Design and Development of Modified Hyaluronic Acid Biopolymers Towards Biomedical Engineering Applications

As a major component of the extracellular matrix (ECM), hyaluronic acid (HA) is an ideal material for hydrogel preparation due to its hydrophilic, biocompatible, cellular signalling, and viscoelastic properties. These characteristics make HA highly suitable for various biomedical applications such as wound healing, drug delivery, and tissue engineering. However, the rapid degradation and poor mechanical properties in vivo of native HA has restricted its application. To address these challenges, researchers have employed chemical modification techniques to HA to control its mechanical properties whilst maintaining its biological effect. These modifications can facilitate cross-linking of HA molecules to create a hydrogel network structure that can better withstand biological environments. Despite these advancements, current HA modification techniques face several limitations, such as the use of toxic crosslinking agents, inconsistent degrees of modification, undesirable side reactions, inefficient use of laboratory resources, and prolonged reaction times. The aim of this thesis is to design and develop improved techniques for functional modified HA.
Chapter One introduces HA and its remarkable properties as a functional biomaterial. Modification techniques, subsequent crosslinking mechanisms and applications are discussed in detail.
Chapter Two presents a novel synthetic route for acrylate modified HA (HA-A-BEA) through a Williamson ether synthesis with 2-bromoethyl acrylate (BEA). This method offers a simple and efficient preparation process, significantly reducing the reaction time and the number of reaction and purification steps when compared to existing techniques, whilst retaining the biocompatibility of HA. The degree of substitution can be tailored from 10% to 40% by modifying the reagent feed ratios. This study demonstrates the ability of HA-A-BEA to form tunable hydrogels, via two techniques, suitable for tissue engineering applications.
Chapter Three describes the development of thiol-modified HA (HA-SH) through conjugation with a disulfide-containing bis-epoxide. The use of EDCI/NHS chemistry is the most common method for developing HA-SH. However, this approach often leads to unwanted molecular rearrangements and the consumption of the carboxyl group, which is crucial for the biological signalling
capacity of HA. In this work, hydroxyethyl disulfide diglycosidyl ether (HEDSGE) is conjugated to HA to form HA-SH-HEDSGE, allowing for adjustable degrees of thiol substitution while preserving the biocompatibility of native HA. HA-SH-HEDSGE was used to prepare in situ forming, stable Michael addition crosslinked hydrogels.
Chapter Four presents the design and development of an HA-based tissue adhesive hydrogel. Conventional techniques for wound closure, such as sutures and staples, have significant drawbacks that can negatively impact wound healing. Tissue adhesives have emerged as promising alternatives, but poor adhesion, low mechanical properties, and toxicity have hindered their widespread clinical adoption. In this chapter, a dual-modified aldehyde and methacrylate HA biopolymer (HA-MA-CHO) has been synthesised through a simplified route for use as a double crosslinked network hydrogel (HA-MA-CHO-DCN) adhesive for the effective closure and sealing of wounds. HA-MA-CHO-DCN crosslinks in two stages, leading to the formation of a self-healing injectable gel. This is followed by secondary crosslinking via UV-initiated polymerisation of the methacrylate (MA) functionality. This stable hydrogel adhesive shows high versatility, with a tunable storage modulus suitable for various wound environments. The new HA-MA-CHO-DCN hydrogel demonstrated excellent adhesive properties by surpassing the burst pressure and lap shear strength of a commercially available tissue adhesive whilst maintaining cell viability.
Chapter Five provides a summary of all the research chapters and discusses limitations and future directions.