An army of new, half-biological-half-mechanical units could be wriggling their ways through people's bodies soon.

Engineers from the University of Illinois have introduced a new type of miniature probe developed from mechanical parts and human cells and capable of swimming through the often highly viscous fluids in the human body.

Described in the Jan. 17 issue of the journal Nature Communications, the robot probes are modeled after single-celled creatures with long tails, called flagella, which propel them forward. An individual probe looks a lot like an individual human sperm cell.

Lead investigator Taher Saif, a mechanical science and engineering professor at the university, explained the microscopic probes, or, sperm-bots, were developed to explore areas in the human body and elsewhere where no human probe has been able to go before.

"Micro-organisms have a whole world that we only glimpse through the microscope," Saif said. "This is the first time that an engineered system has reached this underworld."

Saif added the sperm-bots require a "minimal amount of engineering -- just a head and a wire."

Each sperm-bot body is crafted from a flexible polymer with cultured heart cells embedded near the junction of its head and tail.

Those heart cells align themselves with each other and then synchronize to beat together, resulting in waves that move down the sperm-bot's tail and push the tiny machine forward.

According to the university, the cells' self-organization is "a remarkable emergent phenomenon" and researchers don't fully understand how the embedded cells are able to communicate with one-another.

Saif's team also built probes with two tails which are capable of swimming even more quickly and navigating through the areas of exploration.

The creation of the sperm-bots -- part of Emergent Behaviors in Integrated Cellular Systems, a National Science Foundation-funded Science and Technology Center dedicated to building living, multi-cellular machines to solve real world problems -- marks the beginning of a long-term effort to see if scientists can "make elementary structures and seed them with stem cells that would differentiate into smart structures to deliver drugs, perform minimally invasive surgery or target cancer," Sarif said.