The indispensable biomaterial

15 February 2012 Widely used in the medical field, mechanically complex silicone elastomers are slowly giving up their secrets and becoming ubiquitous. When we think of biomaterials used in medical devices or for surgery, polymers like polyethylene or similar plastics…

15 February 2012

Widely used in the medical field, mechanically complex silicone elastomers are slowly giving up their secrets and becoming ubiquitous.

When we think of biomaterials used in medical devices or for surgery, polymers like polyethylene or similar plastics used in prosthetics or joint repair spring to mind. Or perhaps ceramics used in tooth fillings. Or, possibly, metals like stainless steel that are widely used in mechanical heart valves, stents, and joint replacements.

One of the oldest clinically applied biomaterials, however, is silicone. Its first recorded use as an implant was in 1946, when it was used to repair bile ducts; since then has been used in a tremendous array of medical applications, both in vivo and ex vivo. Silicone is also a polymer, but its specific backbone is built of repeating silicon-to-oxygen bonds. The material can be modified by linking silicon atoms to organic groups.

Cross-linking silicone produces an elastomer, and when it is chemically treated with silanes to enhance matrix integration, it becomes remarkably strong and resistant to tears. Today, silicone elastomers have advanced far beyond their best known application, breast implants, and have become one of the key enabling technologies for medical devices.

Silicone: Indispensable inside the body, and out
In February 2011, Biocoat Inc., Horsham, Pa., introduced its latest product, HydroSleek. Based on the company’s HYDAK silicone coatings technology, HydroSleek has been engineered to be extremely slippery to liquids, particularly water, and is intended for medical device firms seeking to reduce surface friction for their devices.

A 21-year-old company, Biocoat was established by technology pioneered at Columbia University, New York, in the early 1980s. Chemist Ellington M. Beavers, PhD, and his team invented a method for immobilizing hyaluronan and other biopolymers. Hyaluronan or hyaluronic acid (HA) was first discovered in the 1930s, and until the 1970s was ingloriously described as a “goo” molecule. The human body has about 15 g of this material at any one time, and it acts as a natural coating around cartilage cells. It’s also the naturally lubricating agent in synovial fluid, which separates most surfaces that slide against each other in tendon sheaths and joints.

Interest in HA has increased greatly in recent years with major clinical applications in ophthalmology, the treatment of degenerative joint disease, and adhesion prevention after surgery. Medical-grade HA is now produced globally in more than a dozen countries.

At Biocoat, HA was commercialized shortly after it was immobilized in Beavers’ laboratory. Since then, it has undergone a steady process of improvements. New additives and derivations have allowed Biocoat to make bioactive surfaces that can repel bacteria. HydroSleek is an example.

“The product is actually made possible by the primer coat, or basecoat, technology, which has been developed at Biocoat since 1991,” says Josh Simon, senior product manager at Biocoat. “The basecoat is a proprietary polyacrylic co-polymer that uses isocyanate chemistry to reliably and consistently attach species that contain hydroxyl, carboxyl, and/or amine groups. This includes hyaluronic acid, which is the naturally occurring polysaccharide found in cartilage and blood, and in HydroSleek as well.”

The basecoat/topcoat combination can be applied to a multitude of substrates, including nylon, silicone, PVC, and polyurethane. The total thickness of both layers can be as thin as 2 to 3 µm when dry, swelling to 10 µm when wet. The application process is the same as Biocoat’s HYDAK coatings and is a relatively simple process using conventional coating equipment and curing ovens. Usually coatings are applied by dip-coating or cured with ultraviolet light.

“The coating has a coefficient of friction ranging from 0.01 to 0.05, after sterilization and aging. This is notable because most coatings lose performance after sterilization. This one does not,” says Simon.

The main departure from previous coatings is that Biocoat has dealt with the traditional tradeoff between lubricity and durability. Usually, the more durable a coating is, the less lubricious it is, and vice versa. HydroSleek has all of the durability and lubricity of previous coatings, says Simon.

Veryst Engineering, a contract design company, performs materials analysis and testing with an emphasis on biomaterials, such as silicone elastomers. One of their most important tools is finite-element software from vendors such as COMSOL, ANSYS, and Simulia. Image: Veryst Engineering

A year after its introduction, the coating has become popular with cardiovascular and neurovascular companies.

“Stent delivery catheters and neurovascular guidewires seem to be the most desired applications to date,” says Simon. “However, this coating can be used in any of the applications of Biocoat’s current customers, which range from intraocular lens cartridges to catheters used in the fertility industry.”

Computation analysis enables biomaterials
Biocoat has achieved market success through decades-long R&D regimens that resulted in successful products. But benchtop science is now giving way to materials design assisted by advanced computer-aided modeling solutions, such as finite-element analysis (FEA). Among biomaterials, elastomers are among the most challenging to design. An FEA solution must deal with highly non-linear material behavior and large deformations.

“We have worked extensively on silicone elastomers for various medical device manufacturers, so we are very familiar with their properties,” says Jorgen Bergstrom, PhD, principal engineer at Veryst Engineering, Needham, Mass.

Veryst is a contract engineering and design firm that helps biomaterials manufacturers develop new products. It offers companies or universities the ability to mechanically test materials under a wide variety of formats and a range of environmental conditions, including tension and compression, bending, lap shear, hysteresis, creep, and volumetric compression. Most of these tests can be performed at temperatures from -80 to 280 C.

The company then feeds this data into FEA software from COMSOL, Burlington, Mass., to simulate the actions of materials such as polyethylene, stainless steel, and silicone. Because the bulk of Veryst’s client base is in the biomaterials or medical device manufacturing business, engineers like Bergstrom have become familiar with the behavior of silicone.

This knowledge has been used by Veryst to create the PolyUMod library, a series of specialized modules that cover the four main categories of polymeric biomaterials: thermopolymers, thermosets, elastomers, and foams. Bergstrom himself created a model for elastomer while at the Massachusetts Institute of Technology, Boston. That module is now included in the PolyUMod library as the Bergstrom-Boyce model. Silicone is one of the most commonly used biomaterials, he says, although it has not been as thoroughly tested as polyethylene.

Silicone elastomer design requires the expertise provided by Veryst, Bergstrom continues, because they are physically complicated. The materials have a long, non-linear stretching capability, and FEA is one of the quickest ways engineers can predict how much a given formulation can be safely stretched, how much it continues to stretch when relaxed, and what its durability is over time. The PolyUMod tools help capture non-linear elastomeric behavior. The biggest challenge for engineers, Bergstrom says, is the long-term degradation of the material that occurs in some environments. Even in the laboratory, experimental data for how a given elastomer breaks down in biological environments is still quite poor.

However, Bergstrom anticipates this situation to improve. “Finite element [analysis] as a tool has been around for 30 years or more, but because biomaterials are complicated, it hasn’t been easy for design engineers to handle,” says Bergstrom. In the last five to 10 years, however, he has seen a dramatic improvement in biomaterials reliability in part because of tools like FEA. “I think in the next five to 10 years we’ll see even more of a marked improvement.”