Photo-click living strategy for controlled, reversible exchange of biochemical ligands

Advancing Synthetic Hydrogels through Nature-Inspired Materials Chemistry

Synthetic extracellular matrix (ECM) mimics that can recapitulate the complex biochemical and mechanical nature of native tissues are needed for advanced models of development and disease. Biomedical research has heavily relied on the use of animal-derived biomaterials, which is now impeding their translational potential and convoluting the biological insights gleaned from in vitro tissue models. Natural hydrogels have long served as a convenient and effective cell culture tool, but advances in materials chemistry and fabrication techniques now present promising new avenues for creating xenogenic-free ECM substitutes appropriate for organotypic models and microphysiological systems. However, significant challenges remain in creating synthetic matrices that can approximate the structural sophistication, biochemical complexity, and dynamic functionality of native tissues. This review summarizes key properties of the native ECM, and discusses recent approaches used to systematically decouple and tune these properties in synthetic matrices. The importance of dynamic ECM mechanics, such as viscoelasticity and matrix plasticity, is also discussed, particularly within the context of organoid and engineered tissue matrices. Emerging design strategies to mimic these dynamic mechanical properties are reviewed, such as multi-network hydrogels, supramolecular chemistry, and hydrogels assembled from biological monomers.

Visible and near-infrared light-induced photoclick reactions

Photoclick reactions combine the advantages offered by light-driven processes, that is, non-invasive and high spatiotemporal control, with classical click chemistry and have found applications ranging from surface functionalization, polymer conjugation, photocrosslinking, protein labelling and bioimaging. Despite these advances, most photoclick reactions typically require near-ultraviolet (UV) and mid-UV light to proceed. UV light can trigger undesirable responses, including cellular apoptosis, and therefore, visible and near-infrared light-induced photoclick reaction systems are highly desirable. Shifting to a longer wavelength can also reduce degradation of the photoclick reagents and products. Several strategies have been used to induce a bathochromic shift in the wavelength of irradiation-initiating photoclick reactions. For instance, the extension of the conjugated π-system, triplet-triplet energy transfer, multi-photon excitation, upconversion technology, photocatalytic and photoinitiation approaches, and designs involving photocages have all been used to achieve this goal. Current design strategies, recent advances and the outlook for long wavelength-driven photoclick reactions are presented.

Chemical and Biological Engineering Strategies to Make and Modify Next-Generation Hydrogel Biomaterials

There is a growing interest in the development of technologies to probe and direct in vitro cellular function for fundamental organoid and stem cell biology, functional tissue and metabolic engineering, and biotherapeutic formulation. Recapitulating many critical aspects of the native cellular niche, hydrogel biomaterials have proven to be a defining platform technology in this space, catapulting biological investigation from traditional two-dimensional (2D) culture into the 3D world. Seeking to better emulate the dynamic heterogeneity characteristic of all living tissues, global efforts over the last several years have centered around upgrading hydrogel design from relatively simple and static architectures into stimuli-responsive and spatiotemporally evolvable niches. Towards this end, advances from traditionally disparate fields including bioorthogonal click chemistry, chemoenzymatic synthesis, and DNA nanotechnology have been co-opted and integrated to construct 4D-tunable systems that undergo preprogrammed functional changes in response to user-defined inputs. In this Review, we highlight how advances in synthetic, semisynthetic, and bio-based chemistries have played a critical role in the triggered creation and customization of next-generation hydrogel biomaterials. We also chart how these advances stand to energize the translational pipeline of hydrogels from bench to market and close with an outlook on outstanding opportunities and challenges that lay ahead.

Fluorescence-readout as a powerful macromolecular characterisation tool

The last few decades have witnessed significant progress in synthetic macromolecular chemistry, which can provide access to diverse macromolecules with varying structural complexities, topology and functionalities, bringing us closer to the aim of controlling soft matter material properties with molecular precision. To reach this goal, the development of advanced analytical techniques, allowing for micro-, molecular level and real-time investigation, is essential. Due to their appealing features, including high sensitivity, large contrast, fast and real-time response, as well as non-invasive characteristics, fluorescence-based techniques have emerged as a powerful tool for macromolecular characterisation to provide detailed information and give new and deep insights beyond those offered by commonly applied analytical methods. Herein, we critically examine how fluorescence phenomena, principles and techniques can be effectively exploited to characterise macromolecules and soft matter materials and to further unravel their constitution, by highlighting representative examples of recent advances across major areas of polymer and materials science, ranging from polymer molecular weight and conversion, architecture, conformation to polymer self-assembly to surfaces, gels and 3D printing. Finally, we discuss the opportunities for fluorescence-readout to further advance the development of macromolecules, leading to the design of polymers and soft matter materials with pre-determined and adaptable properties.

Stimuli-Responsive Hydrogels: The Dynamic Smart Biomaterials of Tomorrow

In the past decade, stimuli-responsive hydrogels are increasingly studied as biomaterials for tissue engineering and regenerative medicine purposes. Smart hydrogels can not only replicate the physicochemical properties of the extracellular matrix but also mimic dynamic processes that are crucial for the regulation of cell behavior. Dynamic changes can be influenced by the hydrogel itself (isotropic vs anisotropic) or guided by applying localized triggers. The resulting swelling-shrinking, shape-morphing, as well as patterns have been shown to influence cell function in a spatiotemporally controlled manner. Furthermore, the use of stimuli-responsive hydrogels as bioinks in 4D bioprinting is very promising as they allow the biofabrication of complex microstructures. This perspective discusses recent cutting-edge advances as well as current challenges in the field of smart biomaterials for tissue engineering. Additionally, emerging trends and potential future directions are addressed.

Tailoring the Swelling-Shrinkable Behavior of Hydrogels for Biomedical Applications

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

Stimuli-responsive dynamic hydrogels: design, properties and tissue engineering applications

The field of tissue engineering and regenerative medicine has been evolving at a rapid pace with numerous novel and interesting biomaterials being reported. Hydrogels have come a long way in this regard and have been proven to be an excellent choice for tissue regeneration. This could be due to their innate properties such as water retention, and ability to carry and deliver a multitude of therapeutic and regenerative elements to aid in better outcomes. Over the past few decades, hydrogels have been developed into an active and attractive system that can respond to various stimuli, thereby presenting a wider control over the delivery of the therapeutic agents to the intended site in a spatiotemporal manner. Researchers have developed hydrogels that respond dynamically to a multitude of external as well as internal stimuli such as mechanics, thermal energy, light, electric field, ultrasonics, tissue pH, and enzyme levels, to name a few. This review gives a brief overview of the recent developments in such hydrogel systems which respond dynamically to various stimuli, some of the interesting fabrication strategies, and their application in cardiac, bone, and neural tissue engineering.

Steering Stem Cell Fate within 3D Living Composite Tissues Using Stimuli-Responsive Cell-Adhesive Micromaterials

Engineered living microtissues such as cellular spheroids and organoids have enormous potential for the study and regeneration of tissues and organs. Microtissues are typically engineered via self-assembly of adherent cells into cellular spheroids, which are characterized by little to no cell-material interactions. Consequently, 3D microtissue models currently lack structural biomechanical and biochemical control over their internal microenvironment resulting in suboptimal functional performance such as limited stem cell differentiation potential. Here, this work report on stimuli-responsive cell-adhesive micromaterials (SCMs) that can self-assemble with cells into 3D living composite microtissues through integrin binding, even under serum-free conditions. It is demonstrated that SCMs homogeneously distribute within engineered microtissues and act as biomechanically and biochemically tunable designer materials that can alter the composite tissue microenvironment on demand. Specifically, cell behavior is controlled based on the size, stiffness, number ratio, and biofunctionalization of SCMs in a temporal manner via orthogonal secondary crosslinking strategies. Photo-based mechanical tuning of SCMs reveals early onset stiffness-controlled lineage commitment of differentiating stem cell spheroids. In contrast to conventional encapsulation of stem cell spheroids within bulk hydrogel, incorporating cell-sized SCMs within stem cell spheroids uniquely provides biomechanical cues throughout the composite microtissues' volume, which is demonstrated to be essential for osteogenic differentiation.

Middle-out methods for spatiotemporal tissue engineering of organoids

Organoids recapitulate many aspects of the complex three-dimensional (3D) organization found within native tissues and even display tissue and organ-level functionality. Traditional approaches to organoid culture have largely employed a top-down tissue engineering strategy, whereby cells are encapsulated in a 3D matrix, such as Matrigel, alongside well-defined biochemical cues that direct morphogenesis. However, the lack of spatiotemporal control over niche properties renders cellular processes largely stochastic. Therefore, bottom-up tissue engineering approaches have evolved to address some of these limitations and focus on strategies to assemble tissue building blocks with defined multi-scale spatial organization. However, bottom-up design reduces the capacity for self-organization that underpins organoid morphogenesis. Here, we introduce an emerging framework, which we term middle-out strategies, that relies on existing design principles and combines top-down design of defined synthetic matrices that support proliferation and self-organization with bottom-up modular engineered intervention to limit the degrees of freedom in the dynamic process of organoid morphogenesis. We posit that this strategy will provide key advances to guide the growth of organoids with precise geometries, structures and function, thereby facilitating an unprecedented level of biomimicry to accelerate the utility of organoids to more translationally relevant applications.

Three-dimensional printing of smart constructs using stimuli-responsive biomaterials: A future direction of precision medicine

Three-dimensional (3D) printing, which is a valuable technique for the fabrication of tissue-engineered constructs and biomedical devices with complex architectures, has brought about considerable progress in regenerative medicine, drug delivery, and diagnosis of diseases. However, because of the static and inanimate properties of conventional 3D-printed structures, it is difficult to use them in therapies for active and precise medicine, such as improved tissue regeneration, targeted or controlled drug delivery, and advanced pathophysiological monitoring. The integration of stimuli-responsive biomaterials into 3D printing provides a potential strategy for designing and building smart constructs that exhibit programmed functions and controllable changes in properties in response to exogenous and autogenous stimuli. These features make 3D-printed smart constructs the next generation of tissue-engineered products. In this review, we introduce the prevalent 3D printing techniques (with an emphasis on the differences between 3D printing and bioprinting, and biomaterials and bioink), the working principle of each technique, and the advantages of using 3D printing for the fabrication of smart constructs. Stimuli-responsive biomaterials that are widely used for 3D printing of smart constructs are categorized, followed by a summary of their applications in tissue regeneration, drug delivery, and biosensors. Finally, the challenges and future perspectives of 3D-printed smart constructs are discussed.

Exploring the role of mesoporous silica nanoparticle in the development of novel drug delivery systems

The biocompatible nature of mesoporous silica nanoparticles (MSN) attracted researchers' attention to deliver therapeutic agents in the treatment of various diseases, where their porous nature, high drug loading efficiency, and suitability to functionalize with a specific ligand of MSN helped to obtain the desired outcome. The application of MSN has been extended to deliver small chemicals to large-sized peptides or proteins to fight against complex diseases. Recently, formulation researches with MSN have been progressed for various non-conventional drug delivery systems, including liposome, microsphere, oro-dispersible film, 3D-printed formulation, and microneedle. Low bulk density, retaining mesoporous structure during downstream processing, and lack of sufficient in vivo studies are some of the important issues towards the success of mesoporous silica-based advanced drug delivery systems. The present review has aimed to evaluate the application of MSN in advanced drug delivery systems to critically analyze the role of MSN in the respective formulation over other functionalized polymers. Finally, an outlook on the future direction of MSN-based advanced drug delivery systems has been drawn against the existing challenges with this platform.

Engineering a new generation of thermoset self-healing polymers based on intrinsic approaches

Objectives

The development of thermosetting polymers with autonomic reparability has become an important research topic since it has the potential to benefit several fields such as biomaterials, tissue engineering, paint and coating technologies, electronics, and soft robotics. In dentistry, the development of restorative materials capable of inhibiting the propagation of microcracks caused by masticatory forces and thermal stress may represent a crucial expansion of the limited clinical lifespan of dental restorations, which is a pressing challenge. Biological systems have inspired the underlying concepts and designs of synthetic polymeric self-healing systems, and different strategies have been used to impart autonomous repair capability in polymers. In this review, the most relevant intrinsic strategies are categorized based on the reaction mechanisms. In general, these strategies rely on the incorporation of latent functionalities capable of undergoing reversible chemical bonds within the polymeric structure (chemically or compositionally tuned).

Search Strategy

The searches were conducted in the databases Scopus, PubMed, and Google Scholar and limited to articles that were written in English and published during the last ten years. A few additional articles were included by complementing the database searches with manual review of the reference lists.

Overall Conclusions

Although intrinsic approaches remain underexplored in dentistry, a wide variety of elegant chemistries with tremendous translational potential employed in other fields to promote autonomic repair are highlighted in this review.

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.

Synthesis, classification and properties of hydrogels: their applications in drug delivery and agriculture

Absorbent polymers or hydrogel polymer materials have an enhanced water retention capacity and are widely used in agriculture and medicine. The controlled release of bioactive molecules (especially drug proteins) by hydrogels and the encapsulation of living cells are some of the active areas of drug discovery research. Hydrogel-based delivery systems may result in a therapeutically advantageous outcome for drug delivery. They can provide various sequential therapeutic agents including macromolecular drugs, small molecule drugs, and cells to control the release of molecules. Due to their controllable degradability, ability to protect unstable drugs from degradation and flexible physical properties, hydrogels can be used as a platform in which various chemical and physical interactions with encapsulated drugs for controlled release in the system can be studied. Practically, hydrogels that possess biodegradable properties have aroused greater interest in drug delivery systems. The original three-dimensional structure gets broken down into non-toxic substances, thus confirming the excellent biocompatibility of the gel. Chemical crosslinking is a resource-rich method for forming hydrogels with excellent mechanical strength. But in some cases the crosslinker used in the synthesis of the hydrogels may cause some toxicity. However, the physically cross-linked hydrogel preparative method is an alternative solution to overcome the toxicity of cross-linkers. Hydrogels that are responsive to stimuli formed from various natural and synthetic polymers can show significant changes in their properties under external stimuli such as temperature, pH, light, ion changes, and redox potential. Stimulus-responsive hydrogels have a wider range of applications in biomedicine including drug delivery, gene delivery and tissue regeneration. Stimulus-responsive hydrogels loaded with multiple drugs show controlled and sustained drug release and can act as drug carriers. By integrating stimulus-responsive hydrogels, such as those with improved thermal responsiveness, pH responsiveness and dual responsiveness, into textile materials, advanced functions can be imparted to the textile materials, thereby improving the moisture and water retention performance, environmental responsiveness, aesthetic appeal, display and comfort of textiles. This review explores the stimuli-responsive hydrogels in drug delivery systems and examines super adsorbent hydrogels and their application in the field of agriculture.

Advanced Hydrogels for Cartilage Tissue Engineering: Recent Progress and Future Directions

Cartilage is a tension- and load-bearing tissue and has a limited capacity for intrinsic self-healing. While microfracture and arthroplasty are the conventional methods for cartilage repair, these methods are unable to completely heal the damaged tissue. The need to overcome the restrictions of these therapies for cartilage regeneration has expanded the field of cartilage tissue engineering (CTE), in which novel engineering and biological approaches are introduced to accelerate the development of new biomimetic cartilage to replace the injured tissue. Until now, a wide range of hydrogels and cell sources have been employed for CTE to either recapitulate microenvironmental cues during a new tissue growth or to compel the recovery of cartilaginous structures via manipulating biochemical and biomechanical properties of the original tissue. Towards modifying current cartilage treatments, advanced hydrogels have been designed and synthesized in recent years to improve network crosslinking and self-recovery of implanted scaffolds after damage in vivo. This review focused on the recent advances in CTE, especially self-healing hydrogels. The article firstly presents the cartilage tissue, its defects, and treatments. Subsequently, introduces CTE and summarizes the polymeric hydrogels and their advances. Furthermore, characterizations, the advantages, and disadvantages of advanced hydrogels such as multi-materials, IPNs, nanomaterials, and supramolecular are discussed. Afterward, the self-healing hydrogels in CTE, mechanisms, and the physical and chemical methods for the synthesis of such hydrogels for improving the reformation of CTE are introduced. The article then briefly describes the fabrication methods in CTE. Finally, this review presents a conclusion of prevalent challenges and future outlooks for self-healing hydrogels in CTE applications.

Light-Responsive Dynamic Protein Hydrogels Based on LOVTRAP

Protein-based hydrogels can mimic many aspects of native extracellular matrices (ECMs) and are promising biomedical materials that find various applications in cell proliferation, drug/cell delivery, and tissue engineering. To be adapted for different tasks, it is important that the mechanical and/or biochemical properties of protein-based hydrogels can be regulated by external stimuli. Light as a regulation stimulus is of advantage because it can be easily applied in demanded spatiotemporal manners. The noncovalent binding between the light-oxygen-voltage-sensing domain 2 (LOV2) and its binding partner ZDark1 (zdk1), named as LOVTRAP, is a light-responsive interaction. The binding affinity of LOVTRAP is much higher in dark than that under blue light irradiation. Taking advantage of these light-responsive interactions, herein we endeavored to use LOVTRAP as a crosslinking mechanism to engineer light-responsive protein hydrogels. Using LOV2-containing and zdk1-containing multifunctional protein building blocks, we successfully engineered a light-responsive protein hydrogel whose viscoelastic properties can change in response to light: in the dark, the hydrogel showed higher storage modulus; under blue light irradiation, the storage modulus decreased. Due to the noncovalent nature of the LOVTRAP, the engineered LOVTRAP protein hydrogels displayed shear-thinning and self-healing properties and served as an excellent injectable protein hydrogel. We anticipated that this new class of light-responsive protein hydrogels will broaden the scope of dynamic protein hydrogels and help develop other light-responsive protein hydrogels for biomedical applications.

Engineering the Cellular Microenvironment of Post-infarct Myocardium on a Chip

Myocardial infarctions are one of the most common forms of cardiac injury and death worldwide. Infarctions cause immediate necrosis in a localized region of the myocardium, which is followed by a repair process with inflammatory, proliferative, and maturation phases. This repair process culminates in the formation of scar tissue, which often leads to heart failure in the months or years after the initial injury. In each reparative phase, the infarct microenvironment is characterized by distinct biochemical, physical, and mechanical features, such as inflammatory cytokine production, localized hypoxia, and tissue stiffening, which likely each contribute to physiological and pathological tissue remodeling by mechanisms that are incompletely understood. Traditionally, simplified two-dimensional cell culture systems or animal models have been implemented to elucidate basic pathophysiological mechanisms or predict drug responses following myocardial infarction. However, these conventional approaches offer limited spatiotemporal control over relevant features of the post-infarct cellular microenvironment. To address these gaps, Organ on a Chip models of post-infarct myocardium have recently emerged as new paradigms for dissecting the highly complex, heterogeneous, and dynamic post-infarct microenvironment. In this review, we describe recent Organ on a Chip models of post-infarct myocardium, including their limitations and future opportunities in disease modeling and drug screening.

Disulfide-yne reaction: controlling the reactivity of a surface by light

In this paper we provide new insight into the disulfide-yne photo reaction, which is similar but different from the well-known thiol-yne photoclick reaction. We show that, unlike the stable product generated from thiol-yne chemistry, the vinyl dithioether structure obtained from disulfide-yne reaction exhibits unique reactivity with thiols and disulfides, which can be used for surface photochemistry to fabricate reactive and dynamic surfaces. The possible mechanism for the unique reactivity of vinyl dithioether structure was discussed. We demonstrated that disulfide-yne reactions are highly compatible with thiol-yne chemistry, but offer the flexibility and dynamic nature that is lacking in thiol-yne chemistry, thus could be a good replenishment for the existing thiol-yne toolbox.

Advances in Extracellular Matrix-Mimetic Hydrogels to Guide Stem Cell Fate

In the fields of regenerative medicine and tissue engineering, stem cells offer vast potential for treating or replacing diseased and damaged tissue. Much progress has been made in understanding stem cell biology, yielding protocols for directing stem cell differentiation toward the cell type of interest for a specific application. One particularly interesting and powerful signaling cue is the extracellular matrix (ECM) surrounding stem cells, a network of biopolymers that, along with cells, makes up what we define as a tissue. The composition, structure, biochemical features, and mechanical properties of the ECM are varied in different tissues and developmental stages, and serve to instruct stem cells toward a specific lineage. By understanding and recapitulating some of these ECM signaling cues through engineered ECM-mimicking hydrogels, stem cell fate can be directed in vitro. In this review, we will summarize recent advances in material systems to guide stem cell fate, highlighting innovative methods to capture ECM functionalities and how these material systems can be used to provide basic insight into stem cell biology or make progress toward therapeutic objectives.

Polymer Coupling via Hetero-Disulfide Exchange and Its Applications to Rewritable Polymer Brushes

An iodide-terminated polymer (Polymer-I) is converted to a thiol-terminated polymer (Polymer-SH) using HSCH₂CH₂SH in a remarkably short time (10 min). Polymer-SH is further converted to a pyridyl disulfide-terminated polymer (Polymer-SS-Py). The hetero-coupling of Polymer-SH and Polymer-SS-Py is successfully achieved to quantitatively generate a polymer disulfide (Polymer-SS-Polymer). Exploiting this efficient hetero-coupling technique, Polymer-SH is attached (grafted) on a Py-SS-immobilized surface to generate a polymer brush via a disulfide (-SS-) linkage (writing process). The -SS- linkage is cleaved by the treatment with dithiothreitol (DTT) to detach the polymer from the surface (erasing process). Subsequently, another Polymer-SH is attached on the surface to generate another polymer brush (rewriting process). Thus, a writable, erasable, and rewritable polymer brush surface is achieved. Hydrophilic, hydrophobic, and super-hydrophobic polymers (Polymer-SH) are attached on the surface, tailoring the surface wettability in the writing-erasing-rewriting cycles. Polymer-SH is also attached on a chain-end Py-SS-functionalized polymer brush surface, generating a rewritable block copolymer brush surface. A patterned block copolymer brush surface is also obtained using photo-irradiation and a photo-mask in the erasing process. The metal-free synthetic procedure, accessibility to patterned brushes, and switchable surface properties via the writing-erasing-rewriting process are attractive features of the present approach.

Stimuli-Responsive Biomaterials: Scaffolds for Stem Cell Control

Stem cell fate is closely intertwined with microenvironmental and endogenous cues within the body. Recapitulating this dynamic environment ex vivo can be achieved through engineered biomaterials which can respond to exogenous stimulation (including light, electrical stimulation, ultrasound, and magnetic fields) to deliver temporal and spatial cues to stem cells. These stimuli-responsive biomaterials can be integrated into scaffolds to investigate stem cell response in vitro and in vivo, and offer many pathways of cellular manipulation: biochemical cues, scaffold property changes, drug release, mechanical stress, and electrical signaling. The aim of this review is to assess and discuss the current state of exogenous stimuli-responsive biomaterials, and their application in multipotent stem cell control. Future perspectives in utilizing these biomaterials for personalized tissue engineering and directing organoid models are also discussed.

A simplified approach to control cell adherence on biologically derived in vitro cell culture scaffolds by direct UV-mediated RGD linkage

In this work, we present a method to fabricate a hyaluronic acid (HA) hydrogel with spatially controlled cell-adhesion properties based on photo-polymerisation cross-linking and functionalization. The approach utilises the same reaction pathway for both steps meaning that it is user-friendly and allows for adaptation at any stage during the fabrication process. Moreover, the process does not require any additional cross-linkers. The hydrogel is formed by UV-initiated radical addition reaction between acrylamide (Am) groups on the HA backbone. Cell adhesion is modulated by functionalising the adhesion peptide sequence arginine-glycine-aspartate onto the hydrogel surface via radical mediated thiol-ene reaction using the non-reacted Am groups. We show that 10 × 10 µm2 squares could be patterned with sharp features and a good resolution. The smallest area that could be patterned resulting in good cell adhesion was 25 × 25 µm2 squares, showing single-cell adhesion. Mouse brain endothelial cells adhered and remained in culture for up to 7 days on 100 × 100 µm2 square patterns. We see potential for this material combination for future use in novel organ-on-chip models and tissue engineering where the location of the cells is of importance and to further study endothelial cell biology.

Recent advances in bio-orthogonal and dynamic crosslinking of biomimetic hydrogels

In recent years, dynamic, 'click' hydrogels have been applied in numerous biomedical applications. Owing to the mild, cytocompatible, and highly specific reaction kinetics, a multitude of orthogonal handles have been developed for fabricating dynamic hydrogels to facilitate '4D' cell culture. The high degree of tunability in crosslinking reactions of orthogonal 'click' chemistry has enabled a bottom-up approach to install specific biomimicry in an artificial extracellular matrix. In addition to click chemistry, highly specific enzymatic reactions are also increasingly used for network crosslinking and for spatiotemporal control of hydrogel properties. On the other hand, covalent adaptable chemistry has been used to recapitulate the viscoelastic component of biological tissues and for formulating self-healing and shear-thinning hydrogels. The common feature of these three classes of chemistry (i.e., orthogonal click chemistry, enzymatic reactions, and covalent adaptable chemistry) is that they can be carried out under ambient and aqueous conditions, a prerequisite for maintaining cell viability for in situ cell encapsulation and post-gelation modification of network properties. Due to their orthogonality, different chemistries can also be applied sequentially to provide additional biochemical and mechanical control to guide cell behavior. Herein, we review recent advances in the use of orthogonal click chemistry, enzymatic reactions, and covalent adaptable chemistry for the development of dynamically tunable and biomimetic hydrogels.

Next-Generation Biomaterials for Culture and Manipulation of Stem Cells

Stem cell fate decisions are informed by physical and chemical cues presented within and by the extracellular matrix. Despite the generally attributed importance of extracellular cues in governing self-renewal, differentiation, and collective behavior, knowledge gaps persist with regard to the individual, synergistic, and competing effects that specific physiochemical signals have on cell function. To better understand basic stem cell biology, as well as to expand opportunities in regenerative medicine and tissue engineering, a growing suite of customizable biomaterials has been developed. These next-generation cell culture materials offer user-defined biochemical and biomechanical properties, increasingly in a manner that can be controlled in time and 3D space. This review highlights recent innovations in this regard, focusing on advances to culture and maintain stemness, direct fate, and to detect stem cell function using biomaterial-based strategies.

Design and Synthesis of Chitosan-Gelatin Hybrid Hydrogels for 3D Printable in vitro Models

The development of 3D printable hydrogels based on the crosslinking between chitosan and gelatin is proposed. Chitosan and gelatin were both functionalized with methyl furan groups. Chemical modification was performed by reductive amination with methyl furfural involving the lysine residues of gelatin and the amino groups of chitosan to generate hydrogels with tailored properties. The methyl furan residues present in both polymers were exploited for efficient crosslinking via Diels-Alder ligation with PEG-Star-maleimide under cell-compatible conditions. The obtained chitosan-gelatin hybrid was employed to formulate hydrogels and 3D printable biopolymers and its processability and biocompatibility were preliminarily investigated.

Reversible control of biomaterial properties for dynamically tuning cell behavior

In the past decade, significant advances in chemistry and manufacturing have enabled the development of increasingly complex and controllable biomaterials. A key innovation is the design of dynamic biomaterials that allow for user-specified, reversible, temporal control over material properties. In this review, we provide an overview of recent advancements in reversible biomaterials, including control of stiffness, chemistry, ligand presentation, and topography. These systems have wide-ranging applications within biomedical engineering, including in vitro disease models and tissue-engineered scaffolds to guide multistep biological processes.

The Effect of Thiol Structure on Allyl Sulfide Photodegradable Hydrogels and their Application as a Degradable Scaffold for Organoid Passaging

Intestinal organoids are useful in vitro models for basic and translational studies aimed at understanding and treating disease. However, their routine culture relies on animal-derived matrices that limit translation to clinical applications. In fact, there are few fully defined, synthetic hydrogel systems that allow for the expansion of intestinal organoids. Here, an allyl sulfide photodegradable hydrogel is presented, achieving rapid degradation through radical addition-fragmentation chain transfer (AFCT) reactions, to support routine passaging of intestinal organoids. Shear rheology to first characterize the effect of thiol and allyl sulfide crosslink structures on degradation kinetics is used. Irradiation with 365 nm light (5 mW cm-2 ) in the presence of a soluble thiol (glutathione at 15 × 10-3 m), and a photoinitiator (lithium phenyl-2,4,6-trimethylbenzoylphosphinate at 1 × 10-3 m), leads to complete hydrogel degradation in less than 15 s. Allyl sulfide hydrogels are used to support the formation of epithelial colonies from single intestinal stem cells, and rapid photodegradation is used to achieve repetitive passaging of stem cell colonies without loss in morphology or organoid formation potential. This platform could support long-term culture of intestinal organoids, potentially replacing the need for animal-derived matrices, while also allowing systematic variations to the hydrogel properties tailored for the organoid of interest.

Toward Stimuli-Responsive Dynamic Thermosets through Continuous Development and Improvements in Covalent Adaptable Networks (CANs)

Covalent adaptable networks (CANs), unlike typical thermosets or other covalently crosslinked networks, possess a unique, often dormant ability to activate one or more forms of stimuli-responsive, dynamic covalent chemistries as a means to transition their behavior from that of a viscoelastic solid to a material with fluid-like plastic flow. Upon application of a stimulus, such as light or other irradiation, temperature, or even a distinct chemical signal, the CAN responds by transforming to a state of temporal plasticity through activation of either reversible addition or reversible bond exchange, either of which allows the material to essentially re-equilibrate to an altered set of conditions that are distinct from those in which the original covalently crosslinked network is formed, often simultaneously enabling a new and distinct shape, function, and characteristics. As such, CANs span the divide between thermosets and thermoplastics, thus offering unprecedented possibilities for innovation in polymer and materials science. Without attempting to comprehensively review the literature, recent developments in CANs are discussed here with an emphasis on the most effective dynamic chemistries that render these materials to be stimuli responsive, enabling features that make CANs more broadly applicable.

Reconfigurable Surface with Photodefinable Physicochemical Properties for User-Designable Cell Scaffolds

Surfaces with specific topography and chemical composition are quite useful in many applications ranging from functional interfaces to cell incubation scaffolds. Although these surfaces can be easily fabricated by combining topography-construction methods and surface-functionalization strategies, their properties are often static after fabrication or merely switchable between "on" and "off" states. Developing surfaces that can be on-demand regulated are quite important for the generation of smart surfaces for future applications. In this paper, we present a reconfigurable surface with adjustable topography and chemical functionality utilizing the photodynamic feature of the disulfide bond. Structured surfaces, composed of disulfide-cross-linked polymer networks, were prepared by using disulfide-containing methacrylate as the monomer. We show that the topography and chemical functionality of the surface can be on-demand regulated after its fabrication, with 254 and 365 nm UV light, respectively, allowing to "define" the physicochemical properties of the surface using light before the usage. We also demonstrate the application of such surface as a user-designable cell scaffold, that different cell scaffolds can be generated from one original surface with a simple exposure process, to define the desired bioactivity onto every point of the surface and therefore exactly control cell behaviors on the scaffold.

3D Extracellular Matrix Mimics: Fundamental Concepts and Role of Materials Chemistry to Influence Stem Cell Fate

Synthetic 3D extracellular matrices (ECMs) find application in cell studies, regenerative medicine, and drug discovery. While cells cultured in a monolayer may exhibit unnatural behavior and develop very different phenotypes and genotypes than in vivo, great efforts in materials chemistry have been devoted to reproducing in vitro behavior in in vivo cell microenvironments. This requires fine-tuning the biochemical and structural actors in synthetic ECMs. This review will present the fundamentals of the ECM, cover the chemical and structural features of the scaffolds used to generate ECM mimics, discuss the nature of the signaling biomolecules required and exploited to generate bioresponsive cell microenvironments able to induce a specific cell fate, and highlight the synthetic strategies involved in creating functional 3D ECM mimics.

Fluorescent Imprintable Hydrogels via Organic/Inorganic Supramolecular Coassembly

Photoresponsive hydrogels with on/off luminescence show a promising application potential in writable information recording and display materials. However, it still remains a tremendous challenge to fabricate such hydrogels on account of the intrinsic fluorescence quenching effect and the lack of suitable responsive groups. Herein, we present fluorescent imprintable hydrogels constructed via organic/inorganic supramolecular coassembly. A photoisomeric cyanostilbene conjugated cationic surfactant exhibited an aggregation-induced emission behavior upon clay (laponite) complexation, along with excellent thixotropism brought by laponite. Macrocyclic cucurbituril[7] and β-cyclodextrin rings capable of forming host-guest complexes with the surfactant were utilized to give ternary hybrid hydrogels with luminescence and photoresponsive properties. On the account of trans-cis photoisomerization of the cyanostilbene unit, the fluorescence of the multicomponent hydrogels could undergo rapid quenching within a short irradiation period under UV light and be recovered when subjected to an annealing process. According to these properties, the imprinted fluorescent patterns using the hybrid hydrogels were erasable and rewritable. Thus, this research successfully integrates host-guest complexation and supramolecular coassembly into the fabrication of fluorescent imprintable hydrogels.

Adaptable and Reprogrammable Surfaces

Establishing control over chemical reactions on interfaces is a key challenge in contemporary surface and materials science, in particular when introducing well-defined functionalities in a reversible fashion. Reprogrammable, adaptable and functional interfaces require sophisticated chemistries to precisely equip them with specific functionalities having tailored properties. In the last decade, reversible chemistries-both covalent and noncovalent-have paved the way to precision functionalize 2 or 3D structures that provide both spatial and temporal control. A critical literature assessment reveals that methodologies for writing and erasing substrates exist, yet are still far from reaching their full potential. It is thus critical to assess the current status and to identify avenues to overcome the existing limitations. Herein, the current state-of-the-art in the field of reversible chemistry on surfaces is surveyed, while concomitantly identifying the challenges-not only synthetic but also in current surface characterization methods. The potential within reversible chemistry on surfaces to function as true writeable memories devices is identified, and the latest developments in readout technologies are discussed. Finally, we explore how spatial and temporal control over reversible, light-induced chemistries has the potential to drive the future of functional interface design, especially when combined with powerful laser lithographic applications.

Spatiotemporal material functionalization via competitive supramolecular complexation of avidin and biotin analogs

Spatiotemporal control over engineered tissues is highly desirable for various biomedical applications as it emulates the dynamic behavior of natural tissues. Current spatiotemporal biomaterial functionalization approaches are based on cytotoxic, technically challenging, or non-scalable chemistries, which has hampered their widespread usage. Here we report a strategy to spatiotemporally functionalize (bio)materials based on competitive supramolecular complexation of avidin and biotin analogs. Specifically, an injectable hydrogel is orthogonally post-functionalized with desthiobiotinylated moieties using multivalent neutravidin. In situ exchange of desthiobiotin by biotin enables spatiotemporal material functionalization as demonstrated by the formation of long-range, conformal, and contra-directional biochemical gradients within complex-shaped 3D hydrogels. Temporal control over engineered tissue biochemistry is further demonstrated by timed presentation and sequestration of growth factors using desthiobiotinylated antibodies. The method's universality is confirmed by modifying hydrogels with biotinylated fluorophores, peptides, nanoparticles, enzymes, and antibodies. Overall, this work provides a facile, cytocompatible, and universal strategy to spatiotemporally functionalize materials.

Enabling Applications of Covalent Adaptable Networks

The ability to behave in a fluidlike manner fundamentally separates thermoset and thermoplastic polymers. Bridging this divide, covalent adaptable networks (CANs) structurally resemble thermosets with permanent covalent crosslinks but are able to flow in a manner that resembles thermoplastic behavior only when a dynamic chemical reaction is active. As a consequence, the rheological behavior of CANs becomes intrinsically tied to the dynamic reaction kinetics and the stimuli that are used to trigger those, including temperature, light, and chemical stimuli, providing unprecedented control over viscoelastic properties. CANs represent a highly capable material that serves as a powerful tool to improve mechanical properties and processing in a wide variety of polymer applications, including composites, hydrogels, and shape-memory polymers. This review aims to highlight the enabling material properties of CANs and the applied fields where the CAN concept has been embraced.

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.

Fluorogenic "Photoclick" Labeling and Imaging of DNA with Coumarin-Fused Tetrazole in Vivo

Photoclickable fluorogenic probes will enable visualization of specific biomolecules with precise spatiotemporal control in their native environment. However, the fluorogenic tagging of DNA with current photocontrolled clickable probes is still challenging. Herein, we demonstrated the fast (19.5 ± 2.5 M^-1 s^-1) fluorogenic labeling and imaging of DNA in vitro and in vivo with rationally designed coumarin-fused tetrazoles under UV LED photoirradiation. With a water-soluble, nuclear-specific coumarin-fused tetrazole (CTz-SO₃), the metabolically synthesized DNA in cultured cells was effectively labeled and visualized, without fixation, via "photoclick" reaction. Moreover, the photoclickable CTz-SO₃ enabled real-time, spatially controlled imaging of DNA in live zebrafish.

Methods for producing microstructured hydrogels for targeted applications in biology

Hydrogels have been broadly studied for applications in clinically motivated fields such as tissue regeneration, drug delivery, and wound healing, as well as in a wide variety of consumer and industry uses. While the control of mechanical properties and network structures are important in all of these applications, for regenerative medicine applications in particular, matching the chemical, topographical and mechanical properties for the target use/tissue is critical. There have been multiple alternatives developed for fabricating materials with microstructures with goals of controlling the spatial location, phenotypic evolution, and signaling of cells. The commonly employed polymers such as poly(ethylene glycol) (PEG), polypeptides, and polysaccharides (as well as others) can be processed by various methods in order to control material heterogeneity and microscale structures. We review here the more commonly used polymers, chemistries, and methods for generating microstructures in biomaterials, highlighting the range of possible morphologies that can be produced, and the limitations of each method. With a focus in liquid-liquid phase separation, methods and chemistries well suited for stabilizing the interface and arresting the phase separation are covered. As the microstructures can affect cell behavior, examples of such effects are reviewed as well. STATEMENT OF SIGNIFICANCE: Heterogeneous hydrogels with enhanced matrix complexity have been studied for a variety of biomimetic materials. A range of materials based on poly(ethylene glycol), polypeptides, proteins, and/or polysaccharides, have been employed in the studies of materials that by virtue of their microstructure, can control the behaviors of cells. Methods including microfluidics, photolithography, gelation in the presence of porogens, and liquid-liquid phase separation, are presented as possible strategies for producing materials, and their relative advantages and disadvantages are discussed. We also describe in more detail the various processes involved in LLPS, and how they can be manipulated to alter the kinetics of phase separation and to yield different microstructured materials.

Chemical Approaches to Dynamically Modulate the Properties of Synthetic Matrices

As knowledge about the dynamic nature of tissues within the human body has increased, the need for cell culture models that mimic the properties of these dynamic microenvironments has grown. Hydrogels are useful platforms for investigating cellular responses to microenvironment cues in disease and regeneration processes and recently have been designed to contain dynamic bonds to regulate the mechanical and biochemical properties of the matrix in three-dimensional cell culture applications. In this Viewpoint, we highlight recent advances in developing hydrogels with dynamic properties for modeling aspects of human tissues, providing control over the properties of the synthetic matrix on multiple length and time scales, and their application for understanding or directing cell response. We conclude by discussing how orthogonal chemistries can be utilized to design dynamic hydrogel platforms for controlling both the mechanical and biochemical environment, affording opportunities to investigate more complex questions associated with disease progression and tissue regeneration.

Radical-Mediated Thiol-Ene Strategy: Photoactivation of Thiol-Containing Drugs in Cancer Cells

Photoactivated drugs provide an opportunity to improve efficacy alongside reducing side-effects in the treatment of severe diseases such as cancer. Described herein is a photoactivation decaging method of isobutylene-caged thiols through a UV-initiated thiol-ene reaction. The method was demonstrated with an isobutylene-caged cysteine, cyclic disulfide-peptide, and thiol-containing drug, all of which were rapidly and efficiently released under mild UV irradiation in the presence of thiol sources and a photoinitiator. Importantly, it is shown that the activity of histone deacetylase inhibitor largazole can be switched off when stapled, but selectively switched on within cancer cells when irradiated with non-phototoxic light.