DNA strand-displacement computer
Realizes: boolean logic / neural network inference (via hybridization cascades)
Single-stranded DNA molecules in solution compute via toehold-mediated strand displacement: a short single-stranded 'toehold' on a partially double-stranded gate complex allows an input strand to invade, displace, and release an output strand. Presence/absence of a strand encodes a bit. Cascades of these reactions implement AND, OR, NOT, NAND, NOR, XOR, and threshold gates without enzymes or moving parts. Qian and Winfree (2011) demonstrated a four-bit square-root circuit from 130 DNA strands; a subsequent paper (Nature, 2011) realized a 30-node Hopfield neural network entirely in DNA solution. Speed: minutes to hours per logic operation (hybridization kinetics). Capacity: ~100-gate circuits demonstrated; massively parallel (each molecule is a gate).
Examples
Scaling Up Digital Circuit Computation with DNA Strand Displacement Cascades (Science, 2011)
Qian & Winfree demonstrate a 4-bit square-root circuit built from 130 DNA strands using seesaw gate architecture
Neural network computation with DNA strand displacement cascades (Nature, 2011)
30-neuron Hopfield neural network implemented entirely in DNA solution, capable of pattern completion