Probing the Singularity
Melding man and machine at the nano-scale
by John Motsinger

Bozhi and Tzahi. Their names alone conjure up images of twins in a circus act, but the two men are up to something even more bizarre. They work in a world of microscopic mystery, at the interstices of the digital and the biological, where the human ends and the machine begins.

Bozhi Tian moved from Shanghai six years ago to study chemistry and materials science; Tzahi Cohen-Karni moved from Israel five years ago to study applied physics and engineering. In a Harvard University nanotechnology research laboratory, the pair has developed impossibly small semiconductor probes that can record detailed electrical activity from within a single cell.

The current standard for making cellular recordings relies on a relatively blunt needle that sucks to the cell membrane to listen in from the outside. The glass needle, known as a microelectrode or patch clamp, contains an ionic solution that responds to changes in the cell’s internal electrical signaling as membrane channels open and close.

In contrast, the tip of Tian and Cohen-Karni’s nanowire device is one hundredth the diameter of a microelectrode and coated with a layer of lipids, which allows it to slip inside the cell membrane to record electrical signals more directly. Using a transistor that’s built into the silicon nanowire, the device can register changes in electrical potential as small as just a few millivolts, all without disturbing the normal response of the cell.

For Tian, the challenge was synthesizing silicon nanowires bent at just the right angle to enter cells easily while still forming a complete electrical circuit. By carefully controlling the temperature and pressure of precursor gases inside a glass tube furnace, Tian was able to reliably produce successive 120-degree kinks during the nanowire growth process. Changes in the flow rates of different gases, called dopants, gave rise to internal conductance changes that produce a tiny functional region at the probe tip. It’s this section, the “field effect transistor,” that’s capable of measuring changes in electrical potential in the surrounding environment.

To record from living tissue, however, Cohen-Karni had to devise a new method of positioning the probe into target cells. Though each nanowire probe is tiny, with hundreds of probes forming contacts in an area half the size of your pinky nail, the overall device is part of an integrated circuit that is relatively large. So Cohen-Karni came up with a way to invert a cultured substrate of cardiac cells and position the complex on top of the probe device using a micro-manipulator. That way the bigger device chip remains fixed while the smaller cell substrate is free to move in all three dimensions.

The innovation has already paved the way for designing cyborg cells from silicon wafers that can track the contractions of individual muscle cells. What lies beyond is implanting neural chips and nano-scale pacemakers into the brain and heart--the seamless fusion of the body’s own circuity with manufactured hardware.