Biomedical micromotors are an interesting generation of medical treatment technology. When equipped with specific propulsion mechanisms, micromotors can be of great potential in performing accurate procedures such as drug delivery and microsurgery. Unfortunately, the transportation of micromotors through the blood circulatory systems has remained a great challenge, mainly due to the low thrust force that is inadequate to overcome the high flow and complex composition of the blood. Besides, the motion and functions of the majority of the reported synthetic micromotors can be significantly influenced by the molecules contained in the blood.
Sperm-based micromotors have been proposed to overcome the above challenges due to their ability to swim against flow and close to boundaries. Additionally, they can be externally controlled to reach the target sites and perform the desired functions. Equipped with this knowledge, German researchers from Leibniz Institute for Solid State and Materials Research: Dr. Haifeng Xu, Dr. Mariana Medina-Sánchez, Dr. Manfred Maitz, Professor Carsten Werner, and Professor Oliver Schmidt developed a streamlined-horned cap hybrid sperm micromotors with the ability to efficiently and controllably swim against the blood flow and perform the desired functions. Their work is currently published in the research journal, ACS Nano (doi.org/10.1021/acsnano.9b07851).
The research team first conducted flow simulations to compare the hydrodynamic characteristics of tabular cap and SHC designs with the same features. This was deemed necessary for the efficient design and manufacture of the sperm micromotor. The hybrid system is comprised of two main parts: the sperm flagellum that provides the required propulsion force, and the synthetic microstructure that performs the functions of magnetic guidance and cargo transportation. The feasibility of the potential performance of the hybrid micromotor in the circulatory system was validated by functionalizing the microcaps with heparin loaded liposomes to realize a localized anticoagulant effect.
The authors found that the synthesized hybrid sperm micromotor could actively swim against the flowing blood, comparable to the real bloodstream of a human body. The streamline-horned sperm micromotors successfully executed the task of heparin cargo delivery in the flowing blood, working both individually and in swarms. The tapered horn specifically functioned to decrease the energy loss due to squeezing through the cells allowing high pulsatile flow motions and self-assembling into a sperm train. Moreover, assembling single sperm micromotors into a train like a carrier offered an efficient alternative approach for the transportation of a controlled number of coupled sperm and medical cargoes to the targeted sites. The functionalized sperm micromotors exhibited significant anticoagulant effects compared to the available nonfunctionalized sperm micromotors.
In summary, the study presents the first successful hybrid sperm micromotors attempt for cargo delivery through the flowing blood. Due to the advantages of sperm flagellum and the synthetic microstructure, the biohybrid system successfully performed the functions of heparin cargo delivery. This work also serves to identify the minimum requirements that should be fulfilled by medical microrobots for safe and efficient operation in the blood vessels. The authors in a statement to Advances in Engineering, mentioned that the study provides useful insights that will not only allow the treatment of clots and other diseases in the circulatory system but also set the stage for wider applications of sperm based micromotors in the medical field.
Xu, H., Medina-Sánchez, M., Maitz, M., Werner, C., & Schmidt, O. (2020). Sperm Micromotors for Cargo Delivery through Flowing Blood. ACS Nano, 14(3), 2982-2993.