Plastic Material Selection For The Medical Industry

In medical advancements, plastic materials as products for medical devices have become indispensable due to their unique traits. Design and engineering effective medical equipment largely depends on appropriate materials. Plastic materials are at the forefront of innovation and help designers develop complex geometries and previously unattainable features. Health care relies mainly on medical-grade plastic. These materials are one of the driving factors in the research and development of plastic polymers and other new materials. With the help of this guide, we will delve into the critical process of plastic selection for medical devices and discover the principles behind choosing the right material.

Consideration In Selecting The Right Plastic Material

Biocompatibility

A material can interact with the body without resulting in adverse reactions or harm. This material should perform its intended purpose without eliciting any damage, like releasing harmful substances into the patient’s system. It should be non-sensitizing or non-irritating enough not to trigger any allergic reaction even after prolonged use. For example, this aspect is crucial in implantable devices, and the polymer must remain stable and inert throughout the intended lifespan.

Sterilization Compatibility

Hospitals and clinics are places where you must be careful about infections because of all the germs and harmful stuff. Frequent sterilization is vital to get rid of germs, prevent diseases, and guarantee the safety of the patients. The materials used in surgical instruments should withstand the sterilization process without altering their properties. It should not change color, degrade, or lose structural integrity during or after sterilization. Methods used in sterilization include: gamma radiation, chemical method (Hydrogen Peroxide Vapor or Plasma Sterilization), Ethylene Oxide (EtO), and autoclaving.

Mechanical Properties

Plastic material must withstand the forces applied to it without failure, permanent deformation, or breakage. Medical instruments are subsequently subjected to stresses and strains during operation. They get squeezed during packaging, bent and twisted when used, and sometimes even have to work inside a patient’s body. Choosing a material with adequate mechanical strength, elasticity, dimensional stability, and flexibility is paramount for device functionality and patient safety.

Chemical Resistance: Health tools often come into contact with various chemicals, including body fluids, medical agents, disinfectants, and packaging materials. Plastic materials need to handle the chemicals and fluids they encounter during use. Exposure to incompatible chemicals can lead to cracks, dissolving, and degrading, which translates to leakage, compromised functionality, or device failure.

Design Flexibility

Plastics offer designers significant design freedom. For medical equipment, they are handy in terms of flexibility and structure. This crucial factor invokes innovative ideas and effective medical solutions for modification. Unlike other materials like metals and ceramics, plastics can be extruded, formed, or molded into complex designs desirable for many medical devices. Plastic material should possess excellent dimensional stability and moldability to allow customization and adaptation to particular applications. This property provides room for complex geometries, miniaturization (for small invasive devices), function integration, etc.

Cost-Effectiveness

Safety and how well they work are the top priorities when designing and making plastic medical devices. While enhancing patient quality of life is a primary goal, sustainable product development also requires balancing clinical benefits with economic viability. The overall cost of the device impacts the accessibility of healthcare for patients, providers, and manufacturers.

Regulatory Compliance

When you’re working in the medical industry, there are tons of strict rules you have to follow. Regulatory bodies like the ISO 10993 series of standards for evaluating biocompatibility, FDA registration, and EU MDR (2017/745) ensure all regulations are followed throughout the product cycle. This factor is non-negotiable in the selection of plastics for medical devices. These regulatory bodies thoroughly test these materials for the safety and effectiveness of medical-grade plastics. Tests include sensitization, irritation, cytotoxicity, or intracutaneous reactivity tests. Once the material meets the set standards, it is designated for use, allowing subsequent stages of product development.

Commonly Used Plastics in Medical Devices

Polyethylene (PE) (HDPE, LDPE, UHMWPE)

This polymer is the most widely used due to its durability, lightness, cost-effectiveness, and excellent chemical resistance. It maintains its structural integrity even after undergoing a series of sterilizations. It comes in various forms, each with its characteristics.

• Low-density polyethylene (LDPE) is a flexible and relatively cheap plastic used in squeeze bottles, tubing, and medical bags.
• High-density polyethylene (HDPE)-provides non-toxic, better chemical properties (than LDPE) and more rigid properties. Used in medical applications, particularly in orthotics, components of some medical devices, support for joint replacement systems, and medical containers. Variations in its molecular weight affect its impact resistance and stiffness.
• Ultra-High Molecular Weight Polyethylene (UHMWPE): It’s highly wear-resistant, making it a crucial component of joint replacements (hip and knee implants).

Tips: Learn more about “LDPE VS. HDPE“.

Polyvinyl Chloride (PVC)

It’s a clear plastic that offers a balance of flexibility and strength. It can withstand various sterilization processes while retaining its structural properties. It is mainly plasticized with DEHP (diethylhexyl phthalate) for applications in blood tubing, IV bags, catheters, and oxygen masks. However, due to health concerns associated with DEHP, there is a significant regulatory and industry trend toward using alternative, non-phthalate plasticizers for medical PVC applications.

Polystyrene (PS)

It is a rigid, transparent plastic that is quickly processed. Despite being cost-effective, its brittleness makes it unsuitable for highly durable applications. Different models of this material include;

• General-purpose PS (GPPS) is a brittle, rigid, transparent thermoplastic. It’s the most basic and extensively used form of polystyrene. Its transparent nature makes it useful in applications where content visibility is essential. It has excellent rigidity and stiffness and is highly cost-effective compared to higher-performance polymers. It applies to culture flasks, Petri dishes, test tubes, and sample containers.
• Manufacturers produce High-impact PS (HIPS) by modifying general-purpose PS (GPPS). They add rubber particles (typically polybutadiene)during the polymerization process. This alteration increases the toughness and strength of the product, addressing the weakness of GPPS. It is suitable for devices that resist breakage under sudden forces, so it is appropriate for equipment housings. Other uses include packaging, organizers, and trays for surgical instruments.

Acrylonitrile Butadiene Styrene (ABS)

Medical-grade ABS plastic balances different qualities of rigidity, toughness, and impact strength. The selection of medical-grade ABS depends on specific grades. Designers choose grades with higher impact resistance for equipment housing, while those with heat deflection capabilities are subject to sterilization processes.

Polycarbonate (PC)

Highly regarded in the medical industry, this material is known for its high impact strength, clarity, good heat resistance, and biocompatibility. However, the biocompatibility and chemical resistance levels vary depending on the grades. It is suitable for making surgical instrument handles, IV connectors (renal dialysis, cardiac surgery), and blood oxygenator housings.

Polymethyl methacrylate (PMMA) (Acrylic)

This material is very useful in the medical industry. It’s biocompatible, rigid, and transparent. It’s very useful in intraocular lenses, certain optical components or light guides in endoscopic devices, and bone cement.

High-Performance Polymers

They demonstrate enhanced mechanical properties, including thermal endurance during sterilization, strong chemical resistance, and superior strength.

• Polyetheretherketone (PEEK) is both lightweight and strong. It has excellent biocompatibility and maintains strength and dimensional stability at high temperatures. PEEK is ideal for implantable devices, such as trauma fixation devices, Spinal implants, and dental prosthetics.
• Polyphenylene Sulfide (PPS): This material is well suited for harsh environmental applications due to its chemical and high-temperature resistance. Medical applications include drug delivery system parts, implantable device carriers, and dental sterilization trays.
• Polysulfone (PSU) and Polyethersulfone (PES) are high-performance plastic polymers with excellent chemical resistance, transparency, and heat stability. They are valuable in medical device applications such as dialysis/filtration membranes, fluid handling components, endoscope components, and Dental devices.

Silicone (polydimethylsiloxane (PDMS))

Silicone is a versatile synthetic polymer that is highly biocompatible, flexible, and non-reactive. Unlike many other plastics, silicone remains functional even when exposed to chemicals, heat, and prolonged sterilization. Apart from being highly biocompatible, it is hemocompatible, meaning it does not trigger blood clots. Medical silicone applications include breast implants, pacemaker leads, catheters, dialysis tubing, seals for surgical instruments, and cochlear implants, among others.

Design and Manufacturing Considerations for Plastic Medical Devices

Material selection must align with practical aspects when bringing a medical device from concept to reality. Product designers must evaluate not only the properties of the material but also its behavior during the manufacturing process and how it facilitates end-product performance.

Processing Techniques

The manufacturing process can impact the final shape and how many you can produce.

Injection molding is a process used to produce intricate and high-volume parts. It prefers thermoplastics with favorable flow characteristics. Product designers must know a material’s melt flow index (MFI) to predict mold-filling characteristics and potential flow-related defects.

Extrusion requires materials that exhibit uniform flow characteristics and exceptional thermal endurance through a prolonged process cycle. These materials are suitable for continuous profiles like tubing.

Blow molding creates hollow parts like containers. The materials used should have sufficient melting strength to allow inflation within the mold.

Thermoforming: Plastics produced through this process should soften when heated up to mold into the right shape.

Design for Assembly and Function

Plastic components must be designed to be easier to understand and interact with.

Bonding involves the use of adhesives and solvents. The materials used must be biocompatible and bond strongly.

Welding involves the use of laser or ultrasonic welding. It creates strong and clean joints but is limited to some thermoplastics. 

Snap fits are neat because they offer interlocking ability. They let one connect parts without needing any tools—for example, device housings and port seals. The material used for these plastics must be flexible, rigid, and strong enough to handle repetitive loading conditions.

Additives and Colorants

Additives enhance the material’s specific qualities. Antioxidants, like preservatives, maintain the material’s stability. UV stabilizers inhibit photodegradation, and radiopacifiers are added to make the plastic visible under X-ray imaging (radiopaque).

Colorants facilitate visual distinction of product recognition or separate different versions or functionalities.

Tip: Want to research metal material selection for the medical industry? Visit the Metal Selection in Medical Industry page.

Conclusion

Plastic has revolutionized the world of design and manufacture of medical devices. For product designers, the challenge of choosing the right plastic material is all about considering some critical factors. But if you approach it thoughtfully and understand the properties of different materials, you can open the door to some amazing new ideas. Choosing the right plastics creates medical devices that are safe and effective and improve patients’ lives.