Medical Device Link.

Originally Published MDDI May 2003

R&D DIGEST

Polylactide containing caffeine and an optical code. Normally colorless, the polymer picks up a characteristic color when cast in a nanostructured porous template consisting of a silicon photonic crystal. Color decay can be monitored through tissue to provide a marker of the amount of drug delivered from the polymer. The inset shows a flexible, biocompatible polymer molded from porous silicon.

Researchers have now found a way to transfer the optical properties of silicon crystal sensors to plastic. They believe that their work could lead to new types of flexible, implantable devices. Such sensors could track drug delivery, measure strains on a weak joint, or assess the dissolving of a suture.

Initially, the team made sensors from dust-sized chips of porous silicon that could detect the presence of biological agents. They also created a nerve gas detector based on a porous silicon chip optical sensor, which changes color in response to certain nerve agents.

Now, Michael J. Sailor, PhD, professor of chemistry at University of California, San Diego, says his team has developed a way to transfer the optical properties of the silicon sensors to a variety of organic polymers. Such properties were once thought to be found only in nanostructured crystalline materials, such as porous silicon. 

Sailor notes that one hurdle in the project was to induce the polymer “to infiltrate nanometer-scale pores of the porous silicon template in order to transfer the interesting photonic properties to the polymer.”
Says Sailor, “While silicon has many benefits, it has its downsides. 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.”

The method used to create the polymer-based sensors is similar to a conventional injection molding process. A silicon wafer is electrochemically etched to produce a porous silicon chip that contains a precise array of nanometer-sized holes. In this way, the chip’s optical properties are made to resemble those of photonic crystal. The crystal is thus given a periodic structure that can precisely control the transmission of light much as a semiconductor controls the transmission of electrons, according to the group.
Melted plastic is cast into the pores of the porous silicon photonic chip, which acts like a mold. The silicon is then dissolved away, producing a replica of the photonic chip. Says Sailor, “It’s essentially a similar process to the one used in making a plastic toy from a mold. But what’s left behind in our method is a flexible, biocompatible nanostructure with the properties of a photonic crystal.”

Such a sensor could provide significant benefits in medical applications, according to the researchers. As Sailor explains, “It provides a noninvasive means of probing the status of a medical fixture that can be incorporated into a wide range of currently available (and approved) implantable materials.” For example, a physician could determine directly whether biodegradable sutures used to close an incision have dissolved. The device might also be used to monitor how much of a drug implanted in a biodegradable polymer is being delivered to a patient.

The researchers explain that the properties of porous silicon enable the sensors to be “tuned” to reflect a range of wavelengths, including some that are not absorbed by human tissue. This allows polymers to be fabricated that can respond to specific wavelengths that penetrate deep within the body. 

For example, as drugs are released from an implanted polymer delivery system, the process can vary from patient to patient. The implantation site or the rate of disease progression can affect drug uptake, explains Sangeeta Bhatia, MD, PhD, a member of Sailor’s team. 

“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,” says Bhatia. “This same approach would be useful for other medical applications, such as evaluating the status of implantable glucose sensors or monitoring the process of tissue repair in tissue engineering.”

To demonstrate use of the device in monitoring drug delivery, the researchers created a polymer sensor impregnated with caffeine. The sensor was made of polylactic acid, which is used in dissolvable sutures and certain implanted devices. When the polymer was dissolved in a solution that mimicked body fluids, the absorption spectrum of the polymer decayed at the same rate as the amount of caffeine in the solution increased.

The researchers suggest that the study confirms that the drug is released on a time scale comparable to polymer degradation. Sailor adds, “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. Such polymers could be used as drug-delivery materials, in which the color provides a surrogate measure of the amount of drug remaining.” 

A number of steps remain before the sensor will be ready for medical applications. Says Sailor, “To bring this into the medical device community we have to prove it in animal models.” 

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