Advancements in manufacturing hydrophilic porous plastics

Ken Nzeribe, Materials Scientist and Engineer, Porvair Sciences discusses the advancements in manufacturing hydrophilic porous plastics for healthcare and pharmaceutical industries. 

Porous plastic polymers are widely used in healthcare and pharmaceutical applications due to its versatile array of mechanical and biochemical properties. Specifically, the composition, structure and surface energy of polymers can be modified and controlled during manufacturing processes to impart hydrophilicity, sometimes referred to as wettability; that is, the ability of a liquid to stay in contact and be absorbed by the porous plastic. 

During healthcare and pharmaceutical product development, hydrophilicity is a crucial factor in determining the product performance limits, and a challenge that porous plastic manufacturers face is that plastic polymers are naturally hydrophobic; that is, liquids bead up on the surface and do not form strong intermolecular bonds with the material. 

Conversely, hydrophilic materials can attract, absorb, dissolve in, mix with, or be wetted by water; for example, the hydrophilicity of porous plastic materials plays an important role in applications such as liquid absorption, controlled liquid flow and delivery (for example, point-of-care [POC] devices such as eye droppers and nasal sprays) and are thus, at a product level, subject to high levels of regulatory approval (for example, biocompatibility, high bacterial filtration efficiency). Hydrophilicity can be achieved by treatment technologies that render polymeric materials susceptible to fluids by covalently bonding highly reactive polar groups to their molecular chain to create water-attracting surfaces. 

There are different methods to render porous plastics hydrophilic: 

• Plasma treatments — this includes plasma cleaning, plasma surface activation, plasma coatings and plasma etching. At very low vacuum, electromagnetic energy is used to evolve highly reactive polar functional groups from gas molecules which are then covalently bonded to the surface of the material.
• Vacuum deposition process — this includes physical and chemical deposition processes.
• Dip-coating —this process involves lowering an article hanging from a support into a liquid coating solution and then pulling it out at a known speed. The coating will stick to the surface of the article as it is drawn up and out of the solution.
• Spray-coating — during spray coating, a driver and a nozzle is used to nebulise the coating solution applying it to the surface of the material as a mist. The coating will stick to the surface of the material and render the surface hydrophilic. This is typically a short­‑term option and is not as robust as plasma treatments or vacuum deposition process.

Hydrophilic Vyon porous plastic 

Vyon is a porous plastic material manufactured using virgin‑grade polyethylene, which is naturally hydrophobic. Vyon is converted from its naturally hydrophobic nature to hydrophilic using a plasma treatment process. The plasma treatment process works in two main steps.

1. Cleaning — Removal of contamination from the surface
2. Activation — Increasing the surface energy by the attachment of oxygen-containing molecules 

How is hydrophilicity measured?

Typically, the level of hydrophilicity is quantified by introducing a drop of a liquid of known surface tension onto a solid treated surface and measuring the interfacial contact angle of the liquid to the treated surface. The contact angle, θ, is the angle formed by a liquid at the three‑phase boundary where the liquid, gas and solid intersect. A surface is considered hydrophilic if the static water contact angle θ on the surface forms a contact angle <90° as shown in Figure 1a. This measurement is done using a goniometer. 

When dealing with porous surfaces, a different approach is necessary as the interfacial interaction between the liquid and the surface is more complex due to its structure. When liquid is introduced onto a hydrophilic porous surface, the liquid will be pulled or absorbed almost instantly into the porous network of the structure before any contact angle measurements can be taken. An alternative method to quantify hydrophilicity is by assessing the number of hydrophilic pores (hydrophilic porous surface) within the porous structure of the material (Figure 1b). 

Hydrophilic Vyon performance

Figure 2a shows the typical hydrophilic porous surface of hydrophilic Vyon. The error bar is the standard deviation of 10 measured samples, with a tolerance of ±2%. The small variation observed with hydrophilic Vyon translates to highly reproducible fluid transfer behaviour; that is, the flow rate of liquid through the hydrophilic porous plastic is predictable and consistent. 

Figure 2b shows the time it takes for the hydrophilic Vyon to be completely saturated by the liquid dye. The hydrophilic Vyon instantly wets out as soon as it gets in contact with the liquid dye, with a maximum time observed of 2 seconds. This is important because in high throughput applications requiring hydrophilic porous plastics such as process‑scale chromatography, a quick and consistent rate of liquid transfer through the porous bed support is necessary for immediate flow of the liquids during a preparation procedure while providing support to the powder bed above. If there are blind areas within the structure of the porous bed support due to poor hydrophilicity, the chromatography steps reliant on liquid flow will be affected negatively. 

Applications of hydrophilic Vyon 

Due to their ubiquitous use and value in modern society and importance in the medical, pharmaceutical and laboratory industries, there is much desire to render plastics hydrophilic for myriad applications. As demonstrated, porous plastics such as Vyon can be efficiently modified to exhibit hydrophilic properties required to increase the wettability of materials for pharmaceutical and healthcare applications. 

Process-scale chromatography required for drug development, rely strongly on precise flow of liquid through the capture media and bed support. Chromatography bed supports are typically designed from stainless steel (as it is typically hydrophilic) and provide stability and strength to support the resin bed. This stainless‑steel filter can be expensive and typically requires multiple cleaning cycles. Hydrophilic porous plastics can be used as an alternative to stainless‑steel bed supports as it demonstrates the stiffness and strength required to support the resin bed. In addition, porous plastics are much more cost-effective yet deliver similar support and filtration characteristics. 

In pharmaceutical applications such as controlled drug delivery POC devices, the filtration media within the assembly of the device allows drug delivery, provides a breathable environment to stop the build-up of pressure after dispensing the contents of the POC device, and preserves the integrity of the drug by filtering/keeping out any air contaminants or bacteria from entering and contaminating the drug in the device reservoir. The filtration media used must have a tightly controlled pore size rating and high hydrophilic porous surface to be able to absorb and deliver the drug immediately when required whist keeping the drug contaminant free. 

Conclusion

The properties and characteristics of hydrophilic Vyon allows the rendered material to deliver on key product functionality in pharmaceutical and healthcare applications. Due to the advancements in manufacturing, processing, and functionalisation of porous plastic materials, it presents a viable solution to meet the technical and regulatory requirements of fast‑evolving markets. For example, with the adoption of automated production, porous filters are robust and rigid enough for fully automated insertion processes (such as, a manufacturing line requiring insertion of the porous plastic into a final component via an automated process), yet also have hydrophilic porous surface properties to fully absorb water and aqueous solutions. Understanding factors such pore size, volume and surface energy are not only important for the rendering process, but also the hydrophilic quality and performance (for example, shelf‑life and absorption time) achievable with the final product. The versatility of porous plastic materials and ability to tightly control its manufacturing process enables continuous innovation of pharmaceutical and healthcare solutions.  

Vyon is a registered product.