Bryan Kaehr and Jason B. Shear at the University of Texas at Austin have used multiphoton nonlinear excitation to fabricate chemically responsive protein structures embedded in a protein hydrogel matrix [Kaehr, et al., PNAS (2008) doi:10.1073/pnas.0709571105].

The researchers start with a hydrogel of common proteins such as bovine serum albumin (BSA) and avidin (Av) at high concentrations with added photoinitiator, then expose the matrix to a focused laser in a confocal microscope. The laser excitation makes sidechain residues on adjacent proteins crosslink with each other, forming embedded monolithic multiprotein structures.

When the fabricated structures are exposed to different pH or ionic conditions, they undergo dramatic conformational changes. Av cylinders reversibly bend as the pH is lowered from 7 to 5 to 2 and straighten out again as the pH is increased to 10. BSA cylinders, on the other hand, bend when the pH is highly acidic or basic.

By varying the laser power during fabrication, the amount of crosslinking can be modulated. This permits tuning such that the rods bend by different amounts, depending on the ionic conditions.

Shear explained to Materials Today that the structures retain their minimal volume close to isoelectric point, but that when the ionic environment changes, the structures change their global conformation.

Shear hopes to apply the technique to hydrogel environments that contain bacteria and cells. He anticipates that arbitrary structures can be fabricated that change their conformation in response to chemical signals originating from cells.