Researchers at the University of California, San Diego 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 is detailed in the March 28 issue of Science by a UCSD team that pioneered the development of a number of novel optical sensors from silicon wafers, the raw starting material for computer chips.
Led by Michael J. 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.
Now Sailor and his team have 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.”
Besides Sailor, the researchers involved in the discovery included UCSD chemists Yang Yang Li, Frederique Cunin, Jamie Link, Ting Gao, Ronald E. Betts and Sarah Reiver; Sangeeta Bhatia, an associate professor of bioengineering at UCSD, and UCSD bioengineer Vicki Chin.
The method Sailor’s 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 the 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 Bhatia, who is a physician. “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 diabetes or monitoring the process of tissue repair in tissue engineering.”
To demonstrate that this process would work in a medical drug delivery simulation, the researchers created a polymer sensor impregnated with caffeine. The sensor was made of polylactic acid, a polymer used in dissolvable sutures and a variety of medically implanted devices. The researchers watched as the polymer dissolved in a solution that mimicked body fluids and found that the absorption spectrum of the polymer decayed in step with the increase of caffeine in the solution.
“This confirms that the drug is released on a time scale comparable to polymer degradation,” the researchers report in the journal.
“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.”
The study was supported by grants from the National Science Foundation, The David and Lucile Packard Foundation and the Air Force Office of Scientific Research.
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