Autoclavable and Injectable Cryogels for Biomedical Applications

Preparation of Autoclavable and Injectable Silk Fibroin Cryogels for Tissue Engineering Applications

A cryogel is a supermacroporous gel network that is generated at subzero temperatures by polymerizing monomers or gelating polymeric precursors. Since cryogels possess inherent characteristics such as interconnected macroporous structures, excellent mechanical properties, and high resistance to autoclave sterilization, they are highly desirable for tissue engineering and regenerative medicine. Silk fibroin, a natural protein obtained from Bombyx mori silkworms, is an excellent raw material for cryogel preparation. The aim of this study is to establish a controlled method for preparing silk fibroin cryogels with suitable properties for application as tissue engineering scaffolds. Using a dual crosslinking strategy consisting of low-temperature radical polymerization coupled with methanol-induced conformational transformation, porous cryogels are prepared. The cryogels display many unique characteristics, such as an interconnected macroporous structure, a high water absorption capacity, water-triggered shape memory, syringe injectability, and strong resilience to autoclave sterilization. Furthermore, the cryogels demonstrate excellent biocompatibility and cell affinity, facilitating cell adhesion, migration, and proliferation. The interconnected supermacroporous architecture resembling the native extracellular matrix, together with their unique physical properties and autoclaving stability, suggests that cryogels are promising candidate scaffolds for tissue engineering and cell therapy.

Impact of Composition and Autoclave Sterilization on the Mechanical and Biological Properties of ECM-Mimicking Cryogels

Cryogels represent a valid strategy as scaffolds for tissue engineering. In order to adequately support adhesion and proliferation of anchorage-dependent cells, different polymers need to be combined within the same scaffold trying to mimic the complex features of a natural extracellular matrix (ECM). For this reason, in this work, gelatin (Gel) and chondroitin sulfate (CS), both functionalized with methacrylic groups to produce CSMA and GelMA derivatives, were selected to prepare cryogel networks. Both homopolymer and heteropolymer cryogels were produced, via radical crosslinking reactions carried out at -12 °C for 2 h. All the scaffolds were characterized for their mechanical, swelling and morphological properties, before and after autoclave sterilization. Moreover, they were evaluated for their biocompatibility and ability to support the adhesion of human gingival fibroblasts and tenocytes. GelMA-based homopolymer networks better withstood the autoclave sterilization process, compared to CSMA cryogels. Indeed, GelMA cryogels showed a decrease in stiffness of approximately 30%, whereas CSMA cryogels of approximately 80%. When GelMA and CSMA were blended in the same network, an intermediate outcome was observed. However, the hybrid scaffolds showed a general worsening of the biological performance. Indeed, despite their ability to withstand autoclave sterilization with limited modification of the mechanical and morphological properties, the hybrid cryogels exhibited poor cell adhesion and high LDH leakage. Therefore, not only do network components need to be properly selected, but also their combination and ability to withstand effective sterilization process should be carefully evaluated for the development of efficient scaffolds designed for tissue engineering purposes.

Cryogel Scaffolds for Tissue-Engineering: Advances and Challenges for Effective Bone and Cartilage Regeneration

Critical-sized bone defects and articular cartilage injuries resulting from trauma, osteonecrosis, or age-related degeneration can be often non-healed by physiological repairing mechanisms, thus representing a relevant clinical issue due to a high epidemiological incidence rate. Novel tissue-engineering approaches have been proposed as an alternative to common clinical practices. This cutting-edge technology is based on the combination of three fundamental components, generally referred to as the tissue-engineering triad: autologous or allogenic cells, growth-stimulating factors, and a scaffold. Three-dimensional polymer networks are frequently used as scaffolds to allow cell proliferation and tissue regeneration. In particular, cryogels give promising results for this purpose, thanks to their peculiar properties. Cryogels are indeed characterized by an interconnected porous structure and a typical sponge-like behavior, which facilitate cellular infiltration and ingrowth. Their composition and the fabrication procedure can be appropriately tuned to obtain scaffolds that match the requirements of a specific tissue or organ to be regenerated. These features make cryogels interesting and promising scaffolds for the regeneration of different tissues, including those characterized by very complex mechanical and physical properties, such as bones and joints. In this review, state-of-the-art fabrication and employment of cryogels for supporting effective osteogenic or chondrogenic differentiation to allow for the regeneration of functional tissues is reported. Current progress and challenges for the implementation of this technology in clinical practice are also highlighted.

Hypoxia-inducing cryogels uncover key cancer-immune cell interactions in an oxygen-deficient tumor microenvironment

Hypoxia is a major factor shaping the immune landscape, and several cancer models have been developed to emulate hypoxic tumors. However, to date, they still have several limitations, such as the lack of reproducibility, inadequate biophysical cues, limited immune cell infiltration, and poor oxygen (O2) control, leading to non-pathophysiological tumor responses. Therefore, it is essential to develop better cancer models that mimic key features of the tumor extracellular matrix and recreate tumor-associated hypoxia while allowing cell infiltration and cancer-immune cell interactions. Herein, hypoxia-inducing cryogels (HICs) have been engineered using hyaluronic acid (HA) to fabricate three-dimensional microtissues and model a hypoxic tumor microenvironment. Specifically, tumor cell-laden HICs have been designed to deplete O2 locally and induce long-standing hypoxia. HICs promoted changes in hypoxia-responsive gene expression and phenotype, a metabolic adaptation to anaerobic glycolysis, and chemotherapy resistance. Additionally, HIC-supported tumor models induced dendritic cell (DC) inhibition, revealing a phenotypic change in the plasmacytoid DC (pDC) subset and an impaired conventional DC (cDC) response in hypoxia. Lastly, our HIC-based melanoma model induced CD8+ T cell inhibition, a condition associated with the downregulation of pro-inflammatory cytokine secretion, increased expression of immunomodulatory factors, and decreased degranulation and cytotoxic capacity of T cells. Overall, these data suggest that HICs can be used as a tool to model solid-like tumor microenvironments and has great potential to deepen our understanding of cancer-immune cell relationship in low O2 conditions and may pave the way for developing more effective therapies.

Integrated Piezoelectric/Conductive Composite Cryogel Creates Electroactive Microenvironment for Enhanced Bone Regeneration

Natural bone tissue possesses inherent electrophysiological characteristics, displaying conductivity and piezoelectricity simultaneously; hence, the reconstruction of local electrical microenvironment at defect site provides an effective strategy to enhance osteogenesis. Herein, a composite cryogel-type scaffold (referred to as Gel-PD-CMBT) is developed for bone regeneration, utilizing gelatin (Gel) in combination with a conductive poly(ethylene dioxythiophene)/polystyrene sulfonate matrix and Ca/Mn co-doped barium titanate (CMBT) nanofibers as the piezoelectric filler. The incorporation of these components results in the formation of an integrated piezoelectric/conductive network within the scaffold, facilitating charge migration and yielding a conductivity of 0.59 S cm-1 . This conductive scaffold creates a promising electroactive microenvironment, which is capable of up-regulating biological responses. Furthermore, the interconnected porous structure of the Gel-PD-CMBT scaffold not only provides mechanical stability but also offered ample space for cellular and tissue ingrowth. This Gel-PD-CMBT scaffold demonstrates a greater capacity to promote cellular osteogenic differentiation in vitro and neo-bone formation in vivo. In summary, the Gel-PD-CMBT scaffold, with its integrated piezoelectricity and conductivity, effectively restores the local electroactive microenvironment, offering an ideal platform for the regeneration of electrophysiological bone tissue.

Autoclavable Albumin-Based Cryogels with Uncompromising Properties

The development of autoclavable hydrogels has been driven by the need for materials that can withstand the rigors of sterilization without compromising their properties or functionality. Many conventional hydrogels cannot withstand autoclave treatment owing to the breakdown of their composition or structure under the high-temperature and high-pressure environment of autoclaving. Here, the effect of autoclaving on the physical, mechanical, and biological properties of bovine serum albumin methacryloyl (BSAMA) cryogels at three protein concentrations (3, 5, and 10%) was extensively studied. We found that BSAMA cryogels at three concentrations remained little changed after autoclaving in terms of gross shape, pore structure, and protein secondary structure. Young's modulus of autoclaved BSAMA cryogels (BSAMAA) at low concentrations (3 and 5%) was similar to that of BSAMA cryogels, whereas 10% BSAMAA exhibited a higher Young's modulus value, compared with 10% BSAMA. Interestingly, BSAMAA cryogels prolonged degradation. Importantly, cell viability, drug release, and hemolytic behaviors were found to be similar among the pre- and post-autoclaved cryogels. Above all, autoclaving proved to be more effective in sterilizing BSAMA cryogels from bacteria contamination than UV and ethanol treatments. Thus, autoclavable BSAMA cryogels with uncompromising properties would be useful for biomedical applications.

Mitigating Metal-Organic Framework (MOF) Toxicity for Biomedical Applications

Metal-organic frameworks (MOFs) are a novel class of crystalline porous materials, consisting of metal ions and organic linkers. These hybrid materials possess exceptional porosity and specific surface area, which have recently garnered significant interest due to their potential applications in gas separation and storage, energy storage, biomedical imaging, and drug delivery. As MOFs are being explored for biomedical applications, it is essential to comprehensively assess their toxicity. Although nearly ninety thousand MOFs have been investigated, evaluating and optimizing their physico-chemical properties in relevant biological systems remain critical for their clinical translation. In this review article, we first provide a brief classification of MOFs based on their chemical structures. We then conduct a comprehensive evaluation of in vitro and in vivo studies that assess the biocompatibility of MOFs. Additionally, we discuss various approaches to mitigate the critical factors associated with MOF toxicity. To this end, the effects of chemistry, particle size, morphology, and particle aggregation are examined. To better understand MOFs' potential toxicity to living organisms, we also delve into the toxicity mechanisms of nanoparticles (NPs). Furthermore, we introduce and evaluate strategies such as surface modification to reduce the inherent toxicity of MOFs. Finally, we discuss current challenges, the path to clinical trials, and new research directions.

Designing Lignin-Based Biomaterials as Carriers of Bioactive Molecules

There is a need to develop circular and sustainable economies by utilizing sustainable, green, and renewable resources in high-tech industrial fields especially in the pharmaceutical industry. In the last decade, many derivatives of food and agricultural waste have gained considerable attention due to their abundance, renewability, biocompatibility, environmental amiability, and remarkable biological features. Particularly, lignin, which has been used as a low-grade burning fuel in the past, recently attracted a lot of attention for biomedical applications because of its antioxidant, anti-UV, and antimicrobial properties. Moreover, lignin has abundant phenolic, aliphatic hydroxyl groups, and other chemically reactive sites, making it a desirable biomaterial for drug delivery applications. In this review, we provide an overview of designing different forms of lignin-based biomaterials, including hydrogels, cryogels, electrospun scaffolds, and three-dimensional (3D) printed structures and how they have been used for bioactive compound delivery. We highlight various design criteria and parameters that influence the properties of each type of lignin-based biomaterial and corelate them to various drug delivery applications. In addition, we provide a critical analysis, including the advantages and challenges encountered by each biomaterial fabrication strategy. Finally, we highlight the prospects and future directions associated with the application of lignin-based biomaterials in the pharmaceutical field. We expect that this review will cover the most recent and important developments in this field and serve as a steppingstone for the next generation of pharmaceutical research.

Designing Silk-Based Cryogels for Biomedical Applications

There is a need to develop the next generation of medical products that require biomaterials with improved properties. The versatility of various gels has pushed them to the forefront of biomaterials research. Cryogels, a type of gel scaffold made by controlled crosslinking under subzero or freezing temperatures, have great potential to address many current challenges. Unlike their hydrogel counterparts, which are also able to hold large amounts of biologically relevant fluids such as water, cryogels are often characterized by highly dense and crosslinked polymer walls, macroporous structures, and often improved properties. Recently, one biomaterial that has garnered a lot of interest for cryogel fabrication is silk and its derivatives. In this review, we provide a brief overview of silk-based biomaterials and how cryogelation can be used for novel scaffold design. We discuss how various parameters and fabrication strategies can be used to tune the properties of silk-based biomaterials. Finally, we discuss specific biomedical applications of silk-based biomaterials. Ultimately, we aim to demonstrate how the latest advances in silk-based cryogel scaffolds can be used to address challenges in numerous bioengineering disciplines.

Green approaches for extraction, chemical modification and processing of marine polysaccharides for biomedical applications

Over the past few decades, natural-origin polysaccharides have received increasing attention across different fields of application, including biomedicine and biotechnology, because of their specific physicochemical and biological properties that have afforded the fabrication of a plethora of multifunctional devices for healthcare applications. More recently, marine raw materials from fisheries and aquaculture have emerged as a highly sustainable approach to convert marine biomass into added-value polysaccharides for human benefit. Nowadays, significant efforts have been made to combine such circular bio-based approach with cost-effective and environmentally-friendly technologies that enable the isolation of marine-origin polysaccharides up to the final construction of a biomedical device, thus developing an entirely sustainable pipeline. In this regard, the present review intends to provide an up-to-date outlook on the current green extraction methodologies of marine-origin polysaccharides and their molecular engineering toolbox for designing a multitude of biomaterial platforms for healthcare. Furthermore, we discuss how to foster circular bio-based approaches to pursue the further development of added-value biomedical devices, while preserving the marine ecosystem.

An Engineered Protein-Based Building Block (Albumin Methacryloyl) for Fabrication of a 3D In Vitro Cryogel Model

Drug-induced liver injury (DILI) is a leading cause of attrition in drug development or withdrawal; current animal experiments and traditional 2D cell culture systems fail to precisely predict the liver toxicity of drug candidates. Hence, there is an urgent need for an alternative in vitro model that can mimic the liver microenvironments and accurately detect human-specific drug hepatotoxicity. Here, for the first time we propose the fabrication of an albumin methacryloyl cryogel platform inspired by the liver’s microarchitecture via emulating the mechanical properties and extracellular matrix (ECM) cues of liver. Engineered crosslinkable albumin methacryloyl is used as a protein-based building block for fabrication of albumin cryogel in vitro models that can have potential applications in 3D cell culture and drug screening. In this work, protein modification, cryogelation, and liver ECM coating were employed to engineer highly porous three-dimensional cryogels with high interconnectivity, liver-like stiffness, and liver ECM as artificial liver constructs. The resulting albumin-based cryogel in vitro model provided improved cell–cell and cell–material interactions and consequently displayed excellent liver functional gene expression, being conducive to detection of fialuridine (FIAU) hepatotoxicity.

Shape-Recoverable Macroporous Nanocomposite Hydrogels Created via Ice Templating Polymerization for Noncompressible Wound Hemorrhage

Uncontrolled hemorrhage resulting from severe trauma or surgical operations remains a challenge. It is highly important to develop functional materials to treat noncompressible wound bleeding. In this work, a shape-recoverable macroporous nanocomposite hydrogel was facilely created through ice templating polymerization. The covalently cross-linked gelatin networks provide a robust framework, while the Laponite nanoclay disperses into the three-dimensional matrix, enabling mechanical reinforcement and hemostatic functions. The resultant macroporous nanocomposite hydrogel possesses an inherent interconnected macroporous structure and rapid deformation recovery. In vitro assessments indicate that the hydrogel displays good cytocompatibility and a low hemolysis ratio. The hydrogel shows a higher coagulation potential and more erythrocyte adhesion compared to the commercial gauze and gelatin sponge. The noncompressible liver hemorrhage models also confirm its promising hemostasis performance. This strategy of combining a nano-enabled solution with ice templating polymerization displays great potential to develop appealing absorbable macroporous biomaterials for rapid hemostasis.

Macroporous Silk Nanofiber Cryogels with Tunable Properties

Cryogels are widely used in tissue regeneration due to their porous structures and friendly hydrogel performance. Silk-based cryogels were developed but failed to exhibit desirable tunable properties to adapt various biomedical applications. Here, amorphous short silk nanofibers (SSFs) were introduced to fabricate silk cryogels with versatile cues. Compared to previous silk cryogels, the SSF cryogels prepared under same conditions showed significantly enhanced mechanical properties. The microporous cryogels were achieved under lower silk concentrations, confirming better tunability. Versatile cryogels with the modulus in the range of 0.5-283.7 kPa were developed through adjusting silk concentration and crosslinking conditions, superior to previous silk cryogel systems. Besides better cytocompatibility, the SSF cryogels were endowed with effective mechanical cues to control osteogenetic differentiation behaviors of BMSCs. The mechanical properties could be further regulated finely through the introduction of β-sheet-rich silk nanofibers (SNFs), which suggested possible optimization of mechanical niches. Bioactive cargo-laden SNFs were introduced to the SSF cryogel systems, bringing biochemical signals without the compromise of mechanical properties. Versatile SNF-based cryogels with different physical and biological cues were developed here to facilitate the applications in various tissue engineering.

Shape-Recoverable Hyaluronic Acid-Waterborne Polyurethane Hybrid Cryogel Accelerates Hemostasis and Wound Healing

Wound dressings that promote quick hemostasis and are highly efficient in healing wounds are urgently needed to meet the increase in clinical demands worldwide. Herein, a dihydrazide-modified waterborne biodegradable polyurethane emulsion (PU-ADH) and oxidized hyaluronic acid (OHA) were autonomously cross-linked to form a hybrid hyaluronic acid-polyurethane (HA-PU) cryogel by hydrazone bonding at -20 °C. Through its specific macroporous structure (which is approximately 220 μm) constructed by aggregated PU-ADH particles and long-chain OHA, a dried cryogel can have a dramatically compressed volume (1/7 of its original volume) with stable fixation, and it can swell rapidly by absorbing water or blood to approximately 22 and 16 times its dried weight, respectively, in a few minutes. This instantaneous shape-recovering ability favors fast hemostasis in minimally invasive surgery. Moreover, this cryogel is superior to gauze, has excellent biocompatibility, and quickly coagulates blood (in approximately 2 min) by activating the endogenous coagulation system. Comparably, an injectable HA-PU hydrogel with the same components as the HA-PU cryogel was prepared at room temperature, and it exhibited good self-healing properties. An in vivo evaluation of a rat liver hemostasis model and rat skin defect model revealed that the cryogel in fast hemostasis has great potential and superior wound-healing abilities, decreases immune inflammation, and promotes the regeneration of angiogenesis and hair follicles. Consequently, this work proposes a versatile method for constructing biodegradable hybrid cryogels via autonomous cross-linking between synthesized polymer emulsions and natural polymers. The hybrid cryogels demonstrated great potential for applications as high-performance wound dressings.

Nanoparticle-Stabilized Emulsion Bioink for Digital Light Processing Based 3D Bioprinting of Porous Tissue Constructs

A challenge for bioprinting tissue constructs is enabling the viability and functionality of encapsulated cells. Rationally designed bioink that can create appropriate biophysical cues shows great promise for overcoming such challenges. Here, a nanoparticle-stabilized emulsion bioink for direct fabrication of porous tissue constructs by digital light processing based 3D bioprinting technology is introduced. The emulsion bioink is integrated by the mixture of aqueous dextran microdroplets and gelatin methacryloyl solution and is further rendered stable by β-lactoglobulin nanoparticles. After bioprinting, the printed tissue constructs create the macroporous structure via removal of dextran, thereby providing favorable biophysical cues to promote the viability, proliferation, and spreading of the encapsulated cells. Moreover, a trachea-shaped construct containing chondrocytes is bioprinted and implanted in vivo. The results demonstrate that the generated macroporous construct is of benefit to cartilage tissue rebuilding. This work offers an advanced bioink for the fabrication of living tissue constructs by activating the cell behaviors and functions in situ and can lead to the development of 3D bioprinting.

Engineering hyaluronic acid-based cryogels for CD44-mediated breast tumor reconstruction

Breast cancer is a major health concern worldwide and is the leading cause of cancer-related death among American women. Traditional therapies, such as surgery, chemotherapy, and radiotherapy, are usually ineffective. Furthermore, cancer recurrence following targeted therapy often results from acquired drug resistance. Therefore, more realistic tumor models than monolayer cell culture for drug screening and discovery in an in vitro setting would facilitate the development of new therapeutic strategies. Toward this goal, we first developed a simple, rapid, low-cost, and high-throughput method for generating uniform multi-cellular tumor spheroids (MCTS) with controllable size. Next, biomimetic cryogel scaffolds fabricated from hyaluronic acid (HA) were utilized as a platform to reconstruct breast tumor microtissues with aspects of the complex tumor microenvironment in three dimensions. Finally, we investigated the interactions between the HA-based cryogels and CD44-positive breast tumor cells, individually or as MCTS. We found that incorporating the adhesive RGD peptide in cryogels led to the formation of a monolayer of tumor cells on the polymer walls, whereas MCTS cultured on RGD-free HA cryogels resulted in the growth of large and dense microtumors, more similar to native tumor masses. As a result, the MCTS-laden HA cryogel system induced a highly aggressive and chemotherapy drug-resistant tumor model. RGD-free HA-based cryogels represent an effective starting point for designing tumor models for preclinical research, therapeutic drug screening, and early cancer diagnosis.

Composite Hydrogels of Alkyl Functionalized Gellan Gum Derivative and Hydroxyapatite/Tricalcium Phosphate Nanoparticles as Injectable Scaffolds for bone Regeneration

An alkyl functionalized gellan gum derivative is here used to produce hydrogels containing hydroxyapatite and tricalcium phosphate nanoparticles as injectable nanostructured scaffolds for bone regeneration. The amphiphilic nature of the polysaccharide derivative along with its thermotropic behavior and ionotropic crosslinking features make possible to produce injectable bone mimetic scaffolds that can be used to release viable cells and osteoinductive biomolecules. The influence of different nanoparticles concentration on the rheological and physicochemical properties of the injectable systems is studied. It is found that the presence of inorganic nanoparticles reinforces the 3D hydrated polymeric networks without influencing their injectability but improving the physicochemical properties of ionotropic crosslinked hydrogels produced with two different curing media. Preliminary cytocompatibility tests performed with murine preosteoblast cells revealed that gellan gum based hydrogels can safely encapsulate viable cells. Loading and release experiments for dexamethasone and stromal cell-derived factor-1 demonstrate the drug delivery features of the obtained injectable systems.

Injectable Lignin-co-Gelatin Cryogels with Antioxidant and Antibacterial Properties for Biomedical Applications

For several biomedical applications, it is essential to develop novel bioactive materials. Such biomaterials could potentially improve wound healing, prevent infections, or be used in immunoengineering. For example, bioactive materials that reduce oxidative stress without relying on antibiotics and other drugs could be beneficial. Hydrogel-based biomaterials, especially those derived from natural polymers, have been regarded as one of the most promising scaffolds for biomedical research. These multifunctional scaffolds can exhibit high water adsorption capacity, biocompatibility, and biomechanical properties that can match native tissues. Cryogels are a special type of hydrogels in which polymers are cross-linked around ice crystals. As a result, cryogels exhibit unique physical features, including a macroporous and interconnected network, flexibility, shape-memory properties, and syringe injectability. Herein, we developed a multifunctional, i.e., antibacterial, antioxidant, and injectable cryogel by combining lignin with gelatin. The cryogel with 0.2% lignin showed a compressive modulus of 25 kPa and a compressive stress of 140 kPa at 80% strain, which is, respectively, 1.8 and 7 times higher than those of the pure gelatin cryogels. Meanwhile, such a cryogel formulation could completely recover its shape after compression up to 90% and was needle-injectable. Additionally, the lignin-co-gelatin cryogel with 0.1-0.2 lignin showed 8-10 mm of inhibition zone against the most common surgical site infection-associated pathogenic bacteria. Furthermore, lignin-co-gelatin cryogel was found to scavenge free radicals and have good cytocompatibility, and the cryogels with up to 0.2% lignin minimally activate naïve mouse bone marrow-derived dendritic cells. Overall, the current approach shows great promise for the design of bioresource-based multifunctional cryogels for a wide range of biomedical applications.

Cryogel-Integrated Biochip for Liver Tissue Engineering

Microfluidic systems and polymer hydrogels have been widely developed for tissue engineering. Yet, only a few tools combining both approaches, especially for in vitro liver models, are being explored. In this study, an alginate-based cryogel-integrated biochip was engineered for dynamic hepatoma cell line culture in three dimensions (3D). The alginate cryogel was covalently cross-linked in the biochip at subzero temperatures (T < 0 °C) to create a scaffold with high mechanical stability and an interconnected macroporous network. By varying the alginate concentration and the cross-linker ratio, Young's modulus of the cryogel can be fine-tuned between 1.5 and 29 kPa, corresponding to the range of stiffness of the different physiological states of the liver. We demonstrated that HepG2/C3A cells can be cultured and maintained as viable under dynamic conditions in this device up to 6 days. Albumin synthesis and glucose consumption increased over the cell culture days. Moreover, a 3D cell structure was observed across the entire height of the biochip, which was preserved following alginate lyase treatment to remove the cryogel-based scaffold. In summary, these results represent a proof of concept of an interesting cell culture technology that should be further investigated to engineer healthy and cirrhotic liver models.

3D-Printed Hydrogel-Filled Microneedle Arrays

Microneedle arrays (MNAs) have been used for decades to deliver drugs transdermally and avoid the obstacles of other delivery routes. Hydrogels are another popular method for delivering therapeutics because they provide tunable, controlled release of their encapsulated payload. However, hydrogels are not strong or stiff, and cannot be formed into constructs that penetrate the skin. Accordingly, it has so far been impossible to combine the transdermal delivery route provided by MNAs with the therapeutic encapsulation potential of hydrogels. To address this challenge, a low cost and simple, but robust, strategy employing MNAs is developed. These MNAs are formed from a rigid outer layer, 3D printed onto a conformal backing, and filled with drug-eluting hydrogels. Microneedles of different lengths are fabricated on a single patch, facilitating the delivery of various agents to different tissue depths. In addition to spatial distribution, temporal release kinetics can be controlled by changing the hydrogel composition or the needles' geometry. As a proof-of-concept, MNAs are used for the delivery of vascular endothelial growth factor (VEGF). Application of the rigid, resin-based outer layer allows the use of hydrogels regardless of their mechanical properties and makes these multicomponent MNAs suitable for a range of drug delivery applications.

Design and Assessment of Biodegradable Macroporous Cryogels as Advanced Tissue Engineering and Drug Carrying Materials

Cryogels obtained by the cryotropic gelation process are macroporous hydrogels with a well-developed system of interconnected pores and shape memory. There have been significant recent advancements in our understanding of the cryotropic gelation process, and in the relationship between components, their structure and the application of the cryogels obtained. As cryogels are one of the most promising hydrogel-based biomaterials, and this field has been advancing rapidly, this review focuses on the design of biodegradable cryogels as advanced biomaterials for drug delivery and tissue engineering. The selection of a biodegradable polymer is key to the development of modern biomaterials that mimic the biological environment and the properties of artificial tissue, and are at the same time capable of being safely degraded/metabolized without any side effects. The range of biodegradable polymers utilized for cryogel formation is overviewed, including biopolymers, synthetic polymers, polymer blends, and composites. The paper discusses a cryotropic gelation method as a tool for synthesis of hydrogel materials with large, interconnected pores and mechanical, physical, chemical and biological properties, adapted for targeted biomedical applications. The effect of the composition, cross-linker, freezing conditions, and the nature of the polymer on the morphology, mechanical properties and biodegradation of cryogels is discussed. The biodegradation of cryogels and its dependence on their production and composition is overviewed. Selected representative biomedical applications demonstrate how cryogel-based materials have been used in drug delivery, tissue engineering, regenerative medicine, cancer research, and sensing.

Injectable Cryogels Associate with Adipose-Derived Stem Cells for Cardiac Healing After Acute Myocardial Infarctions

Treatment of adipose-derived stem cells (ADSCs) provides support for novel methods of conveying baseline cell protein endothelial cells to promote acute myocardial infarction in gelatin sericin (GS) lamin-coated antioxidant systems (GS@L). The ratio of fixity modules, pores, absorption, and inflammation in the range of ka (65 ka), 149 ±39.8 μm, 92.2%, 42 ± 1.38, and 29 ± 1.9 were observed in the synthesized frames for GS. Herein, ADSC-GS@L was prepared, and the relevant substance for the development of cardiac regenerative applications was stable and physically chemical. In vitro assessments of ADSC-GS@L injectable cryogels established the enhanced survival rates of the cell and improved pro- angiogenic factors as well as pro-inflammatory expression, confirming the favorable outcomes of fractional ejections, fibro-areas, and vessel densities with reduced infraction dimensions. The novel ADSC-injecting cryogel method could be useful for successful heart injury therapies during acute myocardial infarction. Additionally, the method could be useful for successful heart injury therapies during coronary heart disease.

Engineering a macroporous fibrin-based sequential interpenetrating polymer network for dermal tissue engineering

The success of skin tissue engineering for deep wound healing relies predominantly on the design of innovative and effective biomaterials. This study reports the synthesis and characterization of a new type of naturally-derived and macroporous interpenetrating polymer network (IPN) for skin repair. These biomaterials consist of a biologically active fibrous fibrin network polymerized within a mechanically robust and macroporous construct made of polyethylene glycol and biodegradable serum albumin (PEGDM-co-SAM). First, mesoporous PEGDM-co-SAM hydrogels were synthesized and subjected to cryotreatment to introduce an interconnected macroporous network. Subsequently, fibrin precursors were incorporated within the cryotreated PEG-based network and then allowed to spontaneously polymerize and form a sequential IPN. Rheological measurements indicated that fibrin-based sequential IPN hydrogels exhibited improved and tunable mechanical properties when compared to fibrin hydrogels alone. In vitro data showed that human dermal fibroblasts adhere, infiltrate and proliferate within the IPN constructs, and were able to secrete endogenous extracellular matrix proteins, namely collagen I and fibronectin. Furthermore, a preclinical study in mice demonstrated that IPNs were stable over 1-month following subcutaneous implantation, induced a minimal host inflammatory response, and displayed a substantial cellular infiltration and tissue remodeling within the constructs. Collectively, these data suggest that macroporous and mechanically reinforced fibrin-based sequential IPN hydrogels are promising three-dimensional platforms for dermal tissue regeneration.

Harnessing biomaterials for therapeutic strategies against COVID-19

With the emergence of the coronavirus disease 2019 (COVID-19), the world is experiencing a profound human health crisis. The number of infections and deaths due to the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) continues to increase every minute, pinpointing major shortcomings in our ability to prevent viral outbreaks. Although several COVID-19 vaccines have been recently approved for emergency use, therapeutic options remain limited, and their long-term potency has yet to be validated. Biomaterials science has a pivotal role to play in pushing the boundaries of emerging technologies for antiviral research and treatment. In this perspective, we discuss how biomaterials can be harnessed to develop accurate COVID-19 infection models, enhance antiviral drug delivery, foster new antiviral strategies, and boost vaccine efficacy. These efforts will not only contribute to stop or mitigate the current pandemic but will also provide unorthodox platforms to understand, prevent, and protect us from future viral outbreaks.

Injectable Cryogels in Biomedicine

Cryogels are interconnected macroporous materials that are synthesized from a monomer solution at sub-zero temperatures. Cryogels, which are used in various applications in many research areas, are frequently used in biomedicine applications due to their excellent properties, such as biocompatibility, physical resistance and sensitivity. Cryogels can also be prepared in powder, column, bead, sphere, membrane, monolithic, and injectable forms. In this review, various examples of recent developments in biomedical applications of injectable cryogels, which are currently scarce in the literature, made from synthetic and natural polymers are discussed. In the present review, several biomedical applications of injectable cryogels, such as tissue engineering, drug delivery, therapeutic, therapy, cell transplantation, and immunotherapy, are emphasized. Moreover, it aims to provide a different perspective on the studies to be conducted on injectable cryogels, which are newly emerging trend.

Hyaluronic Acid-Based Shape-Memory Cryogel Scaffolds for Focal Cartilage Defect Repair

Traumatic joint injuries can result in significant cartilage defects, which can greatly increase the risk of osteoarthritis development. Due to the limited self-healing capacity of avascular cartilage, tissue engineering approaches are required for filling defects and promoting cartilage regeneration. Current approaches utilize invasive surgical procedures for extraction and implantation of autologous chondrocytes; therefore, injectable biomaterials have gained interest to minimize the risk of infection as well as patient pain and discomfort. In this study, we engineered biomimetic, hyaluronic acid (HA)-based cryogel scaffolds that possess shape-memory properties as they contract and regain their shape after syringe injection to noninvasively fill cartilage defects. The cryogels, fabricated with HA and glycidyl methacrylate at -20°C, resulted in an elastic, macroporous, and highly interconnected network that provided a conducive microenvironment for chondrocytes to remain viable and metabolically active after injection through a syringe needle. Chondrocytes seeded within cryogels and cultured for 15 days exhibited enhanced cell proliferation, metabolism, and production of cartilage extracellular matrix glycosaminoglycans compared with HA-based hydrogels. Furthermore, immunohistochemical staining revealed production of collagen type II from chondrocyte-seeded cryogels, indicating the maintenance of cell phenotype. These results demonstrate the potential of chondrocyte-seeded, HA-based, injectable cryogel scaffolds to promote regeneration of cartilage tissue for nonsurgically invasive defect repair. Impact statement Hyaluronic acid-based shape-memory cryogels provide a conducive microenvironment for chondrocyte adhesion, proliferation, and matrix biosynthesis for use in repair of cartilage defects. Due to their sponge-like elastic properties, cryogels can fully recover their original shape back after injection while not impacting metabolism or viability of encapsulated cells. Clinically, they provide an opportunity for filling focal cartilage defects by using a single, minimally invasive injection of a cell encapsulating biocompatible three-dimensional scaffold that can return to its original structure to fit the defect geometry and enable matrix regeneration.

Implantable and Injectable Biomaterial Scaffolds for Cancer Immunotherapy

Cancer immunotherapy has become an emerging strategy recently producing durable immune responses in patients with varieties of malignant tumors. However, the main limitation for the broad application of immunotherapies still to reduce side effects by controlling and regulating the immune system. In order to improve both efficacy and safety, biomaterials have been applied to immunotherapies for the specific modulation of immune cells and the immunosuppressive tumor microenvironment. Recently, researchers have constantly developed biomaterials with new structures, properties and functions. This review provides the most recent advances in the delivery strategies of immunotherapies based on localized biomaterials, focusing on the implantable and injectable biomaterial scaffolds. Finally, the challenges and prospects of applying implantable and injectable biomaterial scaffolds in the development of future cancer immunotherapies are discussed.

Needle-injectable microcomposite cryogel scaffolds with antimicrobial properties

Porous three-dimensional hydrogel scaffolds have an exquisite ability to promote tissue repair. However, because of their high water content and invasive nature during surgical implantation, hydrogels are at an increased risk of bacterial infection. Recently, we have developed elastic biomimetic cryogels, an advanced type of polymeric hydrogel, that are syringe-deliverable through hypodermic needles. These needle-injectable cryogels have unique properties, including large and interconnected pores, mechanical robustness, and shape-memory. Like hydrogels, cryogels are also susceptible to colonization by microbial pathogens. To that end, our minimally invasive cryogels have been engineered to address this challenge. Specifically, we hybridized the cryogels with calcium peroxide microparticles to controllably produce bactericidal hydrogen peroxide. Our novel microcomposite cryogels exhibit antimicrobial properties and inhibit antibiotic-resistant bacteria (MRSA and Pseudomonas aeruginosa), the most common cause of biomaterial implant failure in modern medicine. Moreover, the cryogels showed negligible cytotoxicity toward murine fibroblasts and prevented activation of primary bone marrow-derived dendritic cells ex vivo. Finally, in vivo data suggested tissue integration, biodegradation, and minimal host inflammatory responses when the antimicrobial cryogels, even when purposely contaminated with bacteria, were subcutaneously injected in mice. Collectively, these needle-injectable microcomposite cryogels show great promise for biomedical applications, especially in tissue engineering and regenerative medicine.

Electroconductive Hydrogels for Tissue Engineering: Current Status and Future Perspectives

Over the past decade, electroconductive hydrogels, integrating both the biomimetic attributes of hydrogels and the electrochemical properties of conductive materials, have gained significant attention. Hydrogels, three-dimensional and swollen hydrophilic polymer networks, are an important class of tissue engineering (TE) scaffolds owing to their microstructural and mechanical properties, ability to mimic the native extracellular matrix, and promote tissue repair. However, hydrogels are intrinsically insulating and therefore unable to emulate the complex electrophysiological microenvironment of cardiac and neural tissues. To overcome this challenge, electroconductive materials, including carbon-based materials, nanoparticles, and polymers, have been incorporated within nonconductive hydrogels to replicate the electrical and biological characteristics of biological tissues. This review gives a brief introduction on the rational design of electroconductive hydrogels and their current applications in TE, especially for neural and cardiac regeneration. The recent progress and development trends of electroconductive hydrogels, their challenges, and clinical translatability, as well as their future perspectives, with a focus on advanced manufacturing technologies, are also discussed.