Medical Device Daily Contributing Writer

BOSTON — Some 14,000 chemical scientists from around the world are attending the American Chemical Society (ACS; Washington) fall national meeting being held this week at the Boston Convention & Exposition Center. One focus of the meeting has been a series of four sessions called “Polymer Science of Everyday Things,” which included 24 presentations about “how medical devices that people encounter and use every day depend on polymer science.”

“Think of a desired application, and/or a desired property, and it is highly likely that a polymer exists or can be designed to perform that task,” said Randy Mrsny, PhD, of Welsh School of Pharmacy at Cardiff University (Cardiff, Wales). “Polymers come in a multitude of forms that can allow for selection of properties based not only on function but also compatibility.”

So, what’s a polymer anyway? “Any material produced by chemical — synthesized or biological — processes that contains a covalently linked, repeating unit,” Mrsny said.

While polymers make possible a plethora of “everyday” products — toothpaste, antiperspirants, vitamins, hair gel, granola bars, condoms, wound dressings, disposable diapers and so forth — applications involving contact lenses, spinal devices and medical device coatings were among the best-attended sessions at the meeting.

A presentation on “The Polymer Chemistry of Contact Lenses: Improving Comfort with Bulk and Surface Modification” by Robert Ward, president and CEO of Polymer Technology Group (Berkeley, California), focused on the bulk and surface properties required in polymers made from silicone hydrogel and the innovative PTG processing that allows rapid scale-up from small R&D formulation to automated manufacturing for producing silicone-hydrogel polymers in bulk quantities — and the resulting positive effect on the market for such contact lenses.

“Silicone-hydrogels make possible a new generation of super-permeable contact lenses that can transmit unprecedented amounts of oxygen to the cornea and, in some cases, enable 30 consecutive days of extended wear without removal,” said Ward. “Silicone-hydrogel contact lenses represent a breakthrough over traditional hydrogel soft contact lenses because silicone permits so much more oxygen to pass through the lens, which is essential for a healthy cornea.”

In fact, he said, “These next-generation lenses allow as much as seven times more oxygen to permeate than previous contact lenses, which is why they are becoming very popular with both wearers and eye care professionals. Most people find silicone-hydrogel lenses much more comfortable to wear than traditional hydrogel lenses, and this is dramatically growing the market for silicone-hydrogels.”

In addition to increased oxygen permeability, comfort is improved through control of surface chemistry, without which high-silicone lenses might actually adhere to the eye, Ward said. Thus, the success of silicone-hydrogel lenses comes from well-known bulk modifications of the lens material (i.e., the inclusion of silicone), combined with sophisticated surface modifications that convert the normally hydrophobic surface of silicone to a very wettable, hydrophilic surface that discourages protein accumulations and supports a normal “tear film” on the outer surface of the lens.

State-of-the-art contact lenses made from silicone-hydrogel polymers that allow for extended wear are expected to represent more than two-thirds of U.S. soft contact lens sales by 2009, according to a Robert W. Baird & Co. equity research report.

A presentation on “The Challenge of Spinal Disc Replacement” by Alastair Clemow, PhD, president and CEO of Nexgen Spine (Whippany, New Jersey), focused on how biomaterials for disc replacement to treat lumbar spinal stenosis and other spinal maladies must be capable of enduring high loads, acidic environments and have a long service life. “Polymers are flexible,” said Clemow, a past president of the Society for Biomaterials (Mt. Laurel, New Jersey). “And they are compliant, which makes a polymer solution more attractive than metals, ceramics and composites.”

The “gold standard” for treating lumbar spinal stenosis today is spinal fusion, with a clinical success rate of 75% over 30 years. However, spinal fusion surgery typically requires a long recuperation period of nine to 15 months, according to Clemow, and an incidence of 10% to 30% for adjacent-level disc degeneration over a period of five to 10 years post-fusion.

Nexgen Spine has designed a total-disc prosthesis constructed of elastomeric polycarbonate polyurethanes (polymers) of differing hardness that the company believes provides “near-physiologic motion patterns.”

“First-generation artificial discs have demonstrated a lack of resistance to motion, which overloads adjacent structures, because the remaining segments of the spine are required to move more in order to provide the necessary mobility,” Clemow said. “Such hypermobility can cause further degeneration of the spine, leading to the need for further spinal surgery.”

Nexgen has said it believes there will be about 302,000 lumbar fusions by 2010, and that some 65% of these patients will be candidates for artificial discs.

“For good reason, there is a lot of interest in generation-2 implants for total disc replacement,” Clemow said. “No doubt, disc replacement represents a difficult materials challenge. With a soft inner nucleus, a stiffer encapsulating nucleus and a rigid outer endplate, we believe our materials are optimized to closely match the intact physiologic disc.”

A third presentation, “From Willow Bark to PolyAspirin: Discovery and Innovations,” by Kathryn Uhrich, PhD, professor of chemistry and chemical biology at Rutgers University (Piscataway, New Jersey), discussed applications for PolyAspirin (a novel polymer form of aspirin, or salicylic acid) used in medical device coatings for cardiac stents, as standalone stents and other products.

“Salicylic acid (SA)-derived polymers inhibit biofilm (bacteria) formation,” said Uhrich. “Initial PolyAspirin uses include a drug-eluting coronary stent coating and carrier for antiproliferative agents developed by Polymerix (also Piscataway). In this application, a stent is combined with a pharmaceutical agent and then placed into a human coronary artery to help prevent restenosis (when scar tissue reblocks arteries).

“In more recent studies sponsored by Bioabsorbable Therapeutics (Menlo Park, California), the same pharmaceutical agents are released from a completely resorbable stent. The stent’s delivery of drug to the wounded artery is controlled by the PolyAspirin coating that gradually releases a drug into the vessel lining to prevent scar tissue growth, a frequent reaction that leads to restenosis. These biodegradable coatings are unique in that they are non-inflammatory and targeted to disappear in six to nine months after providing their therapeutic effects and delivering other agents,” Uhrich said.

According to BCC Research (Norwalk, Connecticut), the value of worldwide sales for all categories of coatings and surface treatment processes used in manufacturing medical devices reached $2.96 billion in 2005 and is expected to grow to $5.31 billion by 2010. The greatest demand for coated medical devices will come from cardiovascular medicine, dentistry, general and plastic surgery, general hospital settings, neurology, ophthalmology, orthopedics and radiology.

BCC Research notes: “The swift acceptance of drug-eluting stents as a ‘gold standard’ treatment by surgeons around the globe suggests that the demand for other coated versions of existing medical devices will be equally swift ... [and] that there will be a correspondingly swift acceptance of new coated devices as they are introduced.”