PULSED-LASER DIRECT WRITING OF BIOLOGICAL MATERIALS

Laser direct writing, developed based on modified laser-induced forward transfer (LIFT), has been emerging as one of the most promising biofabrication techniques. While some studies have been conducted to investigate laser direct writing of biological materials, there are some key challenges such as mechanisms of bubble formation, droplet formation and process-induced cell injury that have not been well elucidated. The objective of this dissertation is to study laser-induced droplet formation dynamics and process-induced cell injury in pulsed-laser direct writing of biological materials, and improve the existing laser direct-write techniques. Phase explosion is identified as the dominant bubble formation mechanism in pulsed-laser direct writing. The phase explosion-induced bubble formation process is modeled using a homogenous nucleation theory. The proposed model can predict the formed bubble diameter and pressure in laser direct writing of glycerol-water solutions. Droplet formation mechanism is systematically studied through investigating the effects of laser fluence and material properties on the transferred droplet diameter. It is found that the transferred droplet diameter is linearly dependent on the laser fluence while there is no systematic dependence on the glycerol concentration. Process-induced cell injury in laser direct writing is systematically elucidated through investigating the effects of operating conditions on the post-transfer cell viability, proliferation and cell injury reversibility. It is found that the post-transfer cell viability decreases as the laser fluence increases and is not dependent on the cell density in direct writing of human colon cancer cells. Reversible process-induced cell injuries are observed in post-transfer yeast cells. An improved laser direct writing approach is proposed using a four-layer structure including an additional metallic foil between the laser transparent donor substrate and the coating to be transferred. It is found that the proposed approach is a promising fabrication technology for making encapsulated microspheres from highly viscous solutions and transferring mammalian cells with high post-transfer cell viability. It can eliminate the direct laser-cell interaction and possible contamination from residual sacrificial layer which may have using other conventional laser direct-write techniques. This dissertation provides a better understanding of the droplet formation dynamics and process-induced cell injury in laser direct writing. This work would help laser direct writing to be a viable biofabrication technology in cell printing and encapsulated microsphere fabrication