A closeup of flexible, biocompatible polymer molded from porous silicon.

Researchers at the University of California, San Diego discovered how to transfer the optical properties of silicon-crystal sensors to plastic. This could lead to flexible, implantable devices able to monitor the delivery of drugs within the body, strains on a weak joint, or the healing of sutures."While silicon has many benefits, it has downsides as well," says Michael Sailor, UCSD professor of chemistry. "It's not particularly biocompatible or flexible, and it can corrode. You need something that possesses all three traits for medical applications."

Researchers treat a silicon wafer with an electrochemical etch, producing a porous silicon chip with a precise array of nanometer-sized holes. This gives the chip optical properties similar to photonic crystals -- a crystal with a periodic structure that can control the transmission of light much as a semiconductor controls the transmission of electrons.

Molten or dissolved plastic is cast into the pores of the finished silicon photonic chip. The chip mold dissolves, leaving behind a flexible biocompatible "replica" of the porous silicon chip. "It's essentially a similar process to the one used in making a plastic toy from a mold," explains Sailor. "But what's left behind in our method is a flexible, biocompatible nanostructure with the properties of a photonic crystal." The properties of the porous silicon let Sailor's team "tune" their sensors to reflect over a wide range of wavelengths, some of which are not absorbed by human tissue. Scientists can fabricate polymers to respond to specific wavelengths that penetrate deep within the body.

To demonstrate how this process would work in a drug-delivery simulation, researchers created a polylactic-acid sensor impregnated with caffeine. The polymer was then dissolved in a solution that mimicked body fluids. Researchers found that the absorption spectrum of the polymer decayed with the increase of caffeine in the "body fluid" solution. "The artificial color code that's embedded in the material can be read through human tissue and provides a noninvasive means of monitoring the status of the fixture," says Sailor. "Such polymers could be used as drug delivery materials, in which the color provides a surrogate measure of the amount of drug remaining," he adds.