The defense of the doctoral thesis titled “Fabrication, modeling, and actuation of soft bioinspired microrobots” by Zihan Wang was held on Wednesday, 12th of March 2025, at 12:45 hours in the Aula of the Academy Building at the University of Groningen. The doctoral thesis is available here. Dr. Zihan Wang conducted his research at the Surgical Robotics Laboratory at the Department of Biomaterials and Biomedical Technology.

 

Thesis summary:

Soft bioinspired microrobots have gained extensive interest in biomedical applications due to their flexibility, maneuverability, and miniaturization. These characteristics enable them to navigate confined spaces within the human body. By emulating the locomotion of biological organisms, these microrobots can use their synthetic flagella to achieve flagellar propulsion. In addition to locomotion ability, another essential feature that soft bioinspired microrobots aim to replicate is the remarkable adaptability of biological organisms to complex environments. Understanding the underlying mechanisms of locomotion and adaptability enables us to design and fabricate soft microrobots that closely mimic their biological counterparts. However, there remains a significant gap between synthetic microrobots and biological organisms. Bridging this gap necessitates further research, particularly in understanding how biological organisms adapt to external disturbances and enhancing the locomotion capabilities of synthetic microrobots.

The objective of this doctoral research is to bridge the gap between biological organisms and soft bioinspired microrobots, advancing these microrobots to resemble their biological counterparts more closely. Chapter 1introduces the development of microrobotics, the origin of microfabrication, and the fabrication methods of bioinspired microrobots. Chapter 2 narrows the focus to soft bioinspired microrobots and presents an overview of their design principles, actuation methods, and biomedical applications. While the soft bioinspired microrobots have shown promise in various biomedical tasks, such as targeted cargo/drug delivery, biopsy, and biofilm eradication, the next phase of research aims to enhance their locomotion efficiency and improve their resemblance to the organisms that inspired their design. To this end, we delve into the environmental adaptability of natural sperm cells via experimental observation and theoretical validation. Chapter 3investigates the flagellar propulsion of sperm cells under external forces and proposes an elastohydrodynamic model to study this process. Our model predicts that both the mean flagellar curvature and the bending amplitude of the wave patterns reduce after experiencing external force, which is consistent with our experimental observation. Furthermore, this model provides a theoretical basis for exploring other interactions’ influence on flagellar propulsion and for improving the locomotion efficiency of synthetic microrobots using traveling waves.

The research focus then shifts to soft microrobots inspired by sperm cells and tadpoles, which acquire propulsion by undulating their soft and flexible tail. The principles of elastohydrodynamics used to study the flagellar propulsion of sperm cells are also applicable to these soft bioinspired microrobots. In particular, step-out frequency is a crucial metric for evaluating the locomotion efficiency of magnetic microrobots. Beyond this frequency, the microrobot lags behind external magnetic fields, thus reducing its velocity. To elaborate on the step-out frequency, Chapter 4 introduces an analytic equation combining elastohydrodynamics and magnetism. This equation establishes a quantitative relationship between the step-out frequency of soft bioinspired microrobots and their magnetic properties, geometry, wave patterns, and the viscosity of the surrounding medium. Validation of this equation involves investigating the swimming performance of electrospun sperm-like microrobots in mediums with varying viscosities. Our theoretical model indicates that the step-out frequency depends on the wave patterns of the sperm-like microrobot. However, it can be accurately predicted by analyzing wave patterns observed without exceeding the step-out threshold. This is due to the slight variations in wave patterns, unlike those observed in natural sperm cells. Our proposed model provides insights into the locomotion efficiency of soft bioinspired microrobots. It has implications for enhancing the experimental adaptability of soft bioinspired microrobots, as the high locomotion efficiency allows these microrobots to overcome external disturbance.

As a result, soft bioinspired microrobots with improved locomotion efficiency and adaptability have significant potential for practical applications. Next, we shift our attention to proof-of-concept studies that explore microrobotic systems. Chapter 5 proposes a microrobotic system with efficient locomotion, dual-motion capabilities, and biodegradability. Magnetic alginate microrobots, including teardrop and tadpole shapes, are fabricated by adjusting centrifugally driven flow. The rolling motion of the teardrop-like microrobots and the stick-slip motion of the tadpole-like microrobots are demonstrated under rotating and oscillating fields, respectively. This dual-motion capability enables them to adapt to different environments, such as obstacle crossing and navigation in confined spaces. Finally, we deploy an ultrasound imaging system to observe their motion and degradation, highlighting their potential for targeted drug delivery in future clinical trials.

Chapter 6 concludes this doctoral thesis by summarizing our findings from Chapters 3-5 on biological organisms and synthetic microrobots. It also presents a vision for future research to develop soft bioinspired microrobots that mimic biological organisms more closely. While this doctoral research primarily addresses laboratory-level challenges in soft bioinspired microrobots, it lays the groundwork for future innovations in medical microrobots.

Promotor

• Prof. Dr. S. Misra (University of Twente/University Medical Center Groningen, The Netherlands)

Supervisor

• Dr. I. S. M. Khalil (University of Twente, The Netherlands)

Assessment Committee

• Prof. Dr. R. Schirhagl (University of Groningen and University Medical Center Groningen)
• Prof. Dr. P. R. Onck (University of Groningen)
• Dr. U K. Cheang (Southern University of Science and Technology)

Opposition Committee

• Dr. P. van Rijn (University of Groningen and University Medical Center Groningen)
• Dr. A. Sadeghi (University of Twente)
• Dr. M. Shahbazi (University of Groningen and University Medical Center Groningen)