Plastic to smile about - thermoplastic polyurethane31 October 2014
Design engineers are increasingly choosing thermoplastic polyurethane (TPU) for medical applications, thanks to the material’s desirable chemical properties, excellent mechanical characteristics and biocompatibility. Prakash Vizzeswarapu, commercial development manager, NAFTA TPU Resins, Bayer Material Science, explains why these materials are having such an impact in the field and why TPU seems the likeliest contender to replace PVC.
In recent years, thermoplastic polyurethane (TPU) has made strong inroads into the medical devices sector. Bridging the gap between rubber and plastics, this class of materials boasts a litany of desirable characteristics, chemical and mechanical in nature. As ever more design engineers discover its benefits, its range of applications is growing fast.
While TPU has uses beyond the medical (not least in automotives, sporting goods and electronics), it has proven itself to be particularly versatile within the healthcare arena. Given that a medical device will actually enter a human body, it's important to take great care in ensuring safety and comfort. This means toughness is only one part of the equation - you also have to consider patient-friendly factors such as flexibility and biocompatibility.
Biomedical polyurethanes score highly on all counts, suiting them to a number of challenging tasks.
Combining patient comfort with ease of use, they have found diverse use, from short-term implants to hospital bedding.
"TPU healthcare grades are currently used to create oxygen masks and medical tubing, high-pressure contrast media tubing, and in a variety of catheters such as central venous, IV and intra-aortic balloon catheters," says Prakash Vizzeswarapu, commercial development manager, NAFTA TPU resins, Bayer Material Science. "They are also used to create soft and pliable post-surgical appliances, stopcocks, component housings, soft-touch grips, dental ligature retainers and dental devices, to name just a few."
Of course, TPU is not the only material developed for these purposes. For many years, the go-to choice was polyvinyl chloride (PVC). While PVC is a rigid material, it can be softened using an agent called a plasticiser, which gives it the necessary malleability for applications like fluid storage bags and medical tubing. It is functional and cheap, and was once hugely popular as a result.
More recently, however, it has fallen out of favour, largely owing to safety concerns surrounding plasticisers. Researchers have discovered that phthalates, the most common of these softening agents, may be deleterious to certain patients. Because the molecules are not chemically bound to the PVC, they may migrate from the material and leach into the patient's bloodstream. At very high levels, phthalate exposure has been associated with health problems including liver issues, cancer, and disruption of the development of the reproductive tract in male infants.
There are environmental concerns too, in that PVC emits harmful chemicals when incinerated. Unfortunately, this is the customary method of disposal for medical tubing and bags.
The evidence is compelling enough that many regulators have urged caution, with FDA, Health Canada, and many European bodies concurring in this assessment. As was explained in a 2001 study in the American Journal of Industrial Medicine: "The observed toxicity of DEHP [a phthalate] and availability of alternatives to many DEHP-containing PVC medical devices presents a compelling argument for moving assertively, but carefully, to the substitution of other materials for PVC in medical devices."
Since then, many hospitals have taken note, with some banning PVC altogether. While PVC is still firmly entrenched in the Asian medical devices market, it has lost ground elsewhere, and other materials such as TPU have stepped into the breach.
TPUs to the rescue
"TPUs possess a wide range of properties that make them suitable for an array of applications," explains Vizzeswarapu. "These include successfully replacing more conventional rubbers or PVC, which require plasticisers to become flexible, in medical applications. Additionally, TPU healthcare grades do not use rubber accelerators and plasticisers that can cause skin irritation or dermatitis."
While various different thermoplastic elastomers (TPEs) may be fit for purpose, TPU presents a robust and high-quality alternative. Significantly stronger than most other options, it allows the creation of thinner bags and tubes, meaning higher durability with less material. It outshines its competitors in flexibility, tear resistance and abrasion resistance - and because it retains the same crystalline clarity as PVC, it allows medical personnel to accurately determine the condition of a fluid.
Unlike PVC, however, it does not require any toxic chemical treatment before use. In fact, TPU is highly responsive to temperature, meaning it can be rigid at the point of insertion and flexible once inside the body.
"When compared with other plastic and elastomer materials, TPUs have excellent non-thrombogenic behavior," says Vizzeswarapu. "The materials also soften slightly at body temperature, allowing products such as catheters to be strong enough for insertion but subsequently soft enough to lend patient comfort."
From a chemical standpoint, TPUs are similar to human proteins. Their molecular configuration gives them a number of advantages: for instance, they are ideally suited to applications requiring adhesive strengths. It also gives them biomimetic properties (the ability to perform the same functions as natural structures), which is an obvious asset within the human body.
More precisely, they consist of a resin called a diisocyanate (a heavy, complex molecule) fused together with a polyol (a long, flexible molecule). Depending on the balance of the two, the resulting material may be rigid or stretchy as appropriate.
"The combination used determines the physical properties and other characteristics of the resulting TPU," says Vizzeswarapu. "The reaction of the isocyanate with the chain extender produces crystalline or semi-crystalline regions that affect rigidity, hardness and high-temperature properties. Amorphous or rubbery regions, formed by the reaction of the high-molecular-weight resin with the isocyanate, impart elasticity, resilience and low-temperature performance."
Its flexible polyol portion can be a polyester, polyether, copolymer or a polycarbonate ester, with the first two options by far the most prevalent. Polyether confers stability in water, and is particularly suited to products that will be exposed to wet or humid conditions.
Meanwhile, polyester, the most common variant, offers superior mechanical properties, cut and scratch resistance, heat stability and better inherent UV stability. This means it is ideal for applications such as the castor wheels on medical carts.
While the precise chemistry does vary according to the healthcare grade, all varieties of TPU are similarly straightforward from a manufacturing point of view.
'Regardless of which chemistry is selected, TPU healthcare grades are characterised by their processing flexibility," says Vizzeswarapu. "They can be injection-moulded, profile-extruded, blown into film, blow-moulded, thermoformed and rotomoulded."
Given that TPUs are so well established - the material, after all, was first developed in the 1950s - it may seem counterintuitive to suggest that the benefits are little known. This said, Vizzeswarapu maintains that education is the largest obstacle to success along the road ahead.
"The biggest challenge right now is raising awareness with new design engineers about the advantages of TPU materials," he says. "Current design engineers know that TPU healthcare grades do not use plasticisers. Market forecasts indicate that the TPU medical market is growing and will continue to grow rapidly in the next five years."
As market penetration increases still further, it is likely that design engineers will grow more creative in their uses of TPU. To take two applications at opposite ends of the spectrum, let's start with the Texin TPU grade from Bayeris, which is highly rigid with exceptional tensile strength. It can be used for medical equipment housings, connectors, hospital bed frames and other structural applications. Meanwhile, breathable soft-touch TPU films are ideally suited for wound care applications and anything requiring skin contact.
From the garments the surgeon is wearing, to the sheets on the hospital bed; from the patient's feeding tube to the surgical drain; from the device implanted to the wound dressing - all of these items and more may contain TPU. The future possibilities remain to be seen, but the material is capable of supporting many newer applications, ranging from artificial hearts to newly patented surgical implants.
As PVC is phased out, and alternatives keenly sought, TPU shows great promise for the years ahead. Polyurethanes have already proved their mettle in the medical devices sector, and it is clear that this trajectory will continue.
"The development of innovative material solutions is creating even more opportunities for the use of TPUs," concludes Vizzeswarapu.