A hybrid thermo-responsive polymer system for application in wound healing
The understanding of wound healing in the current decade has evolved from merely recognizing the three phases of the wound healing cascade, to having to take into consideration the complex cellular reactions and dynamic processes that promote the physiological wound healing process. Wounds are a global burden, and inadequate wound care has detrimental outcomes in terms of the patient’s quality of life, increased hospitalizations due to infections, and added financial strain on the economy. Traditional wound management systems focus primarily on covering the wound and preventing infection. While this is important, these systems play a limited role in actively promoting the wound healing process. A better approach would be to develop a wound healing system that creates an optimal environment for the wound healing process to occur, and actively promotes this process. This approach has been adopted by researchers in the field of wound management. As such, hydrogels have been deemed a favourable option in dealing with wounds as they provide the optimal environment for rehydration, gaseous exchange, angiogenesis, proliferation of connective tissue and re-epithelialization. In recent years, advancements in the field of pharmaceutics have seen the development of in-situ hydrogel forming systems with the use of “smart” stimuli responsive materials. Targeted and controlled drug delivery employing “smart materials” is a widely investigated field, within which stimuli-responsive polymers, particularly those which are thermo responsive, have received considerable attention. Thermo-responsive polymers have facilitated the formulation of in situ gel forming systems which undergo a sol-gel transition at physiological body temperature, and have revolutionized the fields of tissue engineering, cell encapsulation, and controlled, sustained delivery of both drugs and genes. However, the use of single thermo-responsive polymers in the creation of these systems has posed numerous problems in terms of physico-mechanical properties, such as poor mechanical strength, high critical gelation concentrations (CGC) resultingin increased production costs and solutions that are too viscous, toxicity, as well as gelation temperatures that are incompatible with physiological body temperatures. Hybridization of these thermo-responsive polymers with other polymers has therefore been employed, resulting in the creation of tailor-made drug delivery systems that have optimal gelation temperatures and concentrations, ideal viscosities, and improved gel strengths. Thus, the aim of this research was to create a hybrid thermo-responsive polymer system that actively encourages and facilitates the wound healing process. This was achieved with use of polymers such as chitosan and cellulose that are thermoresponsive but are also known to have positive effects in terms of promoting wound healing. Hybridizing cellulose with chitosan allowed for the development of a thermoresponsive polymer system that displayed gelation characteristics suitable for in-situ gelation, adequate gel strength, was bio-erodible and bio-compatible. Rheological analysis was conducted to determine the gelation kinetics of the formulation, as well as its suitability in terms of mechanical strength, structural behaviour in response to temperature and viscosity for ease of application. Analytical methods such as Fourier Transform Infrared Spectroscopy (FTIR), Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA) and morphological analysis were conducted to determine the physico-chemical nature of the system. The swelling and erosion behaviour, as well as the release kinetics of the system were also evaluated. Finally, in-vitro cytotoxicity analysis, migration studies and in-vivo animal studies were conducted to determine the efficacy of the system in healing wounds. The chitosan-cellulose hybrid thermo-responsive system was able to form a gel at physiological temperature within twelve minutes, while remaining in a sol state at room temperature. The low viscosity of the system at room temperature enables ease of application, while the increase in viscosity and mechanical strength of the system at 37°C will enable the system to withstand any physiological stressors at the wound site. The system proved to be bio-erodible and was also able to release the incorporated trans ferulic acid during the initial stages of wound healing, when it is needed. The chitosan—cellulose hybrid system proved to not only be non-cytotoxic, but stimulated cell proliferation as well. Favourable results in terms of efficacy of the system were seen both in-vitro and in-vivo. In-vivo analysis confirmed the wound healing potential of the system, in that it actively encouraged the wound healing process, displayed inherent bioactivity, accelerated the rate of wound closure, and promoted tissue regeneration as opposed to healing with mere tissue repair and the formation of scar tissue. The greatest percentage closure at each time point was exhibited by the pristine chitosancellulose based gel treatment group, with a 61% closure on day three of healing, 88% closure on day seven of healing, and a 98% closure rate at the end of fourteen days, with a notable normal epidermis displayed at the end of the wound healing period.
A dissertation submitted in fulfilment of the requirements for the degree of Master of Pharmacy to the Faculty of Health Sciences, School of Therapeutic Sciences, University of the Witwatersrand, Johannesburg, 2022