A gellan-based fluid gel carrier to enhance topical spray delivery

Autologous cell transplantation was introduced to clinical practice nearly four decades ago to enhance burn wound re-epithelialisation. Autologous cultured or uncultured cells are often delivered to the surface in saline-like suspensions. This delivery method is limited because droplets of the sprayed suspension form upon deposition and run across the wound bed, leading to uneven coverage and cell loss. One way to circumvent this problem would be to use a gel-based material to enhance surface retention. Fibrin systems have been explored as co-delivery system with keratinocytes or as adjunct to 'seal' the cells following spray delivery, but the high costs and need for autologous blood has impeded its widespread use. Aside from fibrin gel, which can exhibit variable properties, it has not been possible to develop a gel-based carrier that solidifies on the skin surface. This is because it is challenging to develop a material that is sprayable but gels on contact with the skin surface. The manuscript reports the use of an engineered carrier device to deliver cells via spraying, to enhance retention upon a wound. The device involves shear-structuring of a gelling biopolymer, gellan, during the gelation process; forming a yield-stress fluid with shear-sensitive behaviours, known as a fluid gel. In this study, a formulation of gellan gum fluid gels are reported, formed with from 0.75 or 0.9% (w/v) polymer and varying the salt concentrations. The rheological properties and the propensity of the material to wet a surface were determined for polymer modified and non-polymer modified cell suspensions. The gellan fluid gels had a significantly higher viscosity and contact angle when compared to the non-polymer carrier. Viability of cells was not impeded by encapsulation in the gellan fluid gel or spraying. The shear thinning property of the material enabled it to be applied using an airbrush and spray angle, distance and air pressure were optimised for coverage and viability. STATEMENT OF SIGNIFICANCE: Spray delivery of skin cells has successfully translated to clinical practice. However, it has not yet been widely accepted due to limited retention and disputable cell viability in the wound. Here, we report a method for delivering cells onto wound surfaces using a gellan-based shear-thinning gel system. The viscoelastic properties allow the material to liquefy upon spraying and restructure rapidly on the surface. Our results demonstrate reduced run-off from the surface compared to currently used low-viscosity cell carriers. Moreover, encapsulated cells remain viable throughout the process. Although this paper studies the encapsulation of one cell type, a similar approach could potentially be adopted for other cell types. Our data supports further studies to confirm these results in in vivo models.

Structuring of Hydrogels across Multiple Length Scales for Biomedical Applications

The development of new materials for clinical use is limited by an onerous regulatory framework, which means that taking a completely new material into the clinic can make translation economically unfeasible. One way to get around this issue is to structure materials that are already approved by the regulator, such that they exhibit very distinct physical properties and can be used in a broader range of clinical applications. Here, the focus is on the structuring of soft materials at multiple length scales by modifying processing conditions. By applying shear to newly forming materials, it is possible to trigger molecular reorganization of polymer chains, such that they aggregate to form particles and ribbon-like structures. These structures then weakly interact at zero shear forming a solid-like material. The resulting self-healing network is of particular use for a range of different biomedical applications. How these materials are used to allow the delivery of therapeutic entities (cells and proteins) and as a support for additive layer manufacturing of larger-scale tissue constructs is discussed. This technology enables the development of a range of novel materials and structures for tissue augmentation and regeneration.

Advances in keratinocyte delivery in burn wound care

This review gives an updated overview on keratinocyte transplantation in burn wounds concentrating on application methods and future therapeutic cell delivery options with a special interest in hydrogels and spray devices for cell delivery. To achieve faster re-epithelialisation of burn wounds, the original autologous keratinocyte culture and transplantation technique was introduced over 3 decades ago. Application types of keratinocytes transplantation have improved from cell sheets to single-cell solutions delivered with a spray system. However, further enhancement of cell culture, cell viability and function in vivo, cell carrier and cell delivery systems remain themes of interest. Hydrogels such as chitosan, alginate, fibrin and collagen are frequently used in burn wound care and have advantageous characteristics as cell carriers. Future approaches of keratinocyte transplantation involve spray devices, but optimisation of application technique and carrier type is necessary.