Top tips for designing medical devices with sintered porous plastics

Hilary Bishop, product manager – Vyon Porous Plastics and Richard Morgan, process manager — Vyon Porous Plastics explain how sintered porous plastic materials can play an integral role in the performance of medical devices. 

New product development (NPD) for medical devices is a lengthy, multistep process often layered with regulatory requirements. In a rapidly evolving market, designers and engineers are under pressure to deliver new and innovative medical devices to meet changing market demands. The global response to the Covid-19 pandemic crisis increased pressure for supply of not only final products, but their components and raw materials. Sintered porous plastics may not always be the first material or even the most familiar material to which engineers turn when designing a medical device. Yet, the versatility and ability to adapt to diverse applications have made sintered porous plastics a key component found at the heart of world leading medical devices.

What is a sintered porous plastic?

Sintered porous plastics are manufactured from thermoplastic polymers to meet different market demands. Through a process of heat and pressure, plastic powders are bonded together to create a porous network composed of tortuous interconnected pathways (Figure 1), a structure that ultimately defines the key characteristics and behaviour of the final material.

Key properties

The top two properties of porous plastics are the size and distribution of pores (pore size). They are key determinants of the material’s permeability, filtration efficiency, porosity and strength. Permeability is a material’s capacity to allow the flow of liquids and gases through the porous structure, whereas filtration efficiency is a measure of the material’s ability to stop particles passing through. Finally, porosity is the measure of pore or void space within the material.

Applications

Filtration: The pore size and tortuous path of sintered porous plastics offers excellent depth filtration for liquids and gases and are, therefore, effective at filtering out a large variety of particulates. In addition, they can act as efficient bacterial filters by preventing microbial ingress — characteristics often desirable for point-of-care and surgical devices. 

Venting: From simple to complex medical devices, venting is typically required to relieve pressure from a closed system or prevent substances from escaping into the external environment. Permeability and pore size play a key role in the material’s ability to efficiently vent a system. 

Absorption: Sintered porous plastics’ uniform porosity enables the controlled wicking of substances such as chemicals, fragrances and drugs. This makes them ideal materials for use in lateral flow tests, which require good capillary flow through the device. Lateral flow devices have always played a significant part in medical testing such as pregnancy tests but are now even more prevalent as they are used for many Covid-19 tests. 

Application: The self-supporting and rigid structure of a sintered porous plastic together with its controlled pore size and porosity enables it to be ideal for use in topical (device to body) applicators, such as wart removal and wound closure devices. This is particularly useful in both drug delivery and wound closure applications. 

Media support: It is the pore size and self-supporting structure of sintered porous plastics that will retain the required media without any breakthrough. Media support is a key attribute in medical devices such as dialysis cartridges.

Top tips to start strong

Collaborate, collaborate, collaborate

Working openly with your partner or supplier is the number one tip for any engineer or product developer. Although this sounds like a no-brainer, often delays and miscommunication can arise during the early stages of the development process. Open collaboration allows innovative and practical design ideas to come to life; most importantly, it ensures you find the best solution. Whether you are replacing a known porous plastic component or exploring porous plastics for the first time as part of a new design, working early in product development process with experienced, knowledgeable, and collaborative partners is key.

Function

As part of your medical device design, you’ll have a plethora of constraints to consider but it is key to know i) exactly what you want the porous plastic to do ii) or whether you want it to fulfil more than one function. Porous plastics designed for filtration can look very different to those used in wicking and absorption applications. Sharing these required functions early will enable you to evaluate which sintered porous plastics are most closely aligned with your application.

Design

This is where working closely with your supplier(s) can pay dividends. Although porous plastics can be formed into a wide range of geometries from large, machined sheets to small discs and complex 3D shapes, you don’t want to complete your design before learning whether it can be effectively manufactured. Conducting manufacturing feasibility from the outset will save you time and cost in your design process. Identifying the key parameters of your product, including critical dimensions and tolerances, will enable them to be incorporated into any test or measurement procedures during the manufacturing process. 

Approvals and regulations

Medical devices and regulations work hand-in-hand. Many sintered porous plastics available meet common FDA, USP and EP regulatory approvals. Knowing what you need will ensure that the chosen material which meets this is used to develop your porous component. This also applies to the chemical compatibility. Some sintered porous plastic materials react with different fluids better than others. 

Assembly

How is the porous plastic component going to be assembled into your final medical device? Sintered porous plastics can be incorporated into devices using a variety of different methods including over moulding, welding and press-fitting. In press-fitting, the degree of interference fit is important to ensure a good seal. 

Sterilisation or post processing

Does your assembled device need to go through a sterilisation cycle? Many porous plastics can be sterilised using, for example, gamma irradiation, ethylene oxide or electron beam. 

In addition, porous plastics can be treated in several ways including hydrophilic plasma treatment and ultraclean treatments to ensure there are low levels of contaminating leachables and extractables. In addition, the material can be treated to bestow chemical and biological functionality for applications such as the attachment of biomolecules.

Timelines and qualification

Familiarise yourself with the timelines and milestone you need to hit throughout the development process. Consider whether your final assembled product including the porous plastic component needs external and internal approvals. With many sintered porous plastics being manufactured to bespoke designs, it is essential to share this information with your supplier early in the process. This will ensure the collaborative team will deliver the sintered porous plastic solution at the right time. 

Summary

Sintered porous plastic materials can play an integral role in the performance of medical devices and so require careful thought and understanding throughout the development process. Collaboration is key and technical accuracy is paramount; harmonising the two ensures greater success in both the manufacturing of sintered porous plastics and design of medical devices.