Adevice the size of your thumb could help people suffering from disorders such as chronic pain, urinary incontinence, and limb dysfunction. Packing less than half the bulk of existing devices, the thumb-sized Micropulse is the tiniest implantable pulse generator yet created. The wireless, rechargeable device sends programmable electrical pulses to nerves that control pain, muscles, and organs. It is expected to enter FDA-required clinical trials early in 2007, and it could be available to patients as soon as 2009.

How it works

NDI Medical Inc., Cleveland, the OEM for the Micropulse, wanted a device that could be customized for several tasks. During clinical trials, the Micropulse will be scrutinized by the FDA for use in three applications:

• Pelvic health care — dealing with urinary incontinence

• Pain management — helping patients cope with chronic pain brought on by illness or injury

• Restorative therapies — bringing back limb function by stimulating muscles

Using minimally-invasive surgery, the Micropulse is implanted into a patient, usually in the lower abdomen or buttock, where the device is most comfortable and least visible. After implantation, a clinician uses a wireless programmer to set the Micropulse's stimulus parameter and timing patterns. The programmer, as well as the patient's controller for the device, has a range of about three feet.

To recharge the device's lithium-ion battery, the patient applies a recharging patch for several hours to the vicinity of the implant. The battery needs recharging from once a month to every few weeks.

The electronics inside

The Micropulse owes its small size to advances in electronic miniaturization, particularly those that make it possible to shrink circuitry. Design innovations allow reducing components to the size of sand grains, making them so small they must be assembled by robots. Not only are these microscopic circuits more complex, but they are also more reliable and have stingier energy needs than conventional implantable circuits.

Many advances in miniature circuit assembly technologies come from working with a bare die — a semiconductor chip. Though its circuitry is tiny, the chip traditionally was put on large packages, negating the benefits of its small size. Valtronic came up with several ways of fixing the bare die directly to a circuit board and tossing the packaging. These assembly techniques allow placing 10 to 20 times the amount of circuitry in the same space.

Valtronic adapted chip-on-board technology followed by chip on chip. These processes use a circuit board and fine wires to configure the required and increasingly compact circuit. Chip-on-board (COB) and chip-on-chip (COC) designs save up to 50% of the space required to make the same circuit in a conventional manner. But engineers saw that despite their advantages, the new techniques also had limitations. One is having to bond wires from the chips to the circuit board, which takes a lot of space. And with COC configurations, the top chip has to be smaller than the bottom.

To address these problems Valtronic engineers use the flip chip process. In these configurations, a chip is mounted onto a circuit board face down, instead of face up as in the COB and COC techniques, and bonded together with solder or adhesives. Contact pads can be distributed all over the chip's surface, not just the edge, which lets the chip hold more input and output contacts. The flip-chip process creates assemblies that use less power and operate more reliably at lower temperatures.

Another important gain in circuit assembly technology is 3-D chip-scale packaging, or 3D-CSP. With this process several flip-chip circuits are assembled on flexible circuit boards. The boards are then folded or rolled into a compact 3D-CSP package. The system produces a 75 to 80% size reduction compared with other assemblies and preserves all advantages of the conventional flip-chip.