Shape-shifting bubble-driven hydrogel microbots

Altunkeyik, Berk ORCID: 0000-0002-8362-2731 (2024). Shape-shifting bubble-driven hydrogel microbots. University of Birmingham. Ph.D.

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Abstract

Microbots are artificial swimmers engineered to perform complex tasks at the microscale including drug delivery, cargo transport, and non-invasive surgery. Historically, the design of microbots has advanced from basic rigid structures to more sophisticated flexible and active particles. This transformation has been driven by the use of soft and flexible materials like hydrogels, which offer low elastic moduli and, consequently, high deformability. These soft active materials hold the potential to enable microbot navigation through shape and volume transformations.

This thesis begins by showing that hydrogel-based bubble-driven rigid microbots with I, U, and S-shapes exhibit distinct swimming characteristics: pumping, translation, and rotation. While the manufacturing and experiments of this study were conducted by our experimental collaborators, we propose a mathematical model to provide physical insight into the bubble growth mechanism at the active tips. The findings of this study inspire us to explore the use of hydrogels to control microbot dynamics through shape transformations.

We proceed by investigating the deformations of active hydrogel-based filaments in Stokes flow. For this investigation, we propose a simple numerical model incorporating a bead and spring system with the method of regularised stokeslets. The bead and spring network captures the elastic deformations, while the method of regularised stokeslets accounts for the non-local hydrodynamic interactions between different segments of the hydrogel body. Simulations of porous filaments, representing hydrogels activated by single or double-tip forces, reveal periodic oscillations and motion patterns such as corkscrew motion and run-and-tumble behaviour, reminiscent of the motility seen in sperm and bacteria.

Finally, the same framework is adapted to model the volume and shape transformations of the responsive hydrogels. By combining responsive and non-responsive sections within the same hydrogel, we are able to control both the function and swimming characteristics of passive and active hydrogel bilayers. Additionally, we propose new smart microbot prototypes, including star-shaped structures, and suggest control mechanisms that integrate responsivity with activity. The findings of this dissertation offer guidance for designing shape-shifting active microbots, and the methodologies developed are broadly applicable to various problems in the study of artificial microswimmers and undulatory motion.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):