杏吧原创

PLASTIC nerves may one day help people with spinal injuries get back on their feet. 杏吧原创s claim to have taken a step in the right direction with thin plastic films designed to behave like nerve-cell membranes, which they say could eventually form part of an artificial nerve.

The goal, says Lindsay Bashford at St George鈥檚 Hospital Medical School in Tooting, was to make a material capable of mimicking the electrochemical reactions that nerves use to convey signals.

In nerve cells, waves of sodium and potassium ions cross the cell membrane in opposite directions in response to electrical signals passing along the membranes. Proteins called ion channels that line the membrane act as conduits through which the different ions pass, and this flow of ions changes the electrical potential of fluids on either side of the cell membrane. This lets the impulse travel along the nerve.

Working with Charles Pasternak and Glenn Alder, Bashford created thin films of plastic containing tiny holes that mimic these channels. When the team tested their membrane in a concentrated potassium solution to see how effective the pores were, they found that ions flooded through as rapidly as in natural channels, even though the pores are a thousand times thicker.

This was encouraging not only because the plastic membrane seemed to work like the real thing but because it also seemed to be using the same mechanism.

Until very recently, little was known about how ions are able to pass through the cell membrane fast enough to convey a nerve impulse without using much energy. But last year, Roderick MacKinnon at the Rockefeller Institute in New York, came up with an explanation (Nature, vol 414, p 43).

In his model, nerve channels fill up with ions鈥攍ike tennis balls in a tube鈥攆ed by the high concentration of ions in the extracellular fluid. Whenever an ion comes along and hits another ion lodged in the top of a channel, a 鈥渒nock-on鈥� effect occurs, pushing yet another ion out on the other side of the membrane (see diagram).

This allows ion exchange to occur very rapidly, because it lets a single incoming ion effectively span the thickness of the membrane in an instant, while using the natural diffusion of ions to provide the energy.

Using marker dyes, Bashford and Pasternak have shown that this also occurs with their plastic membrane. 鈥淣ow we have discovered that this property exists, the challenge is to exploit it,鈥� says Bashford.

The pores themselves are created by bombarding sheets of polyethylene terephthalate (PET), only 10 micrometres thick, with high-velocity gold ions from a particle accelerator. 鈥淭hese just punch their way through it,鈥� says Bashford, leaving a trail of 鈥渂roken bonds鈥� in their wake.

These bonds react with a solution of sodium hydroxide to leave a perfectly cylindrical pore lined with carboxyl groups. These act like the lining of ion channels, holding the positively charged ions in place.

But a lot of work is needed before the plastic membranes could be used in an artificial nerve, Bashford says. 鈥淲e鈥檇 like to make them more nerve-like by making them selective to different ions,鈥� he explains. For example, it鈥檚 still unclear why the pores in the polymer allow potassium ions to pass through slightly quicker than sodium ions, he says. This and other questions need to be answered first.