Silicone materials can be compounded to create materials that are both, bio-inert and fully compatible with biological systems. Medical silicones are compounded in a similar manner as general purpose silicones to obtain certain characteristics, such as durometer, elongation and tear strength. However, medical silicones undergo additional processing to remove unwanted residuals. This allows for the purest silicones to be manufactured for use in medical devices.
Silicone manufacturers have created systems to determine which materials should be used for a variety of bio-contact applications. These suppliers have created in-house certifications that are based on the positive results of a series of USP (U.S. Pharmacopeial Convention) Tests. There are six separate USP tests, from Level I to Level VI. Materials that meet each of the six tests, are often referred to as “Grade 6” materials.
To better understand the requirements of each USP test, please see the table below.
Silicone manufacturers have created several groups in which to place these various medical silicones based on their anticipated use.
- Food Grade Silicones: Silicones of this group must meet USP tests, as well as FDA and USDA guidelines and criteria. Materials from this group would commonly be found in food processing equipment in the form of seals or even spatula blades.
- Medical Non-Implantable: Silicones of this type do not see use inside the body. Typical applications include tubing and one-time use disposables.
- Medical Short Term Implantable: These particular silicones can be implanted into the body for up to 29 days. A few applications include catheters and surgical tools.
- Medical Long Term Implantable: Silicones from this group can be implanted for a period greater than 29 days. Typically these materials are used in cardiovascular implants and remain in the body until end of life.
- Pharmaceutical: Silicone materials used in the pharmaceutical space, must pass all six USP tests and must be compatible with any drug coming into contact with the part. Drug delivery devices, such as punctal plugs, are typical applications in this group.
Determining which medical grade silicone material is required is based on the function and use of the medical device to be manufactured. Albright’s staff of engineers can assist you in material selection so that your medical application’s requirements are met. Please remember that although many silicone suppliers have conducted USP bio-compatibility tests on their respective materials, all finished medical devices will still need further USP testing. To learn more about medical silicone materials visit our Silicone Material page.
Silicones are playing an increasingly important role supporting improvement, innovation and progress in a wide range of industry sectors, from cars to electronics to textiles.
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The goal, when adhering to skin, is to hold the device inplace until it is time to remove it and to not damage the skin, either during wear or at the time of removal. Using methods developed for determining the surface energy of plastics and other materials, the surface energy for human skin has been measured in the low twenties [dynes/cm] – in other words, skin is as difficult to stick to as untreated polyolefins or even fluoropolymers. Low surface energy, as a property of human skin, is generally great for most of the things skin is expected to do, such as easy removal of contaminants with simple soap and water. The downside is that tapes must balance between adequate adhesion levels for the majority of users – the middle of the bell curve – and the ends of that curve. When the adhesion is too low, the device may not stay in place long enough for the full therapeutic effect and if it adheres too well, the tape may cause some mechanical trauma at removal.
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In addition to the LSR, HCR, and RTV silicone materials that we work with on a day-to-day basis, silicone can take on a variety of physical forms, ranging from solids to water-thin liquids and semi-viscous pastes, greases and oils. Click here to read some interesting examples.
Last month Susan Windham-Bannister, Ph.D., President and CEO of Massachusetts Life Sciences Center, participated In the latest installment of the WBZ NewsRadio 1030 Business Breakfast series. The panelists of business leaders & experts discussed the importance of making products and profits in Massachusetts. The group also discussed how the state’s manufacturing sector is staging an epic turnaround. The event examined and discussed the stories behind manufacturing success and how the state is helping to foster this growth and the beneficial ripple effect it is creating for the Commonwealth and beyond.
Click here to watch the video of the Business Breakfast.
Click here to learn more about Massachusetts Life Sciences Center.
While it depends on the specific application, generally molded silicone parts will have a considerable life expectancy. Once fully cross-linked a silicone rubber part will be virtually inert, meaning it won’t degrade or react chemically with most anything in the environment, aggressive solvents can break silicone down. Compared to thermoplastic elastomers and other rubbers silicone tends to retain its physical properties for much longer periods of time, and over numerous cycles of use, hundreds, thousands, millions (again this is somewhat dependent upon the application).
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Question: What criteria should people consider in selecting the hardness of a silicone compound?
Answer: Criteria for hardness may be best answered by considering what it means. Durometer is a measure of the resistance to indentation by a probe tip. Most silicones fall into shore A range although there are exceptions. Lower durometers feel softer like pressing a rubber band versus harder like an eraser. Neither of which are silicone but make the point.
The durometer does not necessarily indicate a materials elongation, modulus, tear strength, chemical resistance, or opacity but in the same product lines higher durometer does often but not always come with greater strength and modulus but lower elongation and viscosity. From one supplier to another and from one product line to another doesn’t always apply.
The criteria to use depends on your need, you may need to look at other properties for highly engineered products with strict performance requirements such as diaphragms, valves, gaskets, etc. On the other hand, products such as handles, covers, and skin contact components may lend themselves to the feel by durometer. Samples of different durometers can be helpful for both types of products to give you a sense of the differences between materials
Question: What shrinkage value should be used when designing a silicone mold?
Answer: Shrinkage is defined as “the amount or proportion by which something shrinks” (http://www.thefreedictionary.com/shrinkage). A material’s shrinkage must be accounted for when designing a mold to produce a silicone part that meets all required dimensions. Silicone normally can shrink from 1% to 4%. The shrinkage analysis is sometimes not provided when we buy silicone from manufacturers. Based on my personal opinion, 2% can usually be used for a standard shrinkage value when designing a silicone mold. Nevertheless, variation between material lots can significantly affect the shrinkage percentage as well as the part’s geometry. For example, a long hollow cylinder part that has a thin wall is going to shrink differently on different axes. Specifically, the long section of the part is going to shrink more than other axes. In this case, the part must be scaled differently on different axes.
The suggested shrinkage value will work most of the time. However, in a case where the material’s shrinkage doesn’t meet the standard shrinkage allowance or a part has a similar geometry to the one described above, educated estimation on shrinkage value should be made when designing a silicone mold.
Article From: Modern Machine Shop, Derek Korn, Senior Editor
Albright Technologies has become adept at micromachining molds for silicone parts such as the one to the right. This has enabled the company to become effective in quickly generating prototypes for medical device manufacturers pressured to speed new products to market. Many of the silicone components it creates are either tiny themselves or have miniscule features measuring just a few thousands of an inch. What’s interesting is that the company has found it can produce prototypes faster by taking a slower, more conservative approach to micromachining molds using end mills that measure just a few thousands of an inch in diameter.
Plus, while one might assume that very high spindle speeds are needed to effectively mill molds using such small tools, the machine that performs micromachining at Albright—a 30-taper VMC—typically spins 0.005-inch-diameter tools at just 9,000 rpm. Although that means feed rates and cycle times are relatively slow, there are a number of reasons why a company focused on quickly turning prototyping work finds this acceptable. David Comeau, Albright’s president, and Robert Waitt, vice president, explained why during a recent visit to the New England-area molder.
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