Polymer Material Selection in Medical Devices

Materials are the physical backbone in every product, andadvancement in materials science and chemical engineering have been a drivingforce behind many of the technological innovations in medical devices. Broadly, medical device materials can becategorized as metals, polymers, ceramics, and composites.This article explores factors to consider when selecting polymers for use inthe development of new medical device products. The vast array of medicalproducts in use today would simply not exist without polymer technology and themanufacturing processes to configure them into their finished form andproperties.

Defining Device Requirements

The clinical intended use is the ground floor for materialselection. Anticipating the tissue contact categories for the product is a keyinput to material selection decisions. ISO 10993-1:2025 defines an updated listof four tissue contact categories: intact skin, intact mucosal membranes,breached or compromised surfaces or internal tissues,and circulating blood other internaltissues, and blood. The contact categories dictate whichbiocompatibility tests are applicable and should be considered early and oftenduring polymer selection planning.

Deciding whether a product should be disposable,reusable, or reposable is a major factor affecting product cost, and theapplicability of cleaning and sterilization requirements. Disposable productsare also referred to as “single-use”, or “single-patient use”. These productsmay use a much wider set of polymer options compared to reusable devices,which, depending on the tissue contact category, may require disinfecting,cleaning, and re-sterilization and the accompanying chemical and heat exposureover many cycles.  Reposable productsemploy a combination of both: the “razorblade and handle” model, wherein thepatient-contacting portion of the device may be single-use, and interconnectwith a reusable non-patient-contacting portion of the device.

If a medical device is to be introduced and used in thesterile field, the material selection must consider the sterilization methodand accompanying polymer compatibility. The three main categories for medicaldevice sterilization and the most common methods in each category include:

-  Gas / Chemical: Ethylene Oxide gas (EtO),Vaporized Hydrogen Peroxide (VPH)

-  Radiation: Gamma, E-Beam

-  Thermal: Steam (autoclave)

The sterilization method is highly influential in materialselection because certain polymers are incompatible with certain sterilizationmethods. For instance, fluoropolymers such as PTFE (common in catheters) areincompatible with typical radiation doses delivered in Gamma sterilization,which is why catheter devices are most commonly sterilized with EtO gas orlow-dose E-beam.  Conversely, manyhospital instruments are sterilized using autoclaving because of its low cost,however the pressurized saturated steam used to achieve the sterilizationtemperatures of 121-135°C exceeds the heat and moisture resistance of mostpolymers.

Engineering performance requirements comprise numerouscategories, including tensile/flexural/torsional strength, stiffness, ultimatetensile strain, temperature resistance, electrical insulation, temperatureresistance, chemical compatibility, adhesive compatibility,bearing and wear performance, coefficient of friction, optical transparency oropacity, and material processability through thermoforming.

Key Factors in Material Selection

- Chemical Structure Categories

Thermoplastic polymers can be subdivided into two maincategories by their chemical structure: amorphous and semi-crystalline.  In amorphous thermoplastics, the polymerchains are arranged in a random, tangled structure, like a bowl of cookedspaghetti.  In semi-crystallinethermoplastics, the polymer has regions where the chains are highly ordered(crystalline structure) mixed within random (amorphous structure) regions.

Thermal behavior is a key distinguishing characteristicbetween amorphous and semi-crystalline thermoplastics.  Amorphous materials have a glass transitiontemperature (T_g), and no true melting point, and gradually soften asthe temperature rises above the T_g. Semi-crystalline materials, bycontrast, have both a glass transition temperature (T_g) and amelting temperature (T_m), and stay relatively stiff until theyapproach the melting temperature and then melt over a narrow temperature range.

- Performance vs. Cost Categories

Another way to further sub-categorize amorphous andsemi-crystalline thermoplastics is by relative performance and cost, andunsurprisingly, higher performance comes at a cost. Commodity thermoplasticsare widely used and are the lowest cost tier. Engineering thermoplastics arethe next tier up in performance and are comparatively moderate cost.High-performance thermoplastics are the highest performing materials andcommand the higher cost.

Source: IAPD Thermoplastic Rectangle

Generally, a useful approach to material selection is todecide whether amorphous or semi-crystalline material properties best match theoverall functional needs, followed by evaluating performance and cost tradeoffsto land in the right quadrant.

Manufacturing Considerations

The intended manufacturing process also is a key factor inselecting materials. For example, PTFE and PEEK are both high-performancesemi-crystalline medical device materials that share common characteristicslike high strength, low coefficient of friction, and toughness. However,whereas PEEK can be injection molded using high-temperature heated moldtooling, PTFE does not become a free-flowing melt when heated above its veryhigh melting point, and therefore it is typically processed by ram extrusion orcompression molding followed by sintering. Different materials also possessdifferent shrink rate ratios for injection molding which must be properlyaccounted for in tooling design and thermal processing to ensure dimensionalaccuracy in the finished  part configuration. For catheterapplications, relatively minor material changes such as modifying durometer oradditives can influence extrusion parameters, reflow processing parameters,adhesive bonding, or coating compatibility. Manufacturing considerations shouldaccount for the full “processing lifecycle” of the material to finished productform.

Managing Design Tradeoffs

Specific material choice, down to the particular trade nameand grade, often comes down to managing tradeoffs of different characteristics;mechanical, thermal, electrical, optical, chemical, and cost. ABS is an exampleof a widely- usedcommodity thermoplastic material that has moderate strength, excellentdimensional stability, is easily colored, and is very low cost. Polyimide (PI)is an example of an imidized thermoplastic that is considered an ultra-high- performancepolymer due to its very high strength, superior dimensional stability,excellent electrical and thermal insulation properties, and low coefficient offriction, but all that performance comes at a premium price. Yet, it is becausePolyimide is uniquely capable for many demanding interventional applicationsthat it is widely used in medical devices, despite the high price point.

The Value of DFM

Material selection decisions are improved by bringingmanufacturing engineering into material discussions early, considering thefabrication process and final assembly steps. DFM input helps avoid redesignsand delays due to impacts from late material changes and supports a smootherpath to commercialization of new products.