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The Critical Questions When Micromolding for an MIS Device

Wed, 08/18/2010 - 5:25am
John Whynott

Asking the right questions when selecting a high performance plastic for micromolding an MIS device could reap some great competitive advantages. This article will review the micromolding process and cover the important aspects of each question that should be addressed by the outsourcing partner, the OEM, or both.

Endoscope_Procedure
A medical team using an endoscope during a surgical procedure.

The growth of high performance plastics has significantly widened the options for designing components and subassemblies used in medical devices for minimally invasive surgery (MIS). It’s not uncommon today to open any medical device trade magazine and see an article touting a new plastic. Advances in polymer science, in conjunction with micromolding technology, allow a range of cost-effective alternatives for MIS devices. Metal machining is no longer the only material and processing option when discussing small, complex components and subassemblies that require high precision tolerances.

A ‘Disruptive’ Technology

Medical devices for minimally invasive surgery are small, complex, high precision multi-part assemblies manufactured using highly skilled manual labor. Many of these devices are assembled under a microscope and use qualitative manufacturing operations, such as bonding and welding. Companies spend years developing and perfecting processes that enable them to ensure compliance, quality, reduced risk, lower costs, and profitability.

Micromolding is a “disruptive” technology. It’s changing the way designers look at developing and sourcing components and subassemblies, providing them with increased flexibility to design smaller, more complex components and subassemblies.

Combining high performance plastics with micromolding can permit designers to reduce the number of components, overall size, assembly complexity, and time required to assemble the device. Perhaps most importantly, it gives them freedom to add more features into a smaller package size, allowing them to acquire product differentiation, sustaining competitive advantages and pricing power.

Plastic Selection Questions for MIS Device Components

Material selection starts with understanding the most important design inputs a plastic will need to satisfy the functional requirements of the component. Addressing the following list of questions will ensure the designer and supplier create an optimum component for the MIS device application.

Is the component suitable for micromolding?

Generally, any component less than one gram can be considered for micromolding. To garner the highest probability for success, an MIS device manufacturer should ask a supplier the following questions: 1) Do they have a business strategy that will suit your needs? 2) Do they have injection molding equipment specifically designed for micromolding? 3) Do they have machining equipment specifically designed for fabricating micromolding tooling? 4) Do they have the capability to measure micromolded components?

Are there CAD data and detailed drawings available? Sample components? Is there any history surrounding this component?

Suppliers rely heavily on 3D CAD data. Without it, the mold tool designer does not have 100% of the information needed to make an accurate assessment of feasibility. The 3D CAD data should also have all specifications dimensioned to nominal. This allows the supplier to easily adjust for mold shrinkage. Samples, prototypes, or past history of micromolding the component can be helpful when determining feasibility or mold design construction.

Are there important functional requirements?

Selecting a plastic is based on a number of traditional material requirements, such as strength, stiffness, or impact resistance. In addition, MIS devices can have their own set of unique material requirements, such as biocompatibility, sterilization, implantability, radiopacity, contamination, and chemical resistance.

“Will the MIS device need to be visible under a fluoroscope or x-ray?” If so, the plastic component must be filled with a radiopaque compound so it can be visible under x-ray imaging and can be used to replace metal components. The most widely used dense metal powders added to base resins are barium sulfate, bismuth, and tungsten. Loading is typically in the range of 5% to 40% of total volume. Too much loading of radiopaque material can affect the mechanical properties of the plastic. Components utilized deeper within the body cavity (arteries) or those with thin walls will need loading on the higher end of the range.

“Does the plastic have embedded particulate requirements?” Plastic components should be clean, dry, and free of grease and foreign contamination. However, some contamination is inherent in the injection molding process. Designers should incorporate that in the form of specification, limiting the number and size of particulate and/or total area of the particulate allowed within the component so that aesthetics or functionality is not compromised.

“Will the components require bonding to another component?” Bonding to another component requires that the contact area has enough surface tension to adhere to the mating component. A surface finish of 1-1.2 Ra is generally adequate. Bonding dissimilar materials (like plastic and steel) may result in stress cracking due to different coefficients of expansion when subjected to heat. Cyanoacrylates (superglues) and light cured adhesives provide the best adhesion to plastics. PC and sulfone-based plastics are the most susceptible to stress cracking.

“How tight are the tolerances?” Amorphous plastics have low, uniform shrinkage that provides good dimensional stability and low warpage to hold tight tolerances in complex components. Dimensionally stable plastics include PC, PEI, PES, PSU, and PPSU. Semi-crystalline plastics have higher, less uniform shrinkage but fillers can be used to retard shrinkage. LCP has one of the lowest shrink rates and provides excellent dimensional stability. Other important functional requirements that should be specified for plastic components are gate vestige and parting line mismatch.

Is material certification required?

When components are subjected to body tissue or fluid contact it is required that the components be biocompatible. The two most common biocompatibility test standards are USP Class VI and ISO 10993–the latter being the most stringent. There are a number of biocompatible plastics that contain the USP Class VI designation. A majority of designers and manufacturers accept the USP VI testing as an acceptable biocompatibility test. However, this is not designed for medical devices. The ISO 10993 is more suited for medical devices. Designers tend to gravitate toward plastics that have already been biocompatibility tested and in existing MIS devices to minimize risk.

Does it need to be micromolded in a controlled environment?

Customer requirements will tend to vary. Some don’t require a controlled environment since the components will ultimately be sterilized or cleaned after final

Orthopedic_operation
Surgeons performing an orthopedic operation.

assembly. They only require that the components are dry as molded (not contaminated with grease, oils, or dirt during the molding process). The balance typically requires a minimum of an ISO Class 8 cleanroom. A cleanroom provides an added level of protection by reducing the level of foreign particles that can potentially find their way onto the plastic component.

Will it have contact with body tissue or fluids? How long?

If a component is subjected to body tissue or fluid contact, the next question is, “For how long?” Body tissue and fluid contact is categorized into three time periods: ?1 day (limited), 2-30 days (prolonged), or >30 days (permanent). The longer the component stays in the body, the fewer plastic options there are from which to choose. Most components are in contact with the body ?1 day. Permanent components can be in the body for as long as twenty years. Plastics subjected to fluids for longer periods need to have low permeability since many MIS devices have electronic circuitry enclosed. Plastics suitable for prolonged or permanent components are PEEK, PPSU, PSU, and SRP.

Single-use medical device? If not, what is the expected usage amount?

If a product is not a single-use device, the plastic components will be subjected to multiple sterilizations before being discarded or reconditioned. Plastics subjected to multiple sterilizations must possess superior toughness and the tendency not to discolor. The tougher the component, the less likely it will need to be replaced, resulting in a lower cost to recondition. PC (except steam), PEEK, PEI, PSU, and PPSU offer outstanding toughness.

Will the component undergo sterilization? What type?

Plastic components that require body tissue or fluid contact require sterilization. Plastics react differently to various sterilization methods. The most common methods of sterilization for MIS devices are radiation (gamma), chemical (EtO), and autoclave (steam). The majority of thermoplastic polymers can handle exposure to EtO without significant changes in properties or color. However, plastics subjected to radiation will be affected and it may change the mechanical properties of the material, such as tensile strength, impact strength, and elongation. Radiation-resistant thermoplastics include PARA, PEEK, PEI, PES, PSU, PPS, PPSU, and TPU. Autoclaving, or steam sterilization, uses a combination of heat and moisture to kill microorganisms. Plastics for autoclaving need the ability to tolerate repeated cycles of moisture/temperature combination. Also, parts that are molded with high residual stress levels may begin to see some stress relaxation (annealing) and, therefore, change dimension or warp when exposed to these temperatures. The amorphous grade materials are more suitable due to their tendency to warp less.

John Whynott is technical product manager of Mikrotech. He is responsible for product planning and marketing. Whynott can be reached at 262-857-5128 or jwhynott@asysttech.com.

For additional information on the technologies and products discussed in the article, see Mikrotech.

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