Polymers: Paving the Road of Medical Device Progress

Originally Published MDDI August 2004

Polymers 



Some dramatic developments in medical polymers have enabled device developers to push the technological envelope.

Len Czuba

Polymers were definitely making their presence known as the obvious material of choice for the growing healthcare industry 25 years ago. Glass intravenous (IV) bottles had largely been replaced within the previous 10 years with flexible PVC IV bags. 

This significant entrée into the area of medical devices led to the further shifting of polymers into the expanding area of accessories and all the various plumbing used in healthcare delivery. As fluid delivery became more sophisticated, the need for newer and better materials grew. Premixed-drug containers, better flow control systems, catheters of all sorts, and a whole variety of monitoring devices were introduced into the marketplace.

Gamma Sterilization

One of the big topics with which the industry wrestled was the move toward gamma sterilization from ethylene oxide gas (EtO) sterilization for dry products. Often, polymers exposed to gamma sterilization would not remain stable throughout the shelf life of the medical device. PVC materials often discolored and polypropylene syringes tended to shatter after 18–24 months in storage.

These materials deficiencies were resolved in the ensuing years, but not before many years of dedicated work in polymer science, the development of stabilizer chemistry, and process engineering were committed to solving this problem.

Implant Woes

The breast-implant fiasco of the early and mid-1990s sent the sourcing of materials for long-term implantable devices into a tailspin.
 
Manufacturers of implants suddenly faced a great reluctance on the part of the materials suppliers to continue to offer them polymers. This era was punctuated with heightened litigation, skyrocketing polymer prices, and, ultimately, a consolidation of products into large corporations that could offer indemnifications to the suppliers.

And the really sad thing is that the only winners in this whole situation were the law firms. The Biomaterials Safety Assurance Act was one measure taken by the U.S. Congress to help shield suppliers from massive lawsuits. To date, I don't believe that this measure has been tested in the courts. There are still only a handful of companies willing to supply polymers for implantable medical devices, and there are many restrictions on the use of these polymers. 

Additive Chemistry

The transition from heavy-metals-based colorants—and the eventual elimination of all nonessential colorants—unfolded during these times of acute sensitivity following the highly publicized breast-implant litigation. This caution resulted in improved safety of both the polymers and the devices made from them. It also forced colorant suppliers to develop alternatives that were safe for the patient and safe for the environment once the devices have served their purpose.

In the 1990s, metallocene catalysts were touted as the basis on which a whole family of promising new engineered polymers could be developed. Although many new materials have been introduced, the scope of properties expected (and originally stated as possible) was never realized.

PVC: A Target for Controversy

The call to find PVC replacements was a constant drone throughout the last 25 years, and sometimes a legitimate reason supported the call. The elimination of vinyl chloride monomer (VCM) certainly warranted the action. Avoiding the loss of absorbable drugs either stored or infused through the PVC also justified the need for finding a replacement. But by far, the call to replace or eliminate PVC was not based on good science. The safety of PVC has been well established, and it has been shown that, in the right application, it provides the best material properties and value.

Replacement materials have been developed using urethane polymers, modified olefin polymers, and a variety of olefin copolymers, each type having its own drawbacks or weaknesses. To date, PVC still provides the best combination of useful properties and value at the most economical price of all suitable medical device materials.

New Areas of Need

The primary focus now is to replace natural rubber latex (NRL) and develop novel elastomers. When a problem with sensitization to latex rubber was recognized as a life-threatening reaction, an extensive effort was initiated to eliminate NRL from all medical devices. By using synthetic polyisoprene elastomers or other nonlatex materials, the safety of medical devices has been vastly improved.

Thermoplastic elastomers (TPEs) have begun to fill the niche long held by thermoset polymers. Many TPEs have superior elevated-temperature resistance and many of the desired mechanical properties necessary for use in medical devices. The newest class of TPEs is thermoplastic vulcanizates (TPVs), which combine the ease of processing found in TPEs with the better physical properties of thermoset materials. 

Enabling Technologies

Rapid prototyping and computerization were powerful tools that dramatically helped accelerate the pace of product development in the medical device industry. Rapid prototyping got its start in the automotive industry, but quickly came to engineering teams in the medical device field. The efficiencies realized with this new tool coupled with computer-aided design (CAD) reduced development times, helped bring marketing input to the early stages of design, and greatly reduced costs associated with new product development.

Continuing improvements in resins and technologies offered better, faster, and stronger rapid prototypes. Drawings can now go directly from the design engineer to a part, and while the part is being made, an FEA (finite element analysis) can be done to determine where any problem areas existed. This is especially important because today's medical instruments are smaller than ever before.

The Advent of Less-Invasive Surgery

As healthcare delivery methods improved, it became apparent that less-invasive surgical techniques reduced trauma to the patient, improved recovery times, and reduced overall costs. These advances drove the need for miniaturization in instruments as well as in the components used in the complex mechanisms needed to perform less-invasive microsurgical techniques. 

Smaller parts also required better materials and new techniques for processing them to meet high tolerances. The load-and-shoot molding methods were not adequate for making such highly engineered precision parts.

New techniques for process monitoring and quality assurance were needed to support this burgeoning segment of the industry. Suddenly the polymer producers were challenged to develop specialty materials with higher tensile strengths, higher temperature resistance, higher abrasion resistance, and greater stiffness.

Polycarbonate was no longer the only game in town in the world of engineering resins. Polyimides, polyetherimide, polysulfones, polyphenyl-
sulfone, polyetheretherketone, polyphenylene sulfide, and liquid-crystal polymer all emerged with characteristics that would enable designers to push the limits of materials and products to new heights. These new materials offered vastly improved properties, but their premium cost limited their use to applications that could justify that added expense. Nevertheless, the miniaturization of the industry drove the use of these new materials, and it continues to the present. 

A Parallel Emerging Technology

Diagnostics have emerged as an adjunct science supporting the healthcare industry. Improved chemistry, more-sensitive analytical techniques, refined imaging technologies, and receptor and binding enhancements are all providing an early glimpse into disease states and improving the monitoring of patient health, often by the patients themselves. A prime example is the development of new blood glucose monitors for diabetic patients. The sampling is simpler, requiring no needle sticks, and it is fast and accurate. Patients are able to maintain proper glucose levels for optimum disease management. 

The next generation of diagnostic devices will be in vivo sensors offering continuous monitoring of a patient's condition; some of these are already available, but not in a size suitable for anything other than research use. In addition, these sensors need to be more sensitive, and they need to provide better accuracy and last far longer in place without fouling.

Improvements in membrane technology will be the basis on which sensors will be built, and some new membrane systems may even incorporate living tissue within the membrane matrix to enhance its performance. This is just one area of what has been called combinatorial science, and it is certainly one that will provide a strong platform for future growth.

Biomaterials

Finally, no review of medical polymers industry would be complete without some discussion of biomaterials. In the early days, biomaterials usually referred to implants: hip, knee, and other joint replacements. However, bioresorbable materials are emerging as a new enabling technology with which bone injuries and degenerative conditions are treated. Polylactic acid and copolymeric compounds are used to replace damaged bone, and these biomaterials allow regrowth of native bone structure as the implant is slowly resorbed. 

New materials are also being used as structures or media on which new tissue is grown ex vivo for later use in a patient. Wound coverings and dermal tissue grafts are only possible because of growth-inducing, tissue-compatible surfaces offered by hydrogel polymers.

Tissue adhesives, both biologically derived and created from synthetic materials, are an amazing new area of development for medical polymers. These polymers offer the ability to close wounds without the need for sutures, staples, or mesh reinforcement. These new materials can be used within the body and are compatible when applied. Some are even bioresorbable. As surgeons develop an understanding of how these materials are used, continued improvements will be made in medical treatments and ultimately in patient outcomes. 

Conclusion

Dramatic developments in polymers for the medical device industry have provided device developers the means to push the technological envelope. Over the last 25 years, the rate of new material developments has peaked and then waned, but this cycle has paralleled the rise and emergence of new technologies. It has been an exciting two-and-a-half decades, and with the improvements in communication and computer technology, the next 25 years promise to be even more exciting. Polymer technology will enable the wildest dreams to become reality, so we all need to keep on dreaming. 

Copyright ©2004 Medical Device & Diagnostic Industry