3D printing hydrogel scaffold and functional hydrogel for tissue engineering application
Date of Issue2017
School of Mechanical and Aerospace Engineering
Ever since its development, tissue engineering has played a very significant role in the medical arena with an ever-growing demand for various tissue donations. One crucial factor in conducting in vitro tissue engineering study is the construction of a desirable artificial three-dimensional (3D) tissue scaffold to act as the extracellular matrix, meeting the complex requirements for specific cell cultures. In view of this, to achieve a more optimal scaffold for tissue engineering, this study addresses two major challenges in tissue engineering nowadays: scaffold fabrication techniques and developing functional hydrogel material for tissue scaffold fabrication. Existing hydrogel scaffold fabrication techniques and systems utilized in constructing extracellular matrix (ECM) are either two-dimensionally limiting or expensive and time consuming. For instance, some simple fabrication methods fabricate structures with morphologies that cannot be accurately controlled, while others may introduce harmful organic solvents into scaffolds during the fabrication processes. In this study, a new 3D printing system, is presented which is capable of printing hydrogel scaffold structure with improved efficiency. The outstanding features of this system are mainly attributed by the transparent diffractive optical elements (DOEs) of linear binary Fresnel lens fabricated to control the luminous intensity distribution. These DOEs of different patterns are arranged in series on a cover slip with each optical element designed to focus and diffract light at a particular plane within the device. Coupled with other components of the system, 3D hydrogel cellular scaffolds can be easily printed via effective and efficient one-step light exposure to the photo- crosslinkable polymer solution upon demand. The combined outstanding features of photo crosslinking and diffractive optical technique incorporated within this system enable the patterning of hydrogel within seconds for not only bulk structures, making large-scale fast production feasible, but also for localized, specific areas of interest, making printing of complex features simple. With this system, 3D two-layered hydrogel woodpile structures were successfully fabricated within 3 seconds in this research. Though the development of conventional in vitro 3D structural support properties of scaffold is crucial, the interests in recent years has steered towards developing multifunctional scaffold to achieve similar biochemical and biophysical features of the natural ECM for further regenerative medicine use. Among various functionalities explored in scaffolds so far, electric stimuli-responsive hydrogel integrated with controllable drug delivery technique has been a state of the art technology for scaffold fabrication. Electric signal plays significant roles in the cell differentiation and signaling among multiple tissues such as nerve, bone and cardiac etc. More importantly, the regeneration process of these tissues needs the cooperation of multiple growth factors, drugs or biomolecules to be delivered to the localized area. Being a pilot study, another focus of this project was placed on investigating ‘smart’ conductive biomaterial that is capable of transiting electric signal as well as delivering incorporated drugs. We integrated hydrogel polyethylene (glycol) diacrylate (PEDGA) with water-soluble reduced graphene oxide (rGO) and created a new trend of ‘smart’ photo-patternable conductive hydrogel. This photo-patternable conductive hydrogel is capable to be printed as conductive 3D tissue scaffold, serving as a conductive 3D template to provide external supports and electric stimulations for cell growth, what is more, its therapeutic controllable drug-delivery capability also allows the hydrogel to be loaded with varied biomolecules, becoming a therapeutic agents carrier, releasing them when needed in a controllable manner. The release of the incorporated agents can be easily achieved via the application of external electric signal with ‘click’ action. Layer-by-layer (LbL) poly-arginine siRNA nano-particles were fabricated and incorporated into the hydrogel, followed by electric stimuli assisted release test. The released nano-particles were then treated to mouse fibroblast cells to confirm if the siRNA loaded nano-particles still retain bioactivity. Confocal microscopy showed successful uptake of the nano-particles into the cells. Western blotting of the treated cells lysates showed intended protein knockdown, indicating that the released nano-particles retained their functionalities, proving the conductive hydrogel’s capability to safe release of sensitive agents. Therefore, this composite polymer is both a favorable cell culture construct and a drug delivery vehicle.