Silicon is the best semiconductor on the periodic table for researchers trying to connect smart devices to living cells, says Bozhi Tian of the University of Chicago. Silicon is biocompatible and biodegradable, and scientists have already developed electronic implants and biosensors using silicon. But conventional single-crystalline silicon is inherently rigid, which can prove irritating to living tissue.
Along with Francisco Bezanilla, Tian and his colleagues have now developed spongy silicon particles that are deformable and, therefore, are less likely to inflame their surroundings (Nat. Mater. 2016, DOI: 10.1038/nmat4673). Another key benefit is that amorphous silicon absorbs light better than single crystals, allowing the team to create efficient links to living cells, Tian explains.
The team demonstrated this by attaching single, micrometer-sized particles of its spongy material to rat neurons grown in a dish. When illuminated with green laser light, the particles cause electrical current to flow into the nerve cells. This, in turn, causes the neurons to fire. Although this may sound similar to the optical neural manipulation made possible by genetically modifying cells, or optogenetics, this material could offer a simpler alternative to studying and modifying neural activity. “There’s no genetics,” Tian explains. “Everything is physical.”
The porous silicon material is a “versatile interface to biology,” says John A. Rogers, an electronic materials expert at the University of Illinois, Urbana-Champaign, who was not involved with the study. The particles “exploit both the excellent electronic properties of silicon and its biocompatible, bioresorbable nature,” he adds.
To create the deformable particles, the team used its homemade chemical vapor deposition system to decompose silane gas. Liberated silicon atoms then congregate in the nanoscopic voids of a mesoporous silicon dioxide template. Within the template, the silicon forms amorphous silicon nanowires. The template also allows nanoscopic bridges to grow between these wires.
After the researchers dissolve the template in hydrofluoric acid, they’re left with microscopic networks of wires and bridges. The gaps in the structures are largely responsible for their sponginess, but the property is boosted by a chemical bonus: Silicon oxidizes in air, which helps the porous particles take on water and become squishier in wet conditions.