Medtech Material Selection For Performance And Feel

By Eric Larson, Art of Mass Production

One of the most challenging aspects of medical device design, or the design of any product for that matter, is proper material selection. This is especially true for plastic materials. With dozens of different material families, and hundreds of grades and versions within each family, there are thousands of different plastic materials available. How do you select the right one?

The process typically involves basic research to obtain material properties for a given number of material candidates, followed by an evaluation of these properties against the performance requirements of the end-use application. Often, other issues must also be considered, including environmental effects, industry and regulatory agency requirements, cost considerations, etc.

But the feel of the material itself is an important aspect of material selection that is often overlooked. “Feel,” in this sense, includes not only what is perceived through the sense of touch, but also what is perceived through sight, hearing, and sometimes even smell and taste. For many products, the feel of the finished product has a significant effect on its function, so it is important to understand how the materials used affect the overall feel.

What Is Feel?

In its most common use, feel refers to tactile perception, that which we can discern through touch. Tactile input is received not just through our fingers, but our feet, our skin, our teeth, muscles, and organs, even our very bones. We also receive input from our other senses, which may overlap with our sense of touch. Quite often, we receive sensory input from many of our senses all at the same time, and we are not really aware of the subtle distinctions.  

 

Feel also is used to describe sensations we are experiencing, responses to sensory input from any of our senses. I feel nauseous. I feel dizzy. I feel warm. These kinds of responses can be visceral and powerful. Fingernails on a chalkboard. Something you taste that makes you think “Yuck!” Something you smell that makes you say, “Eww, that stinks.” And while we may call them feelings, they are not emotions.

These sensations may also trigger memories and emotions. The sensory input may remind you of a past event (good or bad), or a time of the year. The warmth you feel inside when you smell fresh baked cookies. A feeling of doom. A feeling of being home. This response to sensory input is an important aspect of feel.

Finally, there are emotions: Anger, fear, sadness, joy, and a whole spectrum of feelings in between. While a material by itself rarely generates an emotion, we need to be aware that the materials used in products often affect the sensory input of the people who interact with those products. This sensory input can create physical sensations that result in a (sometimes powerful) emotional response.

If we integrate all of this – tactile and other sensory input, sensations, emotions, thoughts, judgements, and life experiences – this is feel.

The Feel Of A Medical Device

You may ask, “Why should a medical device designer be concerned about feel?” The answer is, “it depends.” If you are designing a device and your concern is limited to safety and efficacy, feel may not be that important. But, a medical device that combines safety and clinical effectiveness with good “feel” can often lead to higher performance, higher user satisfaction, and a great path to improved sales and market share.

The feel of a device involves all of the sensory input we receive when we interact with it, all of the sensations we experience from that input, and all of the memories and emotions that are evoked. One advantage of plastic materials is that they offer a unique opportunity to affect every aspect of feel.

Performance

We typically evaluate performance of a medical device based on measurable, quantifiable data, and we often conduct clinical trials to obtain that data. It is not ironic that one definition of the word clinical is “scientifically detached; strictly objective.” Design teams often rely on ergonomic data to enhance the “ease of use” of a device for different segments of the user population, with the specific goal of improved performance for all users.

What is sometimes overlooked is that feel can be an important aspect of product performance. In some applications, feel is so integral to performance that it becomes part of the product specification. This is especially true in the sporting goods and automobile industries. In other applications, feel is an afterthought, something to check at the very end. If poorly addressed, this can lead to user frustration, confusion, and inappropriate or ineffective use — with a resulting loss of performance. When questioned, users often can’t verbalize the reason for their frustration, and say something like, “It just doesn’t feel right.”

Yet, it is rare in the medical device industry for design teams to evaluate the feel of a device, or to consider the effects of material selection on feel. 

Safety

Product safety goes hand-in-hand with performance, and it generally is a device designer’s primary objective. The secondary objective is to design against improper use, often accomplished through fail-safe mechanisms, redundant systems, and other safety protocols. Finally, we strive to make a device “idiot proof,” so that no matter what, the device cannot be used improperly. Ironically, this is a challenging task, because — as any good designer can tell you — sometimes those idiots can be so clever.

My fundamental belief is that by using the right materials — not just materials that are FDA, UL, or Canadian Standards Association (CSA) approved, but materials that are fundamentally the right materials for the application — we can design products that are  inherently safe and perform as intended. For example, a device that not only has the right balance, it absorbs vibration where it needs to be absorbed and transmits vibration where it needs to be transmitted. It offers the tactile feel we need, the acoustic properties we need, the thermal conductivity we need, etc., and it provides our intended users with feelings of safety, comfort, and reliability.  

Treatment Compliance

Typically, treatment compliance is evaluated based on how patients adhere to (or depart from) an established treatment protocol. This view of compliance is based on a simple question: Is a patient doing what they are being told to do, exactly the way they are supposed to be doing it?  

Some medical treatments have incredibly high patient compliance — vision care, for example. Based on information available from the Vision Council of America, the National Eye Institute, and the CDC, one could estimate treatment compliance for vision care in the U.S. at somewhere between 95 percent and 99 percent — regardless of whether the treatment is eye surgery, contacts, or eyeglasses. Why? I would postulate that it is a combination of factors: The treatment is affordable and readily available, post-treatment improvement is almost immediate, and the user typically receives additional positive reinforcement. (i.e., Your new glasses make you look smart / attractive / professional.) 

No matter what treatment protocol the user selects, it is more often than not facilitated through material selection, specifically plastics. Contacts and eyeglasses made from plastics are lightweight, comfortable to use, and make us feel good. At the opposite end of the spectrum are treatment protocols that don’t feel good — a painful medical procedure, a drug that makes you nauseous, or a medical device that is uncomfortable or provides an unpleasant overall experience. 

Consider the work of Temple Grandin, a professor of animal science at Colorado State University. Temple was diagnosed with autism and is hyper-sensitive to sound and sensory stimuli. Her life experiences have provided her insight into animal behavior, and she is a recognized expert in designing systems that reduce the stress of livestock during grazing, handling, and transport. If we can consider animals’ environmental response as they are being led to slaughter, why can’t we consider human beings’ response as a part of medical device design?

Another aspect of compliance is practitioner adherence: Are treatment providers doing what they are supposed to do, exactly the way they are supposed to be doing it?

One example of practitioner compliance involves a simple procedure – medical service provider hand-washing, prior to interacting with a patient after surgery. In 2008, after his wife had knee-replacement surgery, Dr. Gerald Hickson, senior VP for quality, safety and risk prevention at Vanderbilt University Medical Center, noticed that the vast majority of providers who were attending to his wife failed to wash their hands.

This simple observation led to a major protocol overhaul for the university — not just procedures, but training, reinforcement, and organizational culture. However, I would surmise that this overhaul did not address the design of the devices involved (washing stations, drying devices, material disposal systems, etc), the use of those devices, or the materials involved.

Regardless of whether we are measuring patient compliance or practitioner compliance, we need to remember that we are evaluating, and hopefully changing, human behavior. While we may often think of this as a design issue, material selection can play a critical role.

Sales And Market Share

Feel also can provide a manufacturer with a competitive advantage in the marketplace. An excellent example of the financial power of “good feel” comes from the golf industry.

In the early 1900s, the golf ball of choice for most players was a three-piece design, consisting of a solid rubber core, wrapped with a series of rubber threads and then covered with an outer layer. This three-piece design, known as a wound ball, was manufactured for decades.

The wound ball’s outer layer was first made using balata, a natural rubber that comes from the sap of a Balata tree. Balata balls were high-performance, providing a combination of distance and control. The soft balata cover allowed a skilled player to put spin on the ball, usually back spin, but also side spin. Good players could shape the flight of the ball, control where it landed, and manipulate how it behaved on landing. However, the balata cover was not durable, and was easily damaged.

In the 1960s, golf ball manufacturers began to experiment with wound balls covered by different, synthetic outer layer materials, as well as different combinations of winding materials and layers. While these golf ball incarnations were cheaper to manufacture or more durable than balata, most skilled players avoided them: They not spin the same, they didn’t sound the same, they didn’t look the same, and they didn’t feel the same. “It’s like hitting a rock,” some said.

In the 1980s, the Titleist Company began making a high-performance wound ball known as the Titleist Professional. It had a synthetic cover, as well, but was softer than other synthetic materials, provided performance similar to the balata ball, and offered greater durability. It also sounded, looked, and felt like a balata ball, and it quickly became a player favorite. Fast-forward to 2000, and Titleist’s Pro V1 — a solid-core ball consisting of various synthetic layers, itself based on a novel Nike ball design — was the most widely used ball in professional golf, and the wound ball became a relic.  

A similar example of market share success is the Humalog/Humalin insulin pen from Eli Lilly. Introduced in 1999, it is not only highly accurate, it is light weight, comfortable, and easy-to-use. Many users (including my mom) comment that it feels right. Today, over 15 years later, it is still in production, and well over 100 million pens have been produced.  

The Bottom Line On Feel

A few years ago, I was writing a book on plastic material selection. When we first began discussing the chapters, my editor asked me, “What methods do people typically use to select thermoplastic materials?” We talked about selection methods based on performance, appearance, and cost. And then, without really thinking, I said something to the effect of, “You can also select thermoplastic materials based on feel.” As soon as I said that, I had a knot in my stomach. A little voice in head said, “You can’t do that. Material selection based on feel is wrong. It’s simply not . . . technical.”

Over time, though, I realized that people select materials based on feel all the time. Products of all kinds, whether consumer, industrial, or medical, have gaskets and vibration absorbers and sound dampeners. And when it comes to cams and mechanisms, engineers often select materials based on the sound they make or the vibration they make (or don’t make). All of these material choices are based on feel.

Rarely though, is there a methodology behind this selection process. I believe that may, in part, be because technical people don’t want to talk about feel, since it lacks a definable analytical process. There is a positive bias to thinking, which is a rational, valid process. Conversely, feeling is considered an irrational, invalid process. However, I believe that thinking and feeling both are valid processes, and that either side of the brain (and hopefully both) can be used in the selection of thermoplastic materials.

Feel is the final hurdle in effective material selection. After performance measurements and cost analyses all are complete, the ultimate determinant of product success often comes down to feel. As Spock might say, it is not logical, but despite our insistence that we are rational, logical creatures who go about our lives making conscious choices, most of us live by feel. We engage in activities that make us feel happy. We choose friends who make us laugh. We buy clothes that feel comfortable. We buy toys and games and cars (especially cars) that make us feel good. Shouldn’t we expect the same from our medical devices?

Evaluating Plastic Materials for Feel

Material selection is about finding one or more suitable materials that — in combination with an effective design, proper processing, and eventual integration into a final system — results in a product that meets its intended use and satisfies the needs of (and hopefully delights) the end user.

One of the great challenges in selecting plastic materials based on feel is establishing the proper evaluation criteria. What material properties should we be concerned with? What are the desired values for each property? How do each of these properties contribute to the feel of the material, as well as to how a user responds to the device? We will explore these questions in greater detail in our next article.

About The Author

Eric R. Larson is a mechanical engineer with over 30 years' experience in plastics. He has helped develop products ranging from boogie boards, water basketball games and SCUBA diving equipment to disposable lighters, cell phones and handheld medical devices. Eric is owner of Art of Mass Production (AMP), an engineering consulting company based in San Diego, CA. AMP provides services to manufacturing companies in the consumer electronics, wireless, and medical device industries. Eric is also moderator of the blog site plasticsguy.com, where he writes about plastics technology and its effect on people and the planet. His newest book, Plastics Materials Selection: A Practical Guide, can be purchased through his blog site.