Thermoplastic elastomers (TPEs) combine the high-volume molding capability of thermoplastics with elastomeric properties of thermoset rubber, so they lend themselves to a broad array of medical devices, equipment, and packaging. TPEs offer freedom to innovate, solve problems, and they address key trends in healthcare.

For starters, thermoplastic elastomers are low-modulus, flexible materials that can be stretched repeatedly to at least twice their original length at room temperature and return close to their original length after releasing the load. Their “grandfathers” are thermoset rubbers, which are being replaced by TPEs in applications where cost cutting comes from molding in conventional, high-volume equipment instead of fabricating and assembling a thermoset part.

Further, the softness and suppleness of TPEs appeal to consumers in particular, making them widely used in home healthcare products. Other important properties of TPEs include clarity, color, good sealing performance, low leachables and extractables, and they are suitable for gamma, E-beam, and autoclave sterilization.

TPEs answer several medical design challenges. Their versatility and high performance allows adapting them to macro trends, such as the aging patient population, the move toward disposable devices, the emphasis on human factors and ergonomics, and increasing stringent regulations. Examining where the material has succeeded may encourage other ideas for design.

Improved safety for elderly patients

As people live longer, healthcare delivery is shifting to homes and nursing homes. For safety and ease of use, older patients, particularly those at home, need devices and equipment with a firm and secure yet comfortable grip. Canes, walker handles, medication bottle caps, and other hand held products are gripped more easily with an overmolding of soft and supple TPEs. These make it easier for frail elders to follow a treatment regimen. Patients with osteoarthritis or rheumatoid arthritis, for example, benefit from cushioned grips that put less stress on joints. Furthermore, the trend toward miniaturization of devices such as glucose monitors makes it even more important to design easy grasping and handling into the product. TPEs are easily colored or given special effects that make them more appealing to patients, potentially encouraging use of the product or device.

Increased consideration of “human factors.”

This engineering discipline evaluates how people interact with machines, technology, and other aspects of their environment, with the goal of accommodating human limitations and preferences. Operating rooms have become tremendously complex thanks to advanced technology. TPEs can enhance human factors' effectiveness in ORs by making products simpler, more intuitive, and easier to use. For example, a microdebrider for surgeons operating on sinuses has an ergonomic hand piece with a grip molded from a TPE. This provides biomechanical control throughout all orientations. The TPE also withstands repeated sterilization.

High-volume production for disposables

To make disposable devices cost effective, manufacturers must produce large volumes using high-speed machines. TPEs help make this possible.

In contrast, thermoset rubbers require separate fabrication and assembly. Parts made from TPEs can be molded on the same equipment and in the same cleanroom as other components. In addition, the precision of injection molding permits greater design latitude. For example, thanks to its high pressure and processing speed, injection molding can make complex geometries not possible with the low-pressure compression molding used for thermosets.

The benefits of TPEs in disposables can be seen in a closure system for prefilled syringes. In place of traditional slip-on rubber tip caps, the tamper-resistant, twist-off closures prevent reuse of the syringe and spilling the contents. The inertness of TPE also prevents the needle from corroding over time, which would make injections painful. Further, the TPE and plastic used for the syringe can be injection-molded in the same process, trimming time, and avoiding secondary operations.

Compliance with regulatory requirements

One FDA effort is in reducing leachables and extractables from rubber parts and plastic packaging that can affect patient health when introduced into drugs or other formulations. The FDA is concerned about nitrosamine and 2-MBT, among other leachables and extractables. For example, health issues around 2-MBT, a vulcanization accelerator for rubber, have led to reformulation of rubber stoppers for drugs. Because TPEs do not undergo curing, they avoid creating by-products such as nitrosamines and other hazardous leachables.

Addressing environmental awareness

Another plus for TPEs lies in replacing polyvinyl chloride (PVC) in tubing, bags, and films. The industry is moving away from PVC due to environmental concerns with halogens, which are being restricted in Europe and other countries. Soft and clear, temperature-resistant TPEs can match or exceed the physical properties of PVC, while offering additional benefits of inertness and a tolerance for various types of sterilization. Further, TPEs can be overmolded without harmful solvents, which are required to bond PVC to other plastics.

A disposable resuscitator uses a soft and clear TPE instead of PVC or expensive silicone for the squeeze bag used for ventilation. Further benefits are provided by the easy to grip surface of the bag and its flexibility, which makes usage less demanding.

A FEW RULES FOR DESIGNING NEW OVERMOLDED COMPONENTS

1. Substrate thickness should be more than twice the TPE overmold thickness
2. Use air vents between 0.013 to 0.025 mm at end of fill
3. Flow ratios (L/T) for new component designs must be from 80:1 to 120:1
4. Start with a small gate to maintain low cycle time and to degate better
5. Use proper flow shutoff of TPE at cavity edge
6. Add surface texture to prevent sticking
7. Balance the runner system
8. Use mechanical interlocks for thin-wall TPE overmolding
9. For molds with 16 or more cavities, consider a hot sprue or preferably hot-runner system to reduce cycle time and TPE runner scrap
10. Externally heat the hot-runner system