We present a novel indentation method for characterizing the viscoelastic properties of alginate and agarose hydrogel based constructs, which are often used as a model system of soft biological tissues. A sensitive long working distance microscope was used for measuring the time-dependent deformation of the thin circular hydrogel membranes under a constant load. The deformation of the constructs was measured laterally. The elastic modulus as a function of time can be determined by a large deformation theory based on Mooney-Rivlin elasticity. A viscoelastic theory, Zener model, was applied to correlate the time-dependent deformation of the constructs with various gel concentrations, and the creep parameters can therefore be quantitatively estimated. The value of Young's modulus was shown to increase in proportion with gel concentration. This finding is consistent with other publications. Our results also showed the great capability of using the technique to measure gels with incorporated corneal stromal cells. This study demonstrates a novel and convenient technique to measure mechanical properties of hydrogel in a non-destructive, online and realtime fashion. Thus this novel technique can become a valuable tool for soft tissue engineering.

The development of hydrogel-based biomaterials represents a promising approach to generating new strategies for tissue engineering and regenerative medicine. In order to develop more sophisticated cell-seeded hydrogel constructs, it is important to understand how cells mechanically interact with hydrogels. In this paper, we review the mechanisms by which cells remodel hydrogels, the influence that the hydrogel mechanical and structural properties have on cell behaviour and the role of mechanical stimulation in cell-seeded hydrogels. Cell-mediated remodelling of hydrogels is directed by several cellular processes, including adhesion, migration, contraction, degradation and extracellular matrix deposition. Variations in hydrogel stiffness, density, composition, orientation and viscoelastic characteristics all affect cell activity and phenotype. The application of mechanical force on cells encapsulated in hydrogels can also instigate changes in cell behaviour. By improving our understanding of cell-material mechano-interactions in hydrogels, this should enable a new generation of regenerative medical therapies to be developed.

tissue-derived decellularized biomaterials are ideal for tissue engineering applications as they mimic the biochemical composition of the native tissue. these materials can be used as hydrogels for cell encapsulation and delivery. the decellularization process can alter the composition of the extracellular matrix (ECM) and thus influence the hydrogels characteristics. The aim of this study was to examine the impact of decellularization protocols in ecM-derived hydrogels obtained from porcine corneas. Porcine corneas were isolated and decellularized with SDS, Triton X-100 or by freeze-thaw cycles. All decellularization methods decreased DNA significantly when measured by PicoGreen and visually assessed by the absence of cell nuclei. Collagen and other ECM components were highly retained, as quantified by hydroxyproline content and sGAG, by histological analysis and by SDS-PAGE. Hydrogels obtained by freeze-thaw decellularization were the most transparent. The method of decellularization impacted gelation kinetics assessed by turbidimetric analysis. All hydrogels showed a fibrillary and porous structure determined by cryoSEM. Human corneal stromal cells were embedded in the hydrogels to assess cytotoxicity. SDS decellularization rendered cytotoxic hydrogels, while the other decellularization methods produced highly cytocompatible hydrogels. Freeze-thaw decellularization produced hydrogels with the overall best properties.The extracellular matrix (ECM) is primarily composed of structural and regulatory proteins and polysaccharides and is generated and maintained by cells. Many cellular functions, such as proliferation, migration or differentiation are regulated by the ECM 1 . Each organ and tissue is composed of a distinctive ECM, in its biochemical composition and structural organization. The properties of ECM are important in the fields of tissue engineering and regenerative medicine, which often aim to replicate the composition and structure of the ECM. By using synthetic or natural materials, three-dimensional scaffolds can be fabricated to repair or restore damaged organs and tissues.One popular approach to generating scaffolds that try to imitate the tissues or organs ECM characteristics is to use decellularization. This technique involves the removal of cellular components from a tissue so that only the ECM remains. Many methods have been examined for performing decellularization and these can be divided into three main categories: physical, chemical and biological 2 . Physical methods include freeze-thawing cycles 3-6 , high hydrostatic pressure 7-9 or supercritical CO 2 10-12 . Chemical agents can involve ionic detergents, such as sodium dodecyl sulphate (SDS) 13,14 or sodium deoxycholate 15 ; non-ionic detergents, such as Triton X-100 16 ; hypertonic or hypotonic salt solutions, such as sodium chloride 17,18 ; and acids and bases, such as peracetic acid 19 or ammonium hydroxide 20 . Enzymes such as trypsin, dispase and phospholipase A2 have been used as biological methods for decellularization 21,22 . Furt...

Control and maintenance of the keratocyte phenotype is vital to developing in vitro tissue engineered strategies for corneal repair. In this study the influence of topographical and chemical cues on the mechanical, phenotypical and genotypical behaviour of adult human derived corneal stromal (AHDCS) cells in three dimensional (3D) multi‐layered organised constructs is examined. Topographical cues are provided via multiple aligned electrospun nanofiber meshes, which are arranged orthogonally throughout the constructs and are capable of aligning individual cells and permitting cell migration between the layers. The influence of chemical cues is examined using different supplements in culture media. A non‐destructive indentation technique and optical coherence tomography are used to determine the matrix elasiticity (elastic modulus) and dimensional changes, respectively. These measurements were indicative of changes in cell phenotype from contractile fibroblasts to quiescent keratocytes over the duration of the experiment and corroborated by qPCR. Constructs containing nanofibers have a higher initial modulus, reduced contraction and organised cell orientation compared to those without nanofibers. Cell‐seeded constructs cultured in serum‐containing media increased in modulus throughout the culture period and underwent significantly more contraction than constructs cultured in serum‐free and insulin‐containing media. This implies that the growth factors present in serum promote a fibroblast‐like phenotype; qPCR data further validates these observations. These results indicate that the synergistic effect of nanofibers and serum‐free media plus insulin supplementation provide the most suitable topographical and chemical environment for reverting corneal fibroblasts to a keratocyte phenotype in a 3D construct.

Corneal blindness is one of the most common causes of vision loss worldwide, affecting millions of people. To treat these patients, researchers have been examining different approaches to engineer corneal scaffolds suitable for transplantation. Scaffolds have been developed to replace part or all of the cornea depending on the patient requirements. Both acellular and cell-seeded scaffolds have been tested in animal models. Materials that have been under investigation for manufacturing scaffolds include collagen, silk fibroin, amniotic membrane, decellularized cornea, fibrin, chitosan, gelatin, agarose, alginate, and hyaluronic acid in addition to several synthetic polymers. Different combinations of materials, fiber crosslinking techniques, and incorporation of bioactive molecules have also been examined. Factors such as the physical properties, cytocompatibility, degradation behavior, and optical characteristics have to be considered when selecting a suitable scaffold material. Recent advancements in materials fabrication techniques such as bioprinting, electrospinning, and different collagen alignment techniques, allow scaffolds to be generated that more accurately mimic the structure of the corneal stroma. A number of scaffolds have commenced clinical trials to determine their suitability for corneal regeneration.

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Given the hazardous nature of many materials and substances, ocular toxicity testing is required to evaluate the dangers associated with these substances after their exposure to the eye. Historically, animal tests such as the Draize test were exclusively used to determine the level of ocular toxicity by applying a test substance to a live rabbit's eye and evaluating the biological response. In recent years, legislation in many developed countries has been introduced to try to reduce animal testing and promote alternative techniques. These techniques include ex vivo tests on deceased animal tissue, computational models that use algorithms to apply existing data to new chemicals and in vitro assays based on two dimensional (2D) and three dimensional (3D) cell culture models. Here we provide a comprehensive overview of the latest advances in ocular toxicity testing techniques, and discuss the regulatory framework used to evaluate their suitability.

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