Plastics continue to find more uses in medical and dental applications by replacing metal, glass and other traditional materials in single-use and reusable medical devices. They offer strong, lightweight performance along with design flexibility, ease of fabrication, ability to differentiate products and brands using colour, and most importantly durability and cost effectiveness compared with traditional materials.
Commodity polymers occupying a major share are mostly used in disposable products, engineering thermoplastics are used in both disposable and non-disposable products and high temperature thermoplastics (high performance polymers) are used in implants, surgical instruments, and components for machines and equipment.
In the past decade, many such polymers as polycarbonates, polypropylene, polyethylene, having shown fine harmony with the above criteria, have been widely adopted as material of choice by the industry; during the past few years, however, high-performance and engineering polymers have seen an increasing penetration in the industry replacing conventional metals and few existing commodity plastics primarily because of the advent of stringent and powerful sanitising agents.
Of the more than 2 million medical devices listed in the Global Universal Device Identification Database (GUDID), 40% are provided as sterile to the users and patients. Compatibility to these sterilisation processes is a critical requirement in the industry. It is important for material suppliers, as well as device manufacturers (OEMs), to demonstrate the compatibility that their products are free from microbial contamination to a specified statistical level. Manufacturers must also be aware of how their material interacts with different sterilising processes.
On this aspect engineering and high-performance plastics provide many advantages compared with standard plastics, such as good malleability, faster production time, low weight, resistance to high impact, flame, shock and chemicals, and better friction reduction. Improving standards and regulations mandating high quality of plastics used in medical applications are responsible for the large market size for HPPs (high-performance polymers), and are the main drivers of their growth. It is expected that these high performance plastics will grow at a much faster pace than commodity plastics.
In response to this trend most speciality polymer manufacturers have developed their own products portfolio of high-performance medical-grade polymers with excellent performance properties and reliability through different sterilisation methods which can cater to variety of applications including handles, knobs grips, trials, cases, trays, forceps, inserters, retractors, extractors, lids and surgery kits.
The application of polymers in the healthcare industry and medical devices in particular is often challenging and has demanding requirements for thermoplastic materials. Besides meeting the highest quality standards and regulations, these materials have to withstand aggressive disinfectants, pharmaceuticals and resist different sterilisation procedures without sacrificing its mechanical performance. In addition, they need to ensure that no harmful contact takes place among the material and any bodily tissue or fluids, or any reaction with pharmaceutical formulations need to be avoided to prevent changes in the composition and activity of the active ingredients. Furthermore, materials for medical applications need to exhibit excellent processability to realise delicate geometries for minimally invasive devices or an accurate fit of connecting parts.
As a result, only a small number of polymers are available as medical grade and an even smaller number qualifies the continuous stringent market demands. A proper material selection is a crucial step in the development process of medical devices.
In view of these industry demands, all the leading material suppliers to the healthcare industry, such as Solvay, SABIC, Arkema, DuPont, have been closely working with OEMs to understand product requirements and thoroughly analyse the suitability of different plastic materials for the required applications. Most have even developed broad portfolio of versatile specialty polymers with cost-effective metal alternatives that offer exceptional impact strength, heat tolerance, stiffness and sterilisation compatibility for a variety of applications in the industry.
The process of sterilisation refers to any action used to eliminate or kill any form of life present on a surface or contained in a liquid. Currently, a variety of sterilisation methods can be used to reduce microbial load on medical devices and pharmaceutical products. Some of the common sterilisation methods include the following:
- Chemicals (EtO, plasma, oxidizing agents such as hydrogen peroxide, chlorine dioxide or liquid sterilants such as glutaraldehyde.)
- Radiation (gamma irradiation and electron beam)
- Heat (steam, dry heat).
(A) GAS STERILISATION (EO/EtO)
Ethylene oxide (EO or EtO) gas is the most common sterilisation method. It currently occupies approximately 50% of the sterilisation market, because the majority of thermoplastic polymers can withstand exposure to toxic EtO without significant changes in their properties or colour. The increasing penetration of high performance polymers with better susceptibility to stringent and effective sterilisation techniques is eroding this market share.
(B) RADIATION STERILISATION
Radiation generated by either electron beam (E-beam) or gamma rays is used to sterilise bulk quantities of disposable healthcare devices. Dosage amounts typically range between 2 to 4 Megarads (Mrad) or 20 to 40 kiloGrays (kGy). Because the dosage can vary depending on packaging density within the sterilisation chamber, some items may be exposed to higher levels. Similar to EtO sterilisation, radiation can penetrate product packaging. That said, this method is less time-consuming; it can also efficiently irradiate dense materials.
Plastic exposure to radiation results in physical changes such as embrittlement, discoloration, odour generation, stiffening, softening, enhancement or reduction of chemical resistance, and an increase or decrease in melt temperature. Such physical changes during the process are common and most plastics can be stabilised to address this. Loss of mechanical properties (tensile strength, impact strength and elongation), however, is a more serious concern and must be taken into account during the assessment of any polymer.
Radiation-resistant thermoplastics include styrenic thermoplastics such as ABS, PARA, PEEK, PEI, PES, PSU, PPSU and TPU. Aliphatic polymers exhibit degrees of resistance depending upon their levels of unsaturation and substitution. In terms of mechanical performance, PC is generally resistant to radiation, but it will discolour with radiation exposure. There are some antioxidant additives that can be used to improve the radiation stability of certain polymers, but the best course of action is to select polymers that are known to perform well.
(C) HEAT STERILISATION
1) Steam autoclaving
All healthcare agencies recommend using steam autoclaving, whenever possible. It is a fast, reliable and inexpensive technique used to sterilise reusable instruments and devices. Yet, because of its incompatibility with current plastics, steam sterilisation is often not the decontamination method of choice, occupying a small share of just 5%.
The limiting factors for many standard polymers are their heat resistance (Tg, dimensional stability), particularly for single-use devices and their hydrolytic resistance in the case of multiple sterilisation cycles as many of them cannot withstand prolonged, repeated exposure to high temperature and steam. For example, polypropylene (PP), polyamide (PA) and polycarbonate (PC) are suitable for only a limited number of cycles (<100 cycles). Conversely, high-performance plastics such as PPSU, PEEK and PAEK can withstand more than 1,000 steam sterilisation cycles without significant loss of mechanical properties and are well-suited for more demanding applications. PSU offers an intermediate level of performance and is suitable for several hundred steam sterilisation cycles (<500 cycles). Styrenes (ABS, PS), polyesters (PBT or PET) and PARA are not usually recommended for steam autoclaving for more than a few cycles as the material quickly loses tensile strength.
2) Dry heat sterilisation
Dry heat sterilisation takes longer than steam sterilisation due to the inefficiencies of heating air with a very low moisture content. This method is, however, the most appropriate for medical devices. Although the process itself takes just 3–15 minutes, medical instruments must be allowed to cool and dry completely over the course of several hours before being used. In addition, the buildup of water droplets inside device components can impair their functioning and corrode materials that aren’t meant to come into contact with water. Plastic and electronic components can also be damaged by exposure to steam, making this sterilisation method unsuitable for most complex medical devices. For this reason, dry heat sterilisation is most appropriate for medical devices that are heat resistant but susceptible to water damage.
This type of sterilisation is highly dependent on the temperature resistance of the polymer. As a result, high-temperature thermoplastics such as PPS, LCP, PEEK, PEI, fluoropolymers or thermosets polymers are well suited over low-heat resistant polymers such as -PP, PE, styrenes, polyesters.
STERILISATION RESISTANCE OF SEVERAL PLASTICS
Plastics sterilisation resistance characterises the ability of polymers to endure repeated sterilisation cycles (chemical, steam or gamma radiation sterilisation) without significant damage.
Medical plastic products have revolutionised the healthcare industry. The increasing penetration of polymers in medical devices has transformed the marketplace, with plastics steadily replacing other material such as glass, ceramics, and metals, wherever applicable. The current market trends toward increased safety and quality, however, has urged the medical industry to select those medical-grade polymers that meet regulatory standards, safety requirements, manufacturability, and functionality requirements for medical products.
Plastics used in medical device applications must meet stringent performance requirements through production, packaging, shipping, end use and disposal. Many devices and device kits are sterilised before shipping as well as multiple times during use. They also come in contact with various chemicals, solvents, bodily fluids, skin, organs and tissues. The materials used in such devices must be resistant to the sterilisation methods, chemicals, and fluids that they encounter, be compatible with bodily fluids, skin, and tissues, and still maintain their safety, effectiveness and functionality. All these requirements pose constraints and challenges on the materials.
Diverse medical applications, product durability and biocompatibility are all important factors that OEMs consider; thus high-performance plastic resins are now making inroads into the market.
In an article published in medical design & outsourcing, Jeff Hrinvak, global business development manager from Solvay, commented on high-performance polymers and their growth in the healthcare industry: “Specialty polymers will only help to fuel the growth of the overall medical plastics market.”
Encompassing PSU, PPSU, PAEK, PEEK, PARA and other advanced polymers, this class of materials collectively offers better mechanical performance than engineering resins, as well as broader chemical resistance, higher thermal properties and often inherent flame retardance without the need for additives. These materials are also compatible with a broad range of sterilisation technologies, such as steam, EtO, vaporised hydrogen peroxide and high-energy gamma radiations. All these properties are feeding their growth, as is evident in the trend towards increasing usage of HPPs in single-use and reusable instrument applications, such as retractors, impactors and other surgical instruments.
The authors — Sanklan Chandak, associate consultant, and Vikash Kumar, project leader, Speciality Polymer & Material — are from ChemBizR.