Medical implants are complex components made from ultra-high molecular weight polyethylene and other plastics, which act as cushions to minimize stress on the bone-metal interface. While softer than old-style metallic implants, plastic implants lack the elasticity needed for motile body features. Silicone rubber has stepped in to fit the bill.
Besides being elastic and flexible, the material is also almost entirely bio-inert. It does not corrode or break down over time in the human body. In addition, while the costs of crude oil continue to skyrocket, raising the costs of plastic and rubber elastomers, advances in chemical engineering are actually bringing the costs of silicone down. Large-scale silicone distributors provide a wide selection of specialty materials to meet growing market demands. Add to this that silicone part tolerances have dropped from 0.010 to 0.0001 in., thereby bringing parts into the micro world.
Types of silicone
Silicone in general comes in the form of either liquid silicone rubber (LSR) or high consistency rubber (HCR). Both can be used to mold intricate geometries. The earlier HCR form lends itself well to most conventional methods of transfer and compression molding. LSR, on the other hand, suits injection molding and processing similar to thermoplastics. Liquid silicone rubber comes in a two-part solution and must be heated to cure, while thermoplastic comes in a one part resin that must be heated to melt and then cooled to set. The same underlying process is similar for both: Inject raw fluid material into a mold that is under pressure, wait a minute, remove a finished part, and repeat.
Contrary to popular belief, LSR materials can be compression molded with no loss of precision or feature detail. Although cycle times are longer partly because the tool must be either shuttled open and closed, or manually opened and closed by an operator, there is no need to spend time creating mold gates and sprues. Also, no time is lost in setting up the mold in the injection machine or processing in the first shot. Compression molding LSR is not practical in large-scale production, but it works well for short-run rapid prototyping. It might take an extra minute or two per part to manually operate and inject a mold, but spending an extra 20 minutes to make a dozen parts is a lot better than spending two days setting up a machine.
LSR injection molding handles complex geometries with microscopic features (features accurately repeated to the sub-micron level). An 0.03125-in. end mill was once considered a “micro” tool bit. Compare this to the 0.005-in. ball mill that is often used in the fabrication of today’s micro molds. In fact, 0.001-in. ball mills often cut the finer mold features.
Cutting and aligning the mold
Tool bits with microscopic diameters have presented mold makers with a new list of fabrication issues. For example, it takes very little shear force to break a 0.005-in. end mill. To prevent this, CAM programs use low feed rates (5 to 10 in./ min) and high rpms (20,000 to 30,000 rev/min) so that the cutter has time to displace any excess material in its path and avoid getting hung up and snapping in two.
Cutting the mold is just the beginning. Aligning the two halves takes extreme skill and patience. Tolerances for mitre (mold alignment) in thermoplastics typically range from ±0.003 to ±0.005 in., depending on the size of the part. However, now that parts are being created as small as 0.010 in., alignment tolerances have tightened to the ±0.0005-in. range. Because of these changes, optical-measurement devices have replaced calipers and micrometers during mold validation. Molders can now see and accurately measure parts that are almost invisible to the naked eye.
While it is no easy feat, creating a micro mold for liquid silicone molding is still only half the battle. Processing and running the mold to repeatedly create microscopic parts and features has traded old problems for new problems. There are distinct advantages to producing infinitesimally small parts, the most obvious being the cost of materials is greatly diminished. A high-volume order used to take gallons of raw material, while a high-volume micro part order only takes grams. This opens the door for the use of composite materials that cost thousands of dollars a pound. For example, chemical engineers can embed highly concentrated expensive medications in a LSR matrix. When surgeons implant the fully cured parts, the medicine releases in a controlled manner, providing the patient a steadier and more accurate dose. This can eliminate forgetting pills, periodically changing IV bags, and pain associated with hypodermic injection.
In micromolding, shrink is a concern, but not a huge worry. LSR shrinks about 1% to 3% of its original size when it cures. Large parts require higher-level math and advanced design software to properly calculate shrink. In a worst case scenario, the mold maker must run multiple tool iterations to create the correctly sized part. A micro silicone part measuring 0.030 in. in length shrinks to about 0.0297 in., typically not enough for the part to go out of tolerance.
While shrink is a relatively minor concern, flash is critical. The largest allowable flash is usually 0.005 in. The simplest solution to removing flash is a secondary process, where the operator manually removes flash using precision tweezers and a microscope. But this approach is time-consuming and cost-prohibitive in long-term runs. Cryogenic tumbling for deflashing is often used as a secondary off-site process, but it becomes a problem when the parts are small enough to be easily lost, or mistaken for debris.
The best answer is to eliminate flash entirely during the molding process. For example, using tooling with features etched and keyed into the mold to provide the flash with a preferred place to flow; increasing the clamping force on the mold; and running the mold under vacuum are a few options. The best method varies with the material durometer and consistency. Ideally, dial-in the shot size precisely to accommodate only the total volume of the sprue and the part itself. Micro injection units today are capable of accurately and repeatedly splitting a milliliter of liquid material.
Silicone comes in commercially available durometers ranging from 1 to 80 Shore A. Lower durometer parts are soft, flexible, and elastic up to roughly 1,000% elongation. Higher durometer parts have a consistency closer to hockey puck rubber. This variability allows medical-device designers to more closely match the physical properties of the surrounding tissues of the body with the silicone implant. Historically silicone has been used to create gaskets, valves, o-rings, and other simple components involved in more complex implant assemblies. More recently, the wider durometer choices let the material be used to create entire stand-alone implants intended for both drug delivery applications and mechanical function.
To date, silicone has been used in biocompatible adhesives, shunts, stent delivery systems, tubes, microfluidic blood testing devices, drains, catheters, punctal plugs, intraocular devices, cannulas, heart valves, and aesthetic implants (such as breast and testicle). As a biocompatible elastomer, silicone rubber has doctors and biomedical engineers dreaming up new applications for use in the human body every day. As technologies in imaging, design, machining, and molding continue to advance, expect to see even more micro medical silicone parts.
Although not technically a micro part, the silicone knuckle implant is becoming widely used as a means of replacing knuckles afflicted with arthritis. These one-piece highly flexible implants lack the moving parts associated with metal joint implant assemblies. No mechanical articulations and fewer material interfaces mean improved implant longevity and increased comfort. Although there are obvious benefits to the integration of silicone materials in dynamic locations in the body, what is most intriguing is the outside-of-the-box thinking responsible for the design. Silicone by itself won’t revolutionize the medical device industry, but there are still many unexplored applications for its use in the human body.
To learn more about micromolded silicone medical parts please visit http://medicaldesign.com/silicone/small-parts-loom-large-silicone-molding