EVER noticed how gadgets like cellphones can nearly double in size and weight
once you slap on a battery pack? Little wonder then that shrinking the size of
batteries is a major preoccupation鈥攚hich is where a new tool called a
鈥渓aser typewriter鈥� comes in. It can print tiny batteries directly onto
electronic circuits.
The problem with big power sources is most acute in the smallest devices. 鈥淎s
you go smaller, space becomes a premium. It鈥檚 like real estate in
Tokyo鈥攊t鈥檚 a really hot commodity,鈥� says Karen Swider-Lyons of the Naval
Research Laboratory in Washington DC. A single battery usually powers the whole
device but that has knock-on effects too: you need extra components to step
battery voltages up and down to power different circuits, plus wires to deliver
the power, so there鈥檚 a risk of electromagnetic interference that can disrupt
operation.
Microbatteries custom-built to power specific circuits would solve many of
these problems by occupying less space, and making transformers and extra wiring
obsolete. The devices themselves could also be more versatile. 鈥淪ay you wanted
to fly an unmanned aerial vehicle with an infrared sensor and a video camera one
week. The next week, you might want to swap the IR sensor for a biosensor,鈥� says
Swider-Lyons. 鈥淚t鈥檇 be a lot easier if each sensor had its own power supply and
didn鈥檛 need to be wired into the main power source. You could just plug and
辫濒补测.鈥�
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To make these dedicated batteries, Swider-Lyons uses what she鈥檚 dubbed a
laser typewriter. Crucial to the process are three ribbons, each one a layer of
鈥渋nk鈥� containing a different battery constituent. The ink is based on ethylene
glycol mixed with a binder called polyvinylidene fluoride. This base holds the
battery鈥檚 constituent parts in place.
The first ribbon, which becomes the battery鈥檚 anode, contains manganese
dioxide doped with potassium hydroxide. The second contains ethyl cellulose, a
substance that can transport hydroxyl ions. A third ribbon, used to form the
cathode, is made of zinc doped with potassium hydroxide. The ribbons are painted
side by side onto the underside of a quartz disc.
To print a battery, you fire a pulse from an ultraviolet laser through the
quartz onto the first ribbon. This laser pulse vaporises some of the ethylene
glycol in the ink, firing a tiny amount of ink onto a gold-coated glass surface
below. Heating the layer burns off any excess ethylene glycol. You print the
next two layers in the same way to form a three-layered microbattery just 1.5
millimetres long (see Diagram).
The layers can then be trimmed to the right
shape with the laser. You could easily print the battery straight onto your
device, say a silicon chip, rather than the glass surface. Swider-Lyons says
microbatteries could supply milliwatts of power for a couple of weeks. Then you
could recharge them with solar cells.
Combining customised batteries with circuitry is a significant breakthrough,
says Alan Evans at the microelectromechanical group at the University of
Southampton. For starters, it means engineers may be able to make smaller,
rechargeable medical implants, he says.