Researchers at the University of California-San Diego (UCSD) have discovered how to transfer the optical properties of silicon crystal sensors to plastic, an achievement that could lead to the development of flexible, implantable devices capable of monitoring the delivery of drugs within the body, the strains on a weak joint or even the healing of a suture.

The discovery was made by a UCSD team that pioneered the development of a number of novel optical sensors from silicon wafers.

Led by Michael Sailor, a professor of chemistry at UCSD, the team recently developed sensors from dust-sized chips of “porous” silicon capable of detecting biological or chemical agents that might be present in a terrorist attack. It also developed a new kind of nerve gas detector based on a porous silicon chip optical sensor that changes color when it reacts to sarin and other nerve agents.

The research team has developed a way to transfer the optical properties of such silicon sensors, once thought to be the exclusive domain of “nanostructured” crystalline materials, such as porous silicon, to a variety of organic polymers.

“While silicon has many benefits, it has its downsides,” explains Sailor. “It’s not particularly biocompatible, it’s not flexible and it can corrode. You need something that possesses all three traits if you want to use it for medical applications. You also need something that’s corrosion resistant if you want to use it as an environmental sensor. This is a new way of making a nanostructured material with the unique optical properties of porous silicon combined with the reliability and durability of plastics.”

According to Sailor, the method his team uses to create the flexible, polymer-based sensors is something similar to the injection-molding process that manufacturers use in creating plastic toys. The scientists first start by treating a silicon wafer with an electrochemical etch to produce a porous silicon chip containing a precise array of tiny, nanometer-sized holes. This gives the chip the optical properties of a photonic crystal—a crystal with a periodic structure that can precisely control the transmission of light much as a semiconductor controls the transmission of electrons.

The scientists then cast a molten or dissolved plastic into the pores of the finished porous silicon photonic chip. The silicon chip mold is dissolved away, 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.”

Those properties could allow a physician to directly see whether the biodegradable sutures used to sew up an incision have dissolved, how much strain is being placed on a newly implanted joint, or how much of a drug implanted in a biodegradable polymer is being delivered to a patient.

This is possible because the properties of porous silicon allow Sailor’s team to “tune” their sensors to reflect over a wide range of wavelengths, some of which are not absorbed by human tissue. In this way, the scientists can fabricate polymers to respond to specific wavelengths that penetrate deep within the body.

A physician monitoring an implanted joint with this polymer would be able to see changes in the reflection spectrum as the joint is stressed at different angles. A physician in need of information about the amount of a drug being delivered by an implanted device can obtain this by seeing how much the reflection spectrum of a biodegradable polymer diminishes as it and the drug dissolve into the body.

Such degradable polymers are used to deliver antiviral drugs, pain

and chemotherapy medications and contraceptives.

“The drugs are released as the polymer carrier degrades, a process that can vary from patient to patient, depending on the site of implantation or the progression of a disease,” says Sangeeta Bhatia, an associate professor of bioengineering at UCSD. “This approach offers a noninvasive way to monitor the degradation of the device, decide on when it needs to be replaced, and evaluate its function. This same approach would be useful for other implantable devices like evaluating the status of implantable glucose sensors or monitoring the process of tissue repair in tissue engineering.”

“The artificial color code embedded in the material can be read through human tissue and provides a noninvasive means of monitoring the status of the fixture,” adds Sailor. “Such polymers could be used as drug delivery materials, in which the color provides a surrogate measure of the amount of drug remaining.”