A STRANGE new material that contracts like a muscle when you shine light on
it could one day make brain surgery safer. The light-activated material could be
used in surgical tools, keeping electricity well away from sensitive brain
tissue.
Most artificial muscles use polymers that contract when a voltage is applied
across them. Others need to be immersed in solutions of ions to work
(快猫短视频, 3 June 2000, p 14).
But Mark Warner at the University of
Cambridge and Heino Finkelmann at Fribourg University in Germany decided to try
to make a muscle react to light.
To a rubbery polymer called PHMS, they added rod-shaped chemical groups that
pack together roughly in parallel. And to stiffen the material, they bridged the
PHMS backbones with cross-linking molecules called azo dyes, which have another
interesting property.
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When you shine ultraviolet light at 365 nanometres onto the material, a bond
linking two nitrogen atoms in each dye molecule absorbs the light. This energy
boost switches the molecule鈥檚 shape from straight to kinked.
These kinked bridges disrupt the neat parallel orientation of the rod-shaped
groups, making the polymer contract in the direction in which the rods were
originally aligned. 鈥淭hey take a second or so to respond, but we can speed that
up with plasticisers,鈥� says Warner. Plasticisers work like butter in spaghetti,
making it much easier for polymer chains to slide past each other. Increasing
the ultraviolet intensity also speeds up contraction.
There are two ways to 鈥渞elax鈥� or return the muscle to its original shape. In
the dark, the kinked molecules slowly spring back straight鈥攔ealigning the
rod-shaped molecules. But this can take a few hours. Alternatively, another shot
of light gives the molecules the energy boost they need to straighten up.
Shining UV light with a wavelength of 450 nanometres on the muscle pumps in the
energy so that when the light is switched off, the molecules quickly return to
their lower-energy, straight shape.
Warner and Finkelmann calculate that light-activated muscles should be able
to contract by between 20 and 75 per cent of their resting length. They believe
that their material could be used to open and close microsurgical grippers and
tweezers, keeping electricity at a safer distance from sensitive brain tissue.
鈥淭here is definitely a need for actuators like this that can be driven by
optical fibres,鈥� says Yoseph Bar-Cohen, NASA鈥檚 artificial muscles expert.
The inventors are now seeking other applications for their technology.
