Biomimetic macroporous hydrogels: protein ligand distribution and cell response to the ligand architecture in the scaffold

Tailoring of Physical Properties in Macroporous Poly(isocyanopeptide) Cryogels

Over the years, synthetic hydrogels have proven remarkably useful as cell culture matrixes to elucidate the role of the extracellular matrix (ECM) on cell behavior. Yet, their lack of interconnected macropores undermines the widespread use of hydrogels in biomedical applications. To overcome this limitation, cryogels, a class of macroporous hydrogels, are rapidly emerging. Here, we introduce a new, highly elastic, and tunable synthetic cryogel, based on poly(isocyanopeptides) (PIC). Introduction of methacrylate groups on PIC facilitated cryopolymerization through free-radical polymerization and afforded cryogels with an interconnected macroporous structure. We investigated which cryogelation parameters can be used to tune the architectural and mechanical properties of the PIC cryogels by systematically altering cryopolymerization temperature, polymer concentration, and polymer molecular weight. We show that for decreasing cryopolymerization temperatures, there is a correlation between cryogel pore size and stiffness. More importantly, we demonstrate that by simply varying the polymer concentration, we can selectively tune the compressive strength of PIC cryogels without affecting their architecture. This unique feature is highly useful for biomedical applications, as it facilitates decoupling of stiffness from other variables such as pore size. As such, PIC cryogels provide an interesting new biomaterial for scientists to unravel the role of the ECM in cellular functions.

Effect of Ethylene Oxide Sterilization on Polyvinyl Alcohol Hydrogel Compared with Gamma Radiation

This study investigated the effects of terminal sterilization of polyvinyl alcohol (PVA) biomaterials using clinically translatable techniques, specifically ethylene oxide (EtO) and gamma (γ) irradiation. While a few studies have reported the possibility of sterilizing PVA with γ-radiation, the use of EtO sterilization of PVA requires additional study. PVA solutions were chemically crosslinked with trisodium trimetaphosphate and sodium hydroxide. The three experimental groups included untreated control, EtO, and γ-irradiation, which were tested for the degree of swelling and water content, and mechanical properties such as radial compliance, longitudinal tensile, minimum bend radius, burst pressure, and suture retention strength. In addition, samples were characterized with scanning electron microscopy, differential scanning calorimetry, X-ray photoelectron spectroscopy, and water contact angle measurements. Cell attachment was assessed using the endothelial cell line EA.hy926, and the sterilized PVA cytotoxicity was studied with a live/dead stain. Platelet and fibrin accumulation was measured using an ex vivo shunt baboon model. Finally, the immune responses of PVA implants were analyzed after a 21-day subcutaneous implantation in rats and a 30-day implantation in baboon. EtO sterilization reduced the PVA graft wall thickness, its degree of swelling, and water content compared with both γ-irradiated and untreated PVA. Moreover, EtO sterilization significantly reduced the radial compliance and increased Young's modulus. EtO did not change PVA hydrophilicity, while γ-irradiation increased the water contact angle of the PVA. Consequently, endothelial cell attachment on the EtO-sterilized PVA showed similar results to the untreated PVA, while cell attachment significantly improved on the γ-irradiated PVA. When exposing the PVA grafts to circulating whole blood, fibrin accumulation of EtO-sterilized PVA was found to be significantly lower than γ-irradiated PVA. The immune responses of γ-irradiated PVA, EtO-treated PVA, and untreated PVA were compared. Implanted EtO-treated PVA showed the least MAC387 reaction. The terminal sterilization methods in this study changed PVA hydrogel properties; nevertheless, based on the characterizations performed, both sterilization methods were suitable for sterilizing PVA. We concluded that EtO can be used as an alternative method to sterilize PVA hydrogel material. Impact statement Polyvinyl alcohol (PVA) hydrogels have been used for a variety of tissue replacements, including neural, cardiac, meniscal, cartilage, muscle, pancreatic, and ocular applications. In addition, PVA can be made into a tubular shape and used as a small-diameter vascular graft. Ethylene oxide (EtO) is one of the Food and Drug Administration-approved methods for sterilization, but its effect on PVA has not been studied extensively. The outcome of this study provides the effects of EtO and γ-irradiation of PVA grafts on both the material properties and the in vivo responses, particularly for vascular applications. Knowledge of these effects may ultimately improve the success rate of PVA vascular grafts.

Bioengineering Models for Breast Cancer Research

Despite substantial advances in early diagnosis, breast cancer (BC) still remains a clinical challenge. Most BC models use complex in vivo models and two-dimensional monolayer cultures that do not fully mimic the tumor microenvironment. The integration of cancer biology and engineering can lead to the development of novel in vitro approaches to study BC behavior and quantitatively assess different features of the tumor microenvironment that may influence cell behavior. In this review, we present tissue engineering approaches to model BC in vitro. Recent advances in the use of three-dimensional cell culture models to study various aspects of BC disease in vitro are described. The emerging area of studying BC dormancy using these models is also reviewed.

New adipose tissue formation by human adipose-derived stem cells with hyaluronic acid gel in immunodeficient mice

BACKGROUND

Currently available injectable fillers have demonstrated limited durability. This report proposes the in vitro culture of human adipose-derived stem cells (hASCs) on hyaluronic acid (HA) gel for in vivo growth of de novo adipose tissue.

METHODS

For in vitro studies, hASCs were isolated from human adipose tissue and were confirmed by multi-lineage differentiation and flow cytometry. hASCs were cultured on HA gel. The effectiveness of cell attachment and proliferation on HA gel was surveyed by inverted light microscopy. For in vivo studies, HA gel containing hASCs, hASCs without HA gel, HA gel alone were allocated and subcutaneously injected into the subcutaneous pocket in the back of nude mice (n=6) in each group. At eight weeks post-injection, the implants were harvested for histological examination by hematoxylin and eosin (H&E) stain, Oil-Red O stain and immunohistochemical staining. The human-specific Alu gene was examined.

RESULTS

hASCs were well attachment and proliferation on the HA gel. In vivo grafts showed well-organized new adipose tissue on the HA gel by histologic examination and Oil-Red O stain. Analysis of neo-adipose tissues by PCR revealed the presence of the Alu gene. This study demonstrated not only the successful culture of hASCs on HA gel, but also their full proliferation and differentiation into adipose tissue.

CONCLUSIONS

The efficacy of injected filler could be permanent since the reduction of the volume of the HA gel after bioabsorption could be replaced by new adipose tissue generated by hASCs. This is a promising approach for developing long lasting soft tissue filler.

The in vitro characterization of a gelatin scaffold, prepared by cryogelation and assessed in vivo as a dermal replacement in wound repair

A sheet gelatin scaffold with attached silicone pseudoepidermal layer for wound repair purposes was produced by a cryogelation technique. The resulting scaffold possessed an interconnected macroporous structure with a pore size distribution of 131 ± 17 μm at one surface decreasing to 30 ± 8 μm at the attached silicone surface. The dynamic storage modulus (G') and mechanical stability were comparable to the clinical gold standard dermal regeneration template, Integra®. The scaffolds were seeded in vitro with human primary dermal fibroblasts. The gelatin based material was not only non-cytotoxic, but over a 28 day culture period also demonstrated advantages in cell migration, proliferation and distribution within the matrix when compared with Integra®. When seeded with human keratinocytes, the neoepidermal layer that formed over the cryogel scaffold appeared to be more advanced and mature when compared with that formed over Integra®. The in vivo application of the gelatin scaffold in a porcine wound healing model showed that the material supports wound healing by allowing host cellular infiltration, biointegration and remodelling. The results of our in vitro and in vivo studies suggest that the gelatin based scaffold produced by a cryogelation technique is a promising material for dermal substitution, wound healing and other potential biomedical applications.

Processing of gelatin-based cryogels with improved thermomechanical resistance, pore size gradient, and high potential for sustainable protein drug release

Porous gelatin (GEL) cryogels were processed by spatiotemporal and temperature-controlled gelation and freezing-lyophilizaton process, followed by zero-length crosslinking, using different molarities of reagents (EDC and NHS) and reaction media (100% PBS or 20/80% PBS/EtOH mixture) for variable time extensions (1-24 h). In this way, tuneable cryogels with gradient microporosity (from 100 µm to 1000 µm) were formed, being mainly influenced by crosslinkers' concentration and EtOH addition. Later affect the pore morphology (from round to ellipsoid), consequently modulating the steady-state physiological swelling profile toward twice lower values (∼ 600%) comparing to stepwise swelling of in 100% PBS media crosslinked cryogels. While the presence of EtOH decelerate the crosslinking kinetic by retaining cryogels' microstructure formed during freezing, the 100% PBS and higher EDC molarity resulted in approximately 40% crosslinking degree, being expressed as a thermal resistance of cryogels up to approximately 73°C. Finally, the tuneable enzymatic resistance allow time-dependent poly-L-Lysine (pL) release profile in up to month period. The processed GEL cryogels have potential in broad range biomedical applications, especially as sustainable, protein-based drug delivery systems.

Microengineered tumor models: insights & opportunities from a physical sciences-oncology perspective

Prevailing evidence has established the fundamental role of microenvironmental conditions in tumorigenesis. However, the ability to identify, interrupt, and translate the underlying cellular and molecular mechanisms into meaningful therapies remains limited, due in part to a lack of organotypic culture systems that accurately recapitulate tumor physiology. Integration of tissue engineering with microfabrication technologies has the potential to address this challenge and mimic tumor heterogeneity with pathological fidelity. Specifically, this approach allows recapitulating global changes of tissue-level phenomena, while also controlling microscale variability of various conditions including spatiotemporal presentation of soluble signals, biochemical and physical characteristics of the extracellular matrix, and cellular composition. Such platforms have continued to elucidate the role of the microenvironment in cancer pathogenesis and significantly improve drug discovery and screening, particularly for therapies that target tumor-enabling stromal components. This review discusses some of the landmark efforts in the field of micro-tumor engineering with a particular emphasis on deregulated tissue organization and mass transport phenomena in the tumor microenvironment.

Cryogels: morphological, structural and adsorption characterisation

Experimental results on polymer, protein, and composite cryogels and data treatment methods used for morphological, textural, structural, adsorption and diffusion characterisation of the materials are analysed and compared. Treatment of microscopic images with specific software gives quantitative structural information on both native cryogels and freeze-dried materials that is useful to analyse the drying effects on their structure. A combination of cryoporometry, relaxometry, thermoporometry, small angle X-ray scattering (SAXS), equilibrium and kinetic adsorption of low and high-molecular weight compounds, diffusion breakthrough of macromolecules within macroporous cryogel membranes, studying interactions of cells with cryogels provides a consistent and comprehensive picture of textural, structural and adsorption properties of a variety of cryogels. This analysis allows us to establish certain regularities in the cryogel properties related to narrow (diameter 0.4<d<2 nm), middle (2<d<50 nm) and broad (50<d<100 nm) nanopores, micropores (100 nm<d<100 μm) and macropores (d>100 μm) with boundary sizes within modified life science pore classification. Particular attention is paid to water bound in cryogels in native superhydrated or freeze-dried states. At least, five states of water - free unbound, weakly bound (changes in the Gibbs free energy-ΔG<0.5-0.8 kJ/mol) and strongly bound (-ΔG>0.8 kJ/mol), and weakly associated (chemical shift of the proton resonance δ(H)=1-2 ppm) and strongly associated (δ(H)=3-6 ppm) waters can be distinguished in hydrated cryogels using (1)H NMR, DSC, TSDC, TG and other methods. Different software for image treatment or developed to analyse the data obtained with the adsorption, diffusion, SAXS, cryoporometry and thermoporometry methods and based on regularisation algorithms is analysed and used for the quantitative morphological, structural and adsorption characterisation of individual and composite cryogels, including polymers filled with solid nano- or microparticles.

Engineering hydrogels as extracellular matrix mimics

Extracellular matrix (ECM) is a complex cellular environment consisting of proteins, proteoglycans, and other soluble molecules. ECM provides structural support to mammalian cells and a regulatory milieu with a variety of important cell functions, including assembling cells into various tissues and organs, regulating growth and cell-cell communication. Developing a tailored in vitro cell culture environment that mimics the intricate and organized nanoscale meshwork of native ECM is desirable. Recent studies have shown the potential of hydrogels to mimic native ECM. Such an engineered native-like ECM is more likely to provide cells with rational cues for diagnostic and therapeutic studies. The research for novel biomaterials has led to an extension of the scope and techniques used to fabricate biomimetic hydrogel scaffolds for tissue engineering and regenerative medicine applications. In this article, we detail the progress of the current state-of-the-art engineering methods to create cell-encapsulating hydrogel tissue constructs as well as their applications in in vitro models in biomedicine.