Engineering biomimetic tough and printable silk-based hybrid hydrogels for tissue engineering applications
Creators: name: DORISHETTY, Pramod
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 340403
subject: Characterisation of biological macromolecules
classificationCode: 400302
subject: Biomaterials
classificationCode: 401212
subject: Non-Newtonian fluid flows (incl. rheology)

description: Hydrogels are the three-dimensional (3D) solid networks made from hydrophilic polymer chains with chemical or physical crosslinking. The search for ideal hydrogel material for tissue engineering applications such as articular cartilage remains a challenging problem in tissue engineering (TE) particularly when the target tissues or organs undergo periodic loading forces as a part of their proper function. In such applications, the hydrogels are not only required to sustain their own weight, but also should tolerate an external force. Thus, an optimal material for these applications is a tough hydrogel that can sustain the applied force and reversibly deform to large extents without failure. The brittleness of synthetic hydrogels is in great contrast to the toughness of biological tissue. Only recently has this combination of properties been achieved with synthetic hydrogels. However, a key shortcoming of many synthetic materials is their bioinertness.  Hydrogels derived from natural sources such as proteins and polypeptides present an excellent choice for various tissue engineering applications because their inherent structures possess biological cues that support cellular activities such as attachment, proliferation and differentiation. However, the mechanical weakness of these hydrogels poses a challenge for demanding load-bearing tissue engineering applications, such as knee joints, fibrocartilage and intervertebral discs replacements. Therefore, one of the key questions is how to create a tough scaffold platform equipped with appropriate molecular cues to achieve a specific biological response. The present research focuses on a pivotal study that will investigate primarily a hydrogel platform with tunable architecture for engineering cell-biomaterial interactions. 

Considering the requisite behaviour of a tough hydrogel, in this research, regenerated silk fibroin (RSF) has been selected due to its excellent characteristics, such as intrinsic biocompatibility, tunable structural (nano-micro) and mechanical properties.  RSF-based hydrogels can be fabricated by several methods/mechanisms, including physical crosslinking, chemical crosslinking, electro-gelation, photocrosslinking, etc., Among these various gels, the photocrosslinked silk hydrogels exhibit excellent water retention, mechanical modulus with good biocompatibility. However, the obtained mechanical properties of the photocrosslinked silk hydrogel do not provide the required toughness for the demanding load-bearing tissue engineering applications. There have been few studies on reinforcing silk hydrogel using synthetic materials. However, the reinforcement achieved is inadequate to replace the load-bearing tissues such as intervertebral discs and the use of synthetic reinforcing materials also compromises the cellular activities. Therefore, the need to make tough photocrosslinked silk hydrogel and sustaining cellular activities demands the new toughening strategies using biocompatible natural materials. Hence it is hypothesised that the photocrosslinked RSF hydrogel reinforced using nature-derived strong and rigid materials can fulfill the mechanical requirements posed by the load-bearing tissues without compromising the cellular activities. Moreover, the rapid photocrosslinking mechanism of RSF hydrogels can have a huge potential in engineering custom-designed scaffolds using advanced fabrication techniques, such as three-dimensional (3D) printing. Thus, this thesis studied three different silk-based hybrid hydrogels using nature-derived rigid materials such as soy protein isolate (SPI), cellulose and graphene. In each study, several mechanical and structural characterizations have been employed to understand the fundamentals of the structural (nano-micro) contributions in tuning the mechanical properties of silk-based tough hybrid hydrogels. Also, the rheological and flow properties of these hydrogel forming silk hybrid inks were investigated to understand their suitability for 3D printing.  In addition, cell-gel interactions of the fabricated silk-based hybrid hydrogels have been investigated to demonstrate their potential for various tissue engineering applications.