Bonding difficult plastics in medical device manufacturing
Peter Swanson, managing director of Intertronics, shares insight on using difficult to bond plastics in medical device applications.
Many medical device manufacturers are turning to plastic substrates specifically formulated to resist harsh chemical and environmental end use conditions and provide functional advantages. However, such plastics may have characteristics that impact the ability of an adhesive to bond to the material.
Plastic parts made from specialist materials are often joined using adhesives; there are advantages over mechanical means, including process speed and efficacy, lower stresses, and aesthetics. As new polymers enter the market, they create new opportunities for design engineers.
Medical device substrates
Polymer selection will depend on strength, ease of sterilisation, chemical resistance, and mechanical and dynamic properties. Commonly used materials are ABS, PMMA and PC. Nylons (PA), for example, are semi-crystalline polyamide materials used in devices for fluid management, surgical procedures and diagnostics, due to their good thermal, chemical and pressure resistance. Vascular catheter shafts, for example, are an application for nylons.
Newer polymers are finding applications in the industry, including polyether block amide (PEBA) and cyclic olefin copolymers (COC/COP). PEBA delivers low friction properties, kink and pressure resistance, softness and flexibility, while being chemically resistant and lightweight. This flexible thermoplastic elastomer is replacing polyurethanes, polyesters and silicones in applications like medical grade tubing, catheters, connectors and medical films.
COC/COP is new class of amorphous polymers that are an alternative to glass due to their clarity and compatibility with sterilisation. COC/COP film laminates are used in the assembly of in-vitro diagnostics (IVD), Lab-on-a-chip (LOC), and other point-of-care medical devices. Formulators have innovated to provide specialist adhesives that bond well to many newer substrates.
While offering desirable properties for the application (e.g. low friction surfaces), many of these plastics have very low surface energies. If the surface energy of a material is below 50 mJ/m2, adhesives will not wet the surface, sitting as a round droplet instead of spreading out. Wetting is a prerequisite of adhesion for the plastic to be bonded effectively, so in this instance the manufacturer can contact an adhesives specialist who can recommend surface preparation techniques to trial.
One surface treatment technology many have found success with is cold atmospheric plasma. The process involves turning a small number of gas molecules into plasma, partially ionized gas, to initiate a multitude of physical and chemical processes that treat the surface, increasing its surface energy, and removing contaminants without the use of additional chemicals. The piezobrush PZ3 handheld plasma surface treatment device is low cost, easy and intuitive to use, but there are options available to suit processes of different scales, volumes and speeds.
Adhesives in medical devices
Though there are many ISO 10993 tested bonding material suitable for consideration in medical device assembly, specifying an adhesive and associated dispensing equipment always involves trade-offs. For example, two-part epoxies give high resistance to heat and the environment, but may require several hours to cure, creating work in progress. Cyanoacrylate adhesives (CAs) cure quickly but have lower thermal and moisture resistance, and may experience blooming, a white discolouration on the surface of a part that impacts aesthetics. Silicone adhesives are very flexible, can withstand repeated sterilisation, and bond well to silicone substrates.
UV curing adhesives are a popular choice, particularly for volume, automated processes, as these materials cure on demand when exposed to the right intensity and wavelength of light. UV curing materials can speed up the assembly process and reduce work in progress and can be formulated with fluorescing properties to allow easy bond line inspection and detection of adhesive coverage. Some manufacturers, such as Dymax, offer colour change UV curing adhesive technology to confirm cure.
One new option on the market is Born2Bond Light Lock, a low-blooming CA that offers dual cure. Designed for applications that require fast fixturing, coating or surface cure, it can be cured by conventional CA curing mechanisms (usually exposure to surface moisture), which ensures fast interfacial cures, and using UV/visible light, to allow rapid bonding through transparent parts or quick curing of exposed fillets of areas.
New medical device materials provide designers with opportunities for their new products but can exacerbate the bonding challenge. With careful consideration of the latest adhesive and cure chemistries, dispensing technology and surface preparation of the substrate, medical device manufacturers can develop repeatable, validatable processes.
Peter Swanson will also be speaking on Day 2 of Med-Tech Innovation Expo on the Introducing Stage. Register at med-techexpo.com.