Creating tissues from stem cells is a finicky process. Researchers are still figuring out how best to coax them into becoming a particular cell type so that they can create tissues for patients with failing organs. Now they have a new tool for prodding stem cells: conductive nanowires, arranged like a bed of nails. Researchers at the University of California, Berkeley, have now demonstrated that mammalian stem cells can grow and develop into beating muscle cells on such an array.
Berkeley chemist Peidong Yang says that there has been extensive research into using chemical and mechanical stimuli–including treatment with growth factors and confinement to polymer scaffolds–to direct stem-cell development. But work with electrical stimulation has been limited. Yang hopes that applying pulses of electricity to the cells using the conductive array of nanowires will prove to be a good way to influence the cells. “We don’t know what will happen,” says Yang.
Silicon nanowires, which can range in diameter from one to several hundred nanometers, are several orders of magnitude smaller than cells, which are about 10 micrometers across. Nanowires can be functionalized–that is, researchers can attach molecules like DNA and proteins to their tips for delivery into cells–and are highly conductive. These properties, and researchers’ ability to precisely control the diameter and placement of nanowires, make them good candidates for connecting to the insides of cells so that their activity can be studied at the molecular level.
Yang’s group grew embryonic human kidney cells and embryonic mouse stem cells on silicon nanowires of varying diameters. Yang found that cell survival correlated with the wires’ diameter: the smaller the wires, the more likely that the cells survived. He also found that embryonic stem cells that developed into muscle cells lived on the arrays for as long as they were monitored–more than a month. The nanowires were also successfully used to deliver a gene for a fluorescent protein, proving that such arrays can be employed to send chemical stimuli to embryonic cells.
Prior to this research, it was not known whether cells could connect to and thrive on vertical arrays of the wires, although scientists have experimented with other configurations. Harvard chemist Charles Lieber, a major researcher in the nanowire field, has connected horizontally oriented nanowires to fully developed neurons in order to take detailed measurements of their electrical activity. (See “Nanowires Listen In on Neurons.”)
“What we’re seeing emerging is that there are many ways to provide stimuli to cells,” says Linda Griffith, professor of biological and mechanical engineering at MIT and a prominent tissue engineer who has investigated chemical and physical means of encouraging adult stem-cell survival. How cells behave is very much a factor of their local environments–things like physical pressure from neighboring cells or chemical signals they receive from distant cells. Whether the signals are physical, chemical, or electrical, says Griffith, the net effects of these stimuli are what govern how a stem cell matures.
Electrical stimulation by nanowires, says Griffith, may “go into a collection of different kinds of cues for controlling cell behavior.”
Although there was no electricity flowing from the Berkeley nanowires into the cells, the wires may have influenced the cells’ behavior. “Penetration of nanowires into the cells is likely to modify to a certain degree the differentiation of stem cells and, generally, gene expression in the cell,” says Nicholas Kotov, a chemical engineer at the University of Michigan. “[The Berkeley] configuration may be particularly interesting for electrically excitable cells,” including neurons, says Kotov. Embryonic mouse stem cells grown on the Berkeley nanowires matured into muscle cells, which are electrically active, although Yang says it’s impossible to determine whether the conductivity of the wires had anything to do with this development.
Understanding how the diameter of the nanowires affects cell survival could have “substantial fundamental importance,” Kotov says, but he adds that their findings also show that the researchers have much yet to learn about the cells’ responses to the composition and structure of nanoscale materials. He is developing retinal implants that connect to neurons using carbon nanotubes. (See “This Is Your Brain on Nanotubes.”)
Harvard’s Lieber cautions that Yang’s group has not yet demonstrated an active electrical interface between the cells and nanowires, as he did with neurons and as Kotov and others have done with carbon nanotubes.
Yang says that turning on the electricity is his group’s next step. “This is the first preliminary data that these nanowire interfaces with cells are okay,” he says. He hopes further research will demonstrate that the nanowires, acting as electrodes and chemical-delivery vehicles, can be used to direct stem-cell fates.