Sperm that don't swim well rank high among the main causes of infertility but these cells may get a boost from motorized "spermbots" that can deliver poor swimmers -- that are otherwise healthy -- to an egg.  

Artificial insemination is a relatively inexpensive and simple technique that involves introducing sperm to a woman's uterus with a medical instrument. Overall, the success rate is on average under 30 percent, according to the Human Fertilisation&Embryology Authority of the United Kingdom. In vitro fertilization can be more effective, but it's a complicated and expensive process.

It requires removing eggs from a woman's ovaries with a needle, fertilizing them outside the body and then transferring the embryos to her uterus or a surrogate's a few days later.

Each step comes with a risk for failure. Mariana Medina-Sánchez, Lukas Schwarz, Oliver G. Schmidt and colleagues from the Institute for Integrative Nanosciences at IFW Dresden in Germany wanted to see if they could come up with a better option than the existing methods.

Building on previous work on micromotors, the researchers constructed tiny metal helices just large enough to fit around the tail of a sperm. Their movements can be controlled by a rotating magnetic field. Lab testing showed that the motors can be directed to slip around a sperm cell, drive it to an egg for potential fertilization and then release it.

The researchers say that although much more work needs to be done before their technique can reach clinical testing, the success of their initial demonstration is a promising start.