Thermoswitching Microgel Carriers Improve Neuronal Cell Growth and Cell Release for Cell Transplantation

A Promising Application of Injectable Hydrogels in Nerve Repair and Regeneration for Ischemic Stroke

Ischemic stroke, a condition that often leads to severe nerve damage, induces complex pathological and physiological changes in nerve tissue. The mature central nervous system (CNS) lacks intrinsic regenerative capacity, resulting in a poor prognosis and long-term neurological impairments. There is no available therapy that can fully restore CNS functionality. However, the utilization of injectable hydrogels has emerged as a promising strategy for nerve repair and regeneration. Injectable hydrogels possess exceptional properties, such as biocompatibility, tunable mechanical properties, and the ability to provide a supportive environment for cell growth and tissue regeneration. Recently, various hydrogel-based tissue engineering approaches, including cell encapsulation, controlled release of therapeutic factors, and incorporation of bioactive molecules, have demonstrated great potential in the treatment of CNS injuries caused by ischemic stroke. This article aims to provide a comprehensive review of the application and development of injectable hydrogels for the treatment of ischemic stroke-induced CNS injuries, shedding light on their therapeutic prospects, challenges, recent advancements, and future directions. Additionally, it will discuss the underlying mechanisms involved in hydrogel-mediated nerve repair and regeneration, as well as the need for further preclinical and clinical studies to validate their efficacy and safety.

Potential injectable hydrogels as biomaterials for central nervous system injury: A narrative review

Numerous modalities exist through which the central nervous system (CNS) may sustain injury or impairment, encompassing traumatic incidents, stroke occurrences, and neurodegenerative diseases such as Alzheimer's disease and Parkinson's disease. Presently available pharmacological and therapeutic interventions are incapable of restoring or regenerating damaged CNS tissue, leading to substantial unmet clinical needs among patients with CNS ailments or injuries. To address and facilitate the recovery of the impaired CNS, cell-based repair strategies encompass multiple mechanisms, such as neuronal replacement, therapeutic factor secretion, and the promotion of host brain plasticity. Despite the progression of cell-based CNS reparation as a therapeutic strategy throughout the years, substantial barriers have impeded its widespread implementation in clinical settings. The integration of cell technologies with advancements in regenerative medicine utilizing biomaterials and tissue engineering has recently facilitated the surmounting of several of these impediments. This comprehensive review presents an overview of distinct CNS conditions necessitating cell reparation, in addition to exploring potential biomaterial methodologies that enhance the efficacy of treating brain injuries.

Evaluation of Polymeric Particles for Modular Tissue Cultures in Developmental Engineering

Developmental engineering (DE) aims to culture mammalian cells on corresponding modular scaffolds (scale: micron to millimeter), then assemble these into functional tissues imitating natural developmental biology processes. This research intended to investigate the influences of polymeric particles on modular tissue cultures. When poly(methyl methacrylate) (PMMA), poly(lactic acid) (PLA) and polystyrene (PS) particles (diameter: 5-100 µm) were fabricated and submerged in culture medium in tissue culture plastics (TCPs) for modular tissue cultures, the majority of adjacent PMMA, some PLA but no PS particles aggregated. Human dermal fibroblasts (HDFs) could be directly seeded onto large (diameter: 30-100 µm) PMMA particles, but not small (diameter: 5-20 µm) PMMA, nor all the PLA and PS particles. During tissue cultures, HDFs migrated from the TCPs surfaces onto all the particles, while the clustered PMMA or PLA particles were colonized by HDFs into modular tissues with varying sizes. Further comparisons revealed that HDFs utilized the same cell bridging and stacking strategies to colonize single or clustered polymeric particles, and the finely controlled open pores, corners and gaps on 3D-printed PLA discs. These observed cell-scaffold interactions, which were then used to evaluate the adaptation of microcarrier-based cell expansion technologies for modular tissue manufacturing in DE.

Injectable Extracellular Matrix Microparticles Promote Heart Regeneration in Mice with Post-ischemic Heart Injury

Ischemic heart injury causes permanent cardiomyocyte loss and fibrosis impairing cardiac function. Tissue derived biomaterials have shown promise as an injectable treatment for the post-ischemic heart. Specifically, decellularized extracellular matrix (dECM) is a protein rich suspension that forms a therapeutic hydrogel once injected and improves the heart post-injury response in rodents and pig models. Current dECM-derived biomaterials are delivered to the heart as a liquid dECM hydrogel precursor or colloidal suspension, which gels over several minutes. To increase the functionality of the dECM therapy, an injectable solid dECM microparticle formulation derived from heart tissue to control sizing and extend stability in aqueous conditions is developed. When delivered into the infarcted mouse heart, these dECM microparticles protect cardiac function promote vessel density and reduce left ventricular remodeling by sustained delivery of biomolecules. Longer retention, higher stiffness, and slower protein release of dECM microparticles are noted compared to liquid dECM hydrogel precursor. In addition, the dECM microparticle can be developed as a platform for macromolecule delivery. Together, the results suggest that dECM microparticles can be developed as a modular therapy for heart injury.

Bioactive injectable hydrogels for on demand molecule/cell delivery and for tissue regeneration in the central nervous system

Currently there are no potential curative therapies that can improve the central nervous system (CNS) regeneration after traumatic injuries or diseases. Indeed, the regeneration of CNS is greatly impaired by limited drug penetration across the blood brain barrier (BBB), poor drug targeting, deficient progenitor neural cells and limited proliferation of mature neural cells. To overcome these limitations, bioengineered injectable hydrogels in combination with drug and cell therapy have been proposed to mimic the complexity of the CNS microenvironment and architecture. Additionally, to enhance relevant CNS regeneration, proper biophysical and biochemical cues are needed. Recently, great efforts have been devoted to tailor stimuli-responsive hydrogels as novel carrier systems which are able to guide neural tissue regeneration. This review provides an extensive overview on the most promising injectable hydrogels for neural tissue engineering. A special emphasis is made to highlight the ability of these hydrogels to deliver bioactive compounds/cells upon the exposure to internal and external stimuli. Bioactive injectable hydrogels have a broad application in central nervous system's (CNS) regeneration. This review gives an overview of the latest pioneering approaches in CNS recovery using stimuli-responsive hydrogels for several neurodegenerative disorders. STATEMENT OF SIGNIFICANCE: This review summarizes the latest innovations on bioactive injectable hydrogels, focusing on tailoring internal/external stimuli-responsive hydrogels for the new injectable systems design, able to guide neural tissue response. The purpose is to highlight the advantages and the limitations of thermo-responsive, photo responsive, magnetic responsive, electric responsive, ultrasound responsive and enzymes-triggered injectable hydrogels in developing customizable neurotherapies. We believe that this comprehensive review will help in identifying the strengths and gaps in the existing literature and to further support the use of injectable hydrogels in stimulating CNS regeneration.

Stretch-responsive adhesive microcapsules for strain-regulated antibiotic release from fabric wound dressings

Bacterial infection of a wound is a major complication that can significantly delay proper healing and even necessitate surgical debridement. Conventional non-woven fabric dressings, including gauzes, bandages and cotton wools, often fail in treating wound infections in a timely manner due to their passive release mechanism of antibiotics. Here, we propose adhesive mechanically-activated microcapsules (MAMCs) capable of strongly adhering to a fibrous matrix to achieve a self-regulated release of antibiotics upon uniaxial stretching of non-woven fabric dressings. To achieve this, a uniform population of polydopamine (PDA)-coated MAMCs (PDA-MAMCs) are prepared using a microfluidics technique and subsequent oxidative dopamine polymerization. The PDA-MAMC allows for robust mechano-activation within the fibrous network through high retention and effective transmission of mechanical force under stretching. By validating the potential of a PDA-MAMCs-laden gauze to release antibiotics in a tensile strain-dependent manner, we demonstrate that PDA-MAMCs can be successfully incorporated into a woven material and create a smart wound dressing for control of bacterial infections. This new mechano-activatable delivery approach will open up a new avenue for a stretch-triggered, on-demand release of therapeutic cargos in skin-mountable or wearable biomedical devices.

A Paradigm Shift in Tissue Engineering: From a Top–Down to a Bottom–Up Strategy

Tissue engineering (TE) was initially designed to tackle clinical organ shortage problems. Although some engineered tissues have been successfully used for non-clinical applications, very few (e.g., reconstructed human skin) have been used for clinical purposes. As the current TE approach has not achieved much success regarding more broad and general clinical applications, organ shortage still remains a challenging issue. This very limited clinical application of TE can be attributed to the constraints in manufacturing fully functional tissues via the traditional top–down approach, where very limited cell types are seeded and cultured in scaffolds with equivalent sizes and morphologies as the target tissues. The newly proposed developmental engineering (DE) strategy towards the manufacture of fully functional tissues utilises a bottom–up approach to mimic developmental biology processes by implementing gradual tissue assembly alongside the growth of multiple cell types in modular scaffolds. This approach may overcome the constraints of the traditional top–down strategy as it can imitate in vivo-like tissue development processes. However, several essential issues must be considered, and more mechanistic insights of the fundamental, underpinning biological processes, such as cell–cell and cell–material interactions, are necessary. The aim of this review is to firstly introduce and compare the number of cell types, the size and morphology of the scaffolds, and the generic tissue reconstruction procedures utilised in the top–down and the bottom–up strategies; then, it will analyse their advantages, disadvantages, and challenges; and finally, it will briefly discuss the possible technologies that may overcome some of the inherent limitations of the bottom–up strategy.

Designing Microgels for Cell Culture and Controlled Assembly of Tissue Microenvironments

Micron-sized hydrogels, termed microgels, are emerging as multifunctional platforms that can recapitulate tissue heterogeneity in engineered cell microenvironments. The microgels can function as either individual cell culture units or can be assembled into larger scaffolds. In this manner, individual microgels can be customized for single or multi-cell co-culture applications, or heterogeneous populations can be used as building blocks to create microporous assembled scaffolds that more closely mimic tissue heterogeneities. The inherent versatility of these materials allows user-defined control of the microenvironments, from the order of singly encapsulated cells to entire three-dimensional cell scaffolds. These hydrogel scaffolds are promising for moving towards personalized medicine approaches and recapitulating the multifaceted microenvironments that exist in vivo.

Recent Developments in Microfluidic Technologies for Central Nervous System Targeted Studies

Neurodegenerative diseases (NDs) bear a lot of weight in public health. By studying the properties of the blood-brain barrier (BBB) and its fundamental interactions with the central nervous system (CNS), it is possible to improve the understanding of the pathological mechanisms behind these disorders and create new and better strategies to improve bioavailability and therapeutic efficiency, such as nanocarriers. Microfluidics is an intersectional field with many applications. Microfluidic systems can be an invaluable tool to accurately simulate the BBB microenvironment, as well as develop, in a reproducible manner, drug delivery systems with well-defined physicochemical characteristics. This review provides an overview of the most recent advances on microfluidic devices for CNS-targeted studies. Firstly, the importance of the BBB will be addressed, and different experimental BBB models will be briefly discussed. Subsequently, microfluidic-integrated BBB models (BBB/brain-on-a-chip) are introduced and the state of the art reviewed, with special emphasis on their use to study NDs. Additionally, the microfluidic preparation of nanocarriers and other compounds for CNS delivery has been covered. The last section focuses on current challenges and future perspectives of microfluidic experimentation.

Hydrogel microparticles for biomedical applications

Hydrogel microparticles (HMPs) are promising for biomedical applications, ranging from the therapeutic delivery of cells and drugs to the production of scaffolds for tissue repair and bioinks for 3D printing. Biologics (cells and drugs) can be encapsulated into HMPs of predefined shapes and sizes using a variety of fabrication techniques (batch emulsion, microfluidics, lithography, electrohydrodynamic (EHD) spraying and mechanical fragmentation). HMPs can be formulated in suspensions to deliver therapeutics, as aggregates of particles (granular hydrogels) to form microporous scaffolds that promote cell infiltration or embedded within a bulk hydrogel to obtain multiscale behaviours. HMP suspensions and granular hydrogels can be injected for minimally invasive delivery of biologics, and they exhibit modular properties when comprised of mixtures of distinct HMP populations. In this Review, we discuss the fabrication techniques that are available for fabricating HMPs, as well as the multiscale behaviours of HMP systems and their functional properties, highlighting their advantages over traditional bulk hydrogels. Furthermore, we discuss applications of HMPs in the fields of cell delivery, drug delivery, scaffold design and biofabrication.

Engineering Hydrogels beyond a Hydrated Network

In recent years, many mechanical, physical, chemical, and biochemical features of biomatrices have emerged as important properties to dictate the fates of cells. To construct chemically defined biomaterials to recapitulate various biological niches for both cell biology research and therapeutic utilities, it has become increasingly clear that a simple hydrated polymer network would not be able to provide the complex cues and signaling required for many types of cells. The researchers are facing a growing list of mechanophysical and biochemical properties, while each of them could be an important cellular trigger. To include all these design parameters in screening and synthesis is practically difficult, if not impossible. Developing novel high throughput screening technology by combining assay miniaturization, computer simulations, and modeling can help researchers to tackle the challenge to identify the most relevant parameters to tailor materials for specific applications.

Nanogels and Microgels: From Model Colloids to Applications, Recent Developments, and Future Trends

Nanogels and microgels are soft, deformable, and penetrable objects with an internal gel-like structure that is swollen by the dispersing solvent. Their softness and the potential to respond to external stimuli like temperature, pressure, pH, ionic strength, and different analytes make them interesting as soft model systems in fundamental research as well as for a broad range of applications, in particular in the field of biological applications. Recent tremendous developments in their synthesis open access to systems with complex architectures and compositions allowing for tailoring microgels with specific properties. At the same time state-of-the-art theoretical and simulation approaches offer deeper understanding of the behavior and structure of nano- and microgels under external influences and confinement at interfaces or at high volume fractions. Developments in the experimental analysis of nano- and microgels have become particularly important for structural investigations covering a broad range of length scales relevant to the internal structure, the overall size and shape, and interparticle interactions in concentrated samples. Here we provide an overview of the state-of-the-art, recent developments as well as emerging trends in the field of nano- and microgels. The following aspects build the focus of our discussion: tailoring (multi)functionality through synthesis; the role in biological and biomedical applications; the structure and properties as a model system, e.g., for densely packed arrangements in bulk and at interfaces; as well as the theory and computer simulation.

Neural tissue engineering with structured hydrogels in CNS models and therapies

The development of techniques to create and use multiphase microstructured hydrogels (granular hydrogels or microgels) has enabled the generation of cultures with more biologically relevant architecture and use of structured hydrogels is especially pertinent to the development of new types of central nervous system (CNS) culture models and therapies. We review material choice and the customisation of hydrogel structure, as well as the use of hydrogels in developmental models. Combining the use of structured hydrogel techniques with developmentally relevant tissue culture approaches will enable the generation of more relevant models and treatments to repair damaged CNS tissue architecture.

Delivery of Mesenchymal Stem Cells from Gelatin-Alginate Hydrogels to Stomach Lumen for Treatment of Gastroparesis

Gastroparesis (GP) is associated with depletion of interstitial cells of Cajal (ICCs) and enteric neurons, which leads to pyloric dysfunction followed by severe nausea, vomiting and delayed gastric emptying. Regenerating these fundamental structures with mesenchymal stem cell (MSC) therapy would be helpful to restore gastric function in GP. MSCs have been successfully used in animal models of other gastrointestinal (GI) diseases, including colitis. However, no study has been performed with these cells on GP animals. In this study, we explored whether mouse MSCs can be delivered from a hydrogel scaffold to the luminal surfaces of mice stomach explants. Mouse MSCs were seeded atop alginate-gelatin, coated with poly-l-lysine. These cell-gel constructs were placed atop stomach explants facing the luminal side. MSCs grew uniformly all across the gel surface within 48 h. When placed atop the lumen of the stomach, MSCs migrated from the gels to the tissues, as confirmed by positive staining with vimentin and N-cadherin. Thus, the feasibility of transplanting a cell-gel construct to deliver stem cells in the stomach wall was successfully shown in a mice stomach explant model, thereby making a significant advance towards envisioning the transplantation of an entire tissue-engineered 'gastric patch' or 'microgels' with cells and growth factors.

Human amniotic epithelial cells combined with silk fibroin scaffold in the repair of spinal cord injury

Treatment and functional reconstruction after central nervous system injury is a major medical and social challenge. An increasing number of researchers are attempting to use neural stem cells combined with artificial scaffold materials, such as fibroin, for nerve repair. However, such approaches are challenged by ethical and practical issues. Amniotic tissue, a clinical waste product, is abundant, and amniotic epithelial cells are pluripotent, have low immunogenicity, and are not the subject of ethical debate. We hypothesized that amniotic epithelial cells combined with silk fibroin scaffolds would be conducive to the repair of spinal cord injury. To test this, we isolated and cultured amniotic epithelial cells, and constructed complexes of these cells and silk fibroin scaffolds. Implantation of the cell-scaffold complex into a rat model of spinal cord injury resulted in a smaller glial scar in the damaged cord tissue than in model rats that received a blank scaffold, or amniotic epithelial cells alone. In addition to a milder local immunological reaction, the rats showed less inflammatory cell infiltration at the transplant site, milder host-versus-graft reaction, and a marked improvement in motor function. These findings confirm that the transplantation of amniotic epithelial cells combined with silk fibroin scaffold can promote the repair of spinal cord injury. Silk fibroin scaffold can provide a good nerve regeneration microenvironment for amniotic epithelial cells.

Tailored and biodegradable poly(2-oxazoline) microbeads as 3D matrices for stem cell culture in regenerative therapies

We present the synthesis of hydrogel microbeads based on telechelic poly(2-oxazoline) (POx) crosslinkers and the methacrylate monomers (HEMA, METAC, SPMA) by inverse emulsion polymerization. While in batch experiments only irregular and ill-defined beads were obtained, the preparation in a microfluidic (MF) device resulted in highly defined hydrogel microbeads. Variation of the MF parameters allowed to control the microbead diameter from 50 to 500 μm. Microbead elasticity could be tuned from 2 to 20 kPa by the POx:monomer composition, the POx chain length, net charge of the hydrogel introduced via the monomer as well as by the organic content of the aqueous phase. The proliferations of human mesenchymal stem cells (hMSCs) on the microbeads were studied. While neutral, hydrophilic POx-PHEMA beads were bioinert, excessive colonization of hMSCs on charged POx-PMETAC and POx-PSPMA was observed. The number of proliferated cells scaled roughly linear with the METAC or SPMA comonomer content. Additional collagen I coating further improved the stem cell proliferation. Finally, a first POx-based system for the preparation of biodegradable hydrogel microcarriers is described and evaluated for stem cell culturing.

Tackling Cell Transplantation Anoikis: An Injectable, Shape Memory Cryogel Microcarrier Platform Material for Stem Cell and Neuronal Cell Growth

Highly macroporous semisynthetic cryogel microcarriers can be synthesized for culturing stem cells and neuronal type cells. Growth factors loaded to heparin-containing microcarriers show near zero-order release kinetics and cell-loaded microcarriers can be injected through a fine gauge cannula without negative effect on the cells. These carriers can be applied for cell transplantation applications.

Accelerated wound healing by injectable microporous gel scaffolds assembled from annealed building blocks

Injectable hydrogels can provide a scaffold for in situ tissue regrowth and regeneration, yet gel degradation before tissue reformation limits the gels' ability to provide physical support. Here, we show that this shortcoming can be circumvented through an injectable, interconnected microporous gel scaffold assembled from annealed microgel building blocks whose chemical and physical properties can be tailored by microfluidic fabrication. In vitro, cells incorporated during scaffold formation proliferated and formed extensive three-dimensional networks within 48 h. In vivo, the scaffolds facilitated cell migration that resulted in rapid cutaneous-tissue regeneration and tissue-structure formation within five days. The combination of microporosity and injectability of these annealed gel scaffolds should enable novel routes to tissue regeneration and formation in vivo.