Sperm-hybrid micromotors: on-board assistance for nature's bustling swimmers

Sperm cells that cannot swim and orient properly compromise male fertility. Such defects are responsible for male infertility regardless of the actual quality of the most important content, the sperm's DNA. Synthetic micromotors are engineered devices that are able to swim in (body) fluids and microscopic environments, similar to flagellated cells like sperm. Coupled together, a sperm-hybrid micromotor embodies the concept of bringing the sperm cell together with artificial components that assist or replace defective functions of the cell, helping it to pursue its goal without interfering with its health, enabling the process of assisted fertilization and further embryo development all inside the body. Non-invasive, remote-controlled in vivo applicability is the key quality of such hybrid microdevices. Assisted reproduction with the help of micromotors is in the focus of this review, although other biomedical applications that arise from the powerful combination of sperm cell and synthetic enhancement are also discussed and summarized. Details are provided about different fabrication processes and cell-material coupling strategies, and the way from proof-of-concept studies to in vivo experiments in animals is outlined.