Author: Jacob Andrews – Technical Director for Filtration Technologies

As described in our previous blog post “Important things to consider when choosing a filter for your medical device application,” there are many aspects to consider when choosing or designing a filter that is fit-for-purpose in your medical device application. In this post we begin to examine the different types of filtration materials commonly employed in medical filter devices, specifically focusing on the differences between two major classes of filtration materials – nonwoven media and cast membranes. 

Figure 1, shows example SEM images of a nonwoven media (Figure 1.a, nonwoven Polypropylene) and a membrane (Figure 1.b, Polyethersulfone). These images depict some key morphological differences between nonwovens and membranes. As a collection of fibers, nonwovens create a tortuous path of pores between the fibers through which fluid can flow and particles can be captured. The size and packing density of the fibers may be tuned to create the desired performance attributes of flow, retention, and throughput necessary for the application. Generally speaking, more densely packed fibers will increase the retention efficiency and decrease the flow rate and smaller sized fibers will increase the flow potential at given retention efficiency. 

Membranes on the other hand appear more as a solid film with a tortuous path of pores created in more discrete channels with less opportunity for cross-flow as compared to nonwovens. The various manufacturing methods employed to make membranes are used to tune the size and quantity of pores, membrane thickness, chemical properties and other attributes as required to achieve the desired flow and retention performance necessary in the application. 

Figure 1: 

1.a 

1.b