyl groups and a polythiol derivative. In this study, we explored this IP-mediated AdSC delivery system. We investigated the cell viability, mRNA expression, and cytokine secretion of AdSCs cultured in the physical or chemical IP hydrogels. Both of these IP hydrogels retained a certain number of viable cells, and RT-PCR and ELISA analyses revealed that mRNA expression and secretion of vascular endothelial growth factor of the AdSCs cultured in the chemical hydrogel were higher than the physical hydrogel. Moreover, AdSCs injected with the chemical hydrogel into ischemic heart model mice showed longer retention of the cells at the injected site and recovery from the ischemic condition. The results mean that the IP system is a promising candidate for a stem cell delivery system that exhibits the recovery of cardiac function for myocardial infarction treatment.

Designing Inherently Photodegradable Cell-Adhesive Hydrogels for 3D Cell Culture

Light-based microfabrication techniques constitute an indispensable approach to fabricate tissue assemblies, benefiting from noncontact spatially and temporarily controlled manipulation of soft matter. Light-triggered degradation of soft materials, such as hydrogels, is important in tissue engineering, bioprinting, and related fields. The photoresponsiveness of hydrogels is generally not intrinsic and requires complex synthetic procedures wherein photoresponsive crosslinking groups are incorporated into the hydrogel. This paper demonstrates a novel biocompatible and inherently photodegradable poly(ethylene glycol) methacrylate (PEGMA)-based gelatin-methacryloyl (GelMA)-containing hydrogel that can be used to culture cells in 3D for at least 14 d. These gels are conveniently and quickly degraded via UV irradiation for 10 min to produce structured hydrogels of various geometries, sizes, and free-standing cell-laden hydrogel particles. These structures can be flexibly produced on demand. In particular, photodegradation can be temporarily delayed from photopolymerization, offering an alternative to hydrogel array production via photopolymerization with a photomask. The paper investigates the influences of hydrogel composition and swelling liquid on both its photodegradability and biocompatibility.

Hot or cold: Bioengineering immune contextures into in vitro patient-derived tumor models

In the past decade, immune checkpoint inhibitors (ICI) have proven to be tremendously effective for a subset of cancer patients. However, it is difficult to predict the response of individual patients and efforts are now directed at understanding the mechanisms of ICI resistance. Current models of patient tumors poorly recapitulate the immune contexture, which describe immune parameters that are associated with patient survival. In this Review, we discuss parameters that influence the induction of different immune contextures found within tumors and how engineering strategies may be leveraged to recapitulate these contextures to develop the next generation of immune-competent patient-derived in vitro models.

Hydrogels: A potential platform for induced pluripotent stem cell culture and differentiation

Induced pluripotent stem cells (iPSCs) can be used to generate desired types of cells that belong to the three germ layers (i.e., ectoderm, endoderm and mesoderm). These cells possess great potential in regenerative medicine. Before iPSCs are used in various biomedical applications, the existing xenogeneic culture methods must be improved to meet the technical standards of safety, cost effectiveness, and ease of handling. In addition to commonly used 2D substrates, a culture system that mimics the native cellular environment in tissues will be a good choice when culturing iPS cells and differentiating them into different lineages. Hydrogels are potential candidates that recapitulate the native complex three-dimensional microenvironment. They possess mechanical properties similar to those of many soft tissues. Moreover, hydrogels support iPSC adhesion, proliferation and differentiation to various cell types. They are xeno-free and cost-effective. In addition to other substrates, such as mouse embryonic fibroblast (MEF), Matrigel, and vitronectin, the use of hydrogel-based substrates for iPSC culture and differentiation may help generate large numbers of clinical-grade cells that can be used in potential clinical applications. This review mainly focuses on the use of hydrogels for the culture and differentiation of iPSCs into various cell types and their potential applications in regenerative medicine.

Self-Assembling Polypeptide Hydrogels as a Platform to Recapitulate the Tumor Microenvironment

Simple Summary

The tumor microenvironment is characterized by increased tissue stiffness, low (acidic) pH, and elevated temperature, all of which contribute to the development of cancer. Improving our in vitro models of cancer, therefore, requires the development of cell culture platforms that can mimic these microenvironmental properties. Here, we study a new biomaterial composed of short amino acid chains that self-assemble into a fibrous hydrogel network. This material enables simultaneous and independent tuning of substrate rigidity, extracellular pH, and temperature, allowing us to mimic both healthy tissues and the tumor microenvironment. We used this platform to study the effect of these conditions on pancreatic cancer cells and found that high substrate rigidity and low pH promote proliferation and survival of cancer cells and activate important signaling pathways associated with cancer progression.

Abstract

The tumor microenvironment plays a critical role in modulating cancer cell migration, metabolism, and malignancy, thus, highlighting the need to develop in vitro culture systems that can recapitulate its abnormal properties. While a variety of stiffness-tunable biomaterials, reviewed here, have been developed to mimic the rigidity of the tumor extracellular matrix, culture systems that can recapitulate the broader extracellular context of the tumor microenvironment (including pH and temperature) remain comparably unexplored, partially due to the difficulty in independently tuning these parameters. Here, we investigate a self-assembled polypeptide network hydrogel as a cell culture platform and demonstrate that the culture parameters, including the substrate stiffness, extracellular pH and temperature, can be independently controlled. We then use this biomaterial as a cell culture substrate to assess the effect of stiffness, pH and temperature on Suit2 cells, a pancreatic cancer cell line, and demonstrate that these microenvironmental factors can regulate two critical transcription factors in cancer: yes-associated protein 1 (YAP) and hypoxia inducible factor (HIF-1A).

3D Bioprinting Photo-Crosslinkable Hydrogels for Bone and Cartilage Repair

Three-dimensional (3D) bioprinting has become a promising strategy for bone manufacturing, with excellent control over geometry and microarchitectures of the scaffolds. The bioprinting ink for bone and cartilage engineering has thus become the key to developing 3D constructs for bone and cartilage defect repair. Maintaining the balance of cellular viability, drugs or cytokines' function, and mechanical integrity is critical for constructing 3D bone and/or cartilage scaffolds. Photo-crosslinkable hydrogel is one of the most promising materials in tissue engineering; it can respond to light and induce structural or morphological transition. The biocompatibility, easy fabrication, as well as controllable mechanical and degradation properties of photo-crosslinkable hydrogel can meet various requirements of the bone and cartilage scaffolds, which enable it to serve as an effective bio-ink for 3D bioprinting. Here, in this review, we first introduce commonly used photo-crosslinkable hydrogel materials and additives (such as nanomaterials, functional cells, and drugs/cytokine), and then discuss the applications of the 3D bioprinted photo-crosslinkable hydrogel scaffolds for bone and cartilage engineering. Finally, we conclude the review with future perspectives about the development of 3D bioprinting photo-crosslinkable hydrogels in bone and cartilage engineering.

Dually cross-linked single networks: structures and applications

Cross-linked polymers have attracted an immense attention over the years, however, there are many flaws of these systems, e.g. softness and brittleness; such materials possess non-adjustable properties and cannot recover from damage and thus are limited in their practical applications. Supramolecular chemistry offers a variety of dynamic interactions that when integrated into polymeric gels endow the systems with reversibility and responsiveness to external stimuli. A combination of different cross-links in a single gel could be the key to tackle these drawbacks, since covalent or chemical cross-linking serve to maintain the permanent shape of the material and to improve overall mechanical performance, whereas non-covalent cross-links impart dynamicity, reversibility, stimuli-responsiveness and often toughness to the material. In the present review we sought to give a comprehensive overview of the progress in design strategies of different types of dually cross-linked single gels made by researchers over the past decade as well as the successful implementations of these advances in many demanding fields where versatile multifunctional materials are required, such as tissue engineering, drug delivery, self-healing and adhesive systems, sensors as well as shape memory materials and actuators.

Cancer theranostic platforms based on injectable polymer hydrogels

Theranostic platforms that combine therapy with diagnosis not only prevent the undesirable biological responses that may occur when these processes are conducted separately, but also allow individualized therapies for patients. Polymer hydrogels have been employed to provide well-controlled drug release and targeted therapy in theranostics, where injectable hydrogels enable non-invasive treatment and monitoring with a single injection, offering greater patient comfort and efficient therapy. Efforts have been focused on applying injectable polymer hydrogels in theranostic research and clinical use. This review highlights recent progress in the design of injectable polymer hydrogels for cancer theranostics, particularly focusing on the elements/components of theranostic hydrogels, and their cross-linking strategies, structures, and performance with regard to drug delivery/tracking. Therapeutic agents and tracking modalities that are essential components of the theranostic platforms are introduced, and the design strategies, properties and applications of the injectable hydrogels developed via two approaches, namely chemical bonds and physical interactions, are described. The theranostic functions of the platforms are highly dependent on the architecture and components employed for the construction of hydrogels. Challenges currently presented by theranostic platforms based on injectable hydrogels are identified, and prospects of acquiring more comfortable and personalized therapies are proposed.

(Macro)molecular self-assembly for hydrogel drug delivery

Hydrogels prepared via self-assembly offer scalable and tunable platforms for drug delivery applications. Molecular-scale self-assembly leverages an interplay of attractive and repulsive forces; drugs and other active molecules can be incorporated into such materials by partitioning in hydrophobic domains, affinity-mediated binding, or covalent integration. Peptides have been widely used as building blocks for self-assembly due to facile synthesis, ease of modification with bioactive molecules, and precise molecular-scale control over material properties through tunable interactions. Additional opportunities are manifest in stimuli-responsive self-assembly for more precise drug action. Hydrogels can likewise be fabricated from macromolecular self-assembly, with both synthetic polymers and biopolymers used to prepare materials with controlled mechanical properties and tunable drug release. These include clinical approaches for solubilization and delivery of hydrophobic drugs. To further enhance mechanical properties of hydrogels prepared through self-assembly, recent work has integrated self-assembly motifs with polymeric networks. For example, double-network hydrogels capture the beneficial properties of both self-assembled and covalent networks. The expanding ability to fabricate complex and precise materials, coupled with an improved understanding of biology, will lead to new classes of hydrogels specifically tailored for drug delivery applications.

Synthesis and characterization of mono S-lipidated peptide hydrogels: a platform for the preparation of reactive oxygen species responsive materials

In this work we report the synthesis of mono lipidated peptides containing a 3-mercaptopropionate linker in the N-terminus by means of a photoinitiated thiol-ene reaction (S-lipidation). We evaluate the self-assembling and hydrogelation properties of a library of mono S-lipidated peptides containing lipid chains of various lengths and demonstrate that hydrogelation was driven by a balance between the lipid chain's hydrophobicity and the peptide's facial hydrophobicity. We further postulate that a simple calculation using estimated values of log D could be used as a predictor of hydrogelation when designing similar systems. A mono S-lipidated peptide containing a short lipid chain that formed hydrogels was fully characterized and a mechanism for the peptide hydrogelation developed. Finally, we demonstrate that the presence of the thioether group in the mono S-lipidated peptide hydrogels, which is a feature lacking in conventional N-acyl lipidated systems, enables the controlled disassembly of the gel via oxidation to the sulfoxide by reactive oxygen species in accordance with a hydrophobicity-modulated strategy. Thus, we conclude that mono S-lipidated peptide hydrogels constitute a novel and simple tool for the development of tissue engineering and targeted drug delivery applications of diseases with overexpression of reactive oxygen species (e.g. degenerative and metabolic diseases, and cancers).

Review of Design Considerations for Brain-on-a-Chip Models

In recent years, the need for sophisticated human in vitro models for integrative biology has motivated the development of organ-on-a-chip platforms. Organ-on-a-chip devices are engineered to mimic the mechanical, biochemical and physiological properties of human organs; however, there are many important considerations when selecting or designing an appropriate device for investigating a specific scientific question. Building microfluidic Brain-on-a-Chip (BoC) models from the ground-up will allow for research questions to be answered more thoroughly in the brain research field, but the design of these devices requires several choices to be made throughout the design development phase. These considerations include the cell types, extracellular matrix (ECM) material(s), and perfusion/flow considerations. Choices made early in the design cycle will dictate the limitations of the device and influence the end-point results such as the permeability of the endothelial cell monolayer, and the expression of cell type-specific markers. To better understand why the engineering aspects of a microfluidic BoC need to be influenced by the desired biological environment, recent progress in microfluidic BoC technology is compared. This review focuses on perfusable blood-brain barrier (BBB) and neurovascular unit (NVU) models with discussions about the chip architecture, the ECM used, and how they relate to the in vivo human brain. With increased knowledge on how to make informed choices when selecting or designing BoC models, the scientific community will benefit from shorter development phases and platforms curated for their application.

Soft Materials by Design: Unconventional Polymer Networks Give Extreme Properties

Hydrogels are polymer networks infiltrated with water. Many biological hydrogels in animal bodies such as muscles, heart valves, cartilages, and tendons possess extreme mechanical properties including being extremely tough, strong, resilient, adhesive, and fatigue-resistant. These mechanical properties are also critical for hydrogels' diverse applications ranging from drug delivery, tissue engineering, medical implants, wound dressings, and contact lenses to sensors, actuators, electronic devices, optical devices, batteries, water harvesters, and soft robots. Whereas numerous hydrogels have been developed over the last few decades, a set of general principles that can rationally guide the design of hydrogels using different materials and fabrication methods for various applications remain a central need in the field of soft materials. This review is aimed at synergistically reporting: (i) general design principles for hydrogels to achieve extreme mechanical and physical properties, (ii) implementation strategies for the design principles using unconventional polymer networks, and (iii) future directions for the orthogonal design of hydrogels to achieve multiple combined mechanical, physical, chemical, and biological properties. Because these design principles and implementation strategies are based on generic polymer networks, they are also applicable to other soft materials including elastomers and organogels. Overall, the review will not only provide comprehensive and systematic guidelines on the rational design of soft materials, but also provoke interdisciplinary discussions on a fundamental question: why does nature select soft materials with unconventional polymer networks to constitute the major parts of animal bodies?

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.

Thiol-Thioester Exchange Reactions in Precursors Enable pH-Triggered Hydrogel Formation

Bio-interactive hydrogel formation in situ requires sensory capabilities toward physiologically relevant stimuli. Here, we report on pH-controlled in situ hydrogel formation relying on latent cross-linkers, which transform from pH sensors to reactive molecules. In particular, thiopeptolide/thio-depsipeptides were capable of pH-sensitive thiol-thioester exchange reactions to yield α,ω-dithiols, which react with maleimide-functionalized multi-arm polyethylene glycol to polymer networks. Their water solubility and diffusibility qualify thiol/thioester-containing peptide mimetics as sensory precursors to drive in situ localized hydrogel formation with potential applications in tissue regeneration such as treatment of inflamed tissues of the urinary tract.

Immunomodulatory biomaterials and their application in therapies for chronic inflammation-related diseases

The degree of tissue injuries such as the level of scarring or organ dysfunction, and the immune response against them primarily determine the outcome and speed of healing process. The successful regeneration of functional tissues requires proper modulation of inflammation-producing immune cells and bioactive factors existing in the damaged microenvironment. In the tissue repair and regeneration processes, different types of biomaterials are implanted either alone or by combined with other bioactive factors, which will interact with the immune systems including immune cells, cytokines and chemokines etc. to achieve different results highly depending on this interplay. In this review article, the influences of different types of biomaterials such as nanoparticles, hydrogels and scaffolds on the immune cells and the modification of immune-responsive factors such as reactive oxygen species (ROS), cytokines, chemokines, enzymes, and metalloproteinases in tissue microenvironment are summarized. In addition, the recent advances of immune-responsive biomaterials in therapy of inflammation-associated diseases such as myocardial infarction, spinal cord injury, osteoarthritis, inflammatory bowel disease and diabetic ulcer are discussed.

Fabrication and Impact of Fouling-Reducing Temperature-Responsive POEGMA Coatings with Embedded CaCO3 Nanoparticles on Different Cell Lines

In the present work, we have successfully prepared and characterized novel nanocomposite material exhibiting temperature-dependent surface wettability changes, based on grafted brush coatings of non-fouling poly(di(ethylene glycol)methyl ether methacrylate) (POEGMA) with the embedded CaCO3 nanoparticles. Grafted polymer brushes attached to the glass surface were prepared in a three-step process using atom transfer radical polymerization (ATRP). Subsequently, uniform CaCO3 nanoparticles (NPs) embedded in POEGMA-grafted brush coatings were synthesized using biomineralized precipitation from solutions of CaCl2 and Na2CO3. An impact of the low concentration of the embedded CaCO3 NPs on cell adhesion and growth depends strongly on the type of studied cell line: keratinocytes (HaCaT), melanoma (WM35) and osteoblastic (MC3T3-e1). Based on the temperature-responsive properties of grafted brush coatings and CaCO3 NPs acting as biologically active substrate, we hope that our research will lead to a new platform for tissue engineering with modified growth of the cells due to the release of biologically active substances from CaCO3 NPs and the ability to detach the cells in a controlled manner using temperature-induced changes of the brush.

Recent Advances on Bioprinted Gelatin Methacrylate-Based Hydrogels for Tissue Repair

Bioprinting of body tissues has gained great attention in recent years due to its unique advantages, including the creation of complex geometries and printing the patient-specific tissues with various drug and cell types. The most momentous part of the bioprinting process is bioink, defined as a mixture of living cells and biomaterials (especially hydrogels). Among different biomaterials, natural polymers are the best choices for hydrogel-based bioinks due to their intrinsic biocompatibility and minimal inflammatory response in body condition. Gelatin methacryloyl (GelMA) hydrogel is one of the high-potential hydrogel-based bioinks due to its easy synthesis with low cost, great biocompatibility, transparent structure that is useful for cell monitoring, photocrosslinkability, and cell viability. Furthermore, the potential of adjusting properties of GelMA due to the synthesis protocol makes it a suitable choice for soft or hard tissues. In this review, different methods for the bioprinting of GelMA-based bioinks, as well as various effective process parameters, are reviewed. Also, several solutions for challenges in the printing of GelMA-based bioinks are discussed, and applications of GelMA-based bioprinted tissues argued as well. Impact statement Bioprinting has been demonstrated as a promising and alternative approach for organ transplantation to develop various types of living tissue. Bioinks, with great biological characteristics similar to the host tissues and rheological/flow features, are the first requirements for the successful bioprinting approach. Gelatin methacryloyl (GelMA) hydrogel is one of the high-potential hydrogel-based bioinks. This review provides a comprehensive look at different methods for the bioprinting of GelMA-based bioinks and applications of GelMA-based bioprinted tissues for tissue repair.

Textile Materials Modified with Stimuli-Responsive Drug Carrier for Skin Topical and Transdermal Delivery

Textile materials, as a suitable matrix for different active substances facilitating their gradual release, can have an important role in skin topical or transdermal therapy. Characterized by compositional and structural variety, those materials readily meet the requirements for applications in specific therapies. Aromatherapy, antimicrobial substances and painkillers, hormone therapy, psoriasis treatment, atopic dermatitis, melanoma, etc., are some of the areas where textiles can be used as carriers. There are versatile optional methods for loading the biologically active substances onto textile materials. The oldest ones are by exhaustion, spraying, and a pad-dry-cure method. Another widespread method is the microencapsulation. The modification of textile materials with stimuli-responsive polymers is a perspective route to obtaining new textiles of improved multifunctional properties and intelligent response. In recent years, research has focused on new structures such as dendrimers, polymer micelles, liposomes, polymer nanoparticles, and hydrogels. Numerous functional groups and the ability to encapsulate different substances define dendrimer molecules as promising carriers for drug delivery. Hydrogels are also high molecular hydrophilic structures that can be used to modify textile material. They absorb a large amount of water or biological fluids and can support the delivery of medicines. These characteristics correspond to one of the current trends in the development of materials used in transdermal therapy, namely production of intelligent materials, i.e., such that allow controlled concentration and time delivery of the active substance and simultaneous visualization of the process, which can only be achieved with appropriate and purposeful modification of the textile material.

3D printing of robust and biocompatible poly(ethylene glycol)diacrylate/nano-hydroxyapatite composites via continuous liquid interface production

Three-dimensional (3D) printing technology with satisfactory speed and accuracy has been a powerful force in biomaterial processing. Early studies on 3D printing of biomaterials mainly focused on their biocompatibility and cellular viability while rarely attempted to produce robust specimens. Nonetheless, the biomedical applications of polymers can be severely limited by their inherently weak mechanical properties particularly in bone tissue engineering. In this study, continuous liquid interface production (CLIP) is applied to construct 3D objects of nano-hydroxyapatite (n-HA) filled polymeric biomaterials with complex architectures. Notably, the bioactive and osteoconductive n-HA endows the 3D prints of poly(ethyleneglycol)diacrylate (PEGDA) composites with a high compression strength of 6.5 ± 1.4 MPa, about 342% improvement over neat PEGDA. This work demonstrates the first successful attempt on CLIP 3D printing of n-HA nanocomposites, providing a feasible, cost-effective and patient-specific solution to various fields in the biomedical industry.

Biomaterials for Three-Dimensional Cell Culture: From Applications in Oncology to Nanotechnology

Three-dimensional cell culture has revolutionized cellular biology research and opened the door to novel discoveries in terms of cellular behavior and response to microenvironment stimuli. Different types of 3D culture exist today, including hydrogel scaffold-based models, which possess a complex structure mimicking the extracellular matrix. These hydrogels can be made of polymers (natural or synthetic) or low-molecular weight gelators that, via the supramolecular assembly of molecules, allow the production of a reproducible hydrogel with tunable mechanical properties. When cancer cells are grown in this type of hydrogel, they develop into multicellular tumor spheroids (MCTS). Three-dimensional (3D) cancer culture combined with a complex microenvironment that consists of a platform to study tumor development and also to assess the toxicity of physico-chemical entities such as ions, molecules or particles. With the emergence of nanoparticles of different origins and natures, implementing a reproducible in vitro model that consists of a bio-indicator for nano-toxicity assays is inevitable. However, the maneuver process of such a bio-indicator requires the implementation of a repeatable system that undergoes an exhaustive follow-up. Hence, the biggest challenge in this matter is the reproducibility of the MCTS and the associated full-scale characterization of this system's components.

Current progress, challenges, and future prospects of testis organoids†

Spermatogenic failure is believed to be a major cause of male infertility. The establishment of a testis organoid model would facilitate the study of such pathological mechanisms and open the possibility of male fertility preservation. Because of the complex structures and cellular events occurring within the testis, the establishment of a compartmentalized testis organoid with a complete spermatogenic cycle remains a challenge in all species. Since the late 20th century, a great variety of scaffold-based and scaffold-free testis cell culture systems have been established to recapitulate de novo testis organogenesis and in vitro spermatogenesis. The utilization of the hydrogel scaffolds provides a 3D microenvironment for testis cell growth and development, facilitating the reconstruction of de novo testis tissue-like structures and spermatogenic differentiation. Using a combination of different strategies, including the use of various scaffolding biomaterials, the incorporation of the living cells with high self-assembling capacity, and the integration of the advanced fabrication techniques, a scaffold-based testis organoid with a compartmentalized structure that supports in vitro spermatogenesis may be achieved. This article briefly reviews the current progress in the development of scaffold-based testis organoids while focusing on the scaffolding biomaterials (hydrogels), cell sources, and scaffolding approaches. Key challenges in current organoid studies are also discussed along with recommendations for future research.