Silicon-Neuron Junction: Capacitive Stimulation of an Individual Neuron on a Silicon Chip.

The integration of electronic circuitry and neuronal networks requires a bidirectional electrical communication between silicon elements and nerve cells. In recent years, the successful assembly of a neuron-to-silicon junction was reported with direct signal transfer from an individual neuron to a microscopic metal-free fieldeffect transistor [1,2]. In the present paper, we describe a complementary system, the direct silicon-to-neuron junction. An individual Retzius cell of the leech (diameter about 70 mm) is stimulated extracellularly from a microspot (diameter about 50 mm) of p-type silicon which is perfectly insulated by silicon oxide (dc resistance . 1 TV) [Fig. 1(a)]. The stimulation was achieved by capacitive coupling which is effective in our system due to the tight association of oxidized silicon and neuron. There is no Faradaic current flowing across the electrode/electrolyte interface in contrast to all systems with a metallic interface [3–8]. Thus any toxic electrochemical effects can be avoided. Silicon electrode.—Chips (30 3 10 mm) were cut from n-type Si wafers with a (100) surface and a resistance of 5–10 V cm (Aurel GmbH, Landsberg, FRG). We opened 16 radial lanes in a mask oxide grown in wet O2 at 1000 ±C by etching with ammonium fluoride solution (AF 87.5.12,5, Merck, Darmstadt). The lanes were 2.5 mm long (from 0.5 to 3 mm from the center of chip). Their width was 60 mm for the first millimeter and widened up to 700 mm at the periphery. These lanes were p-doped with boron [1,2]. Their dc resistance was about 400 V. A field oxide (1 mm) was grown at 1000 ±C. Stimulation spots (diameter 20–50 mm) were etched at the inner end of the lanes and covered by a thin layer of oxide (about 10 nm) grown in dry oxygen at 1100 ±C in a rapid thermal processing oven (AST, Dornstadt, FRG). After opening contact sites at the peripheral ends of the lanes, they were wire bonded. Finally, a Plexiglass chamber (radius 1.5 mm) was attached using silicone glue (Raumedic, Rehau, FRG). It separates the region of the stimulating spots from the ends of the conductive lanes which were connected to external electronics. Neuron-chip assembly.—Retzius cells were dissociated from Hirudo medicinalis [9] and kept in a plastic petri dish in a culture medium (L-15, Gibco, Life Technologies