Medical Device Link.

MATERIALS

Diverse applications

Image: iStockphoto

An expanding number of medical devices contain electronic functionality. It is important that those who integrate electronics into medical devices understand the materials that are used in both systems. This article focuses on the significant properties that silicones offer electronics. It examines the relationship between silicones used in electronics applications and those used in the health care industry. It is helpful to view electronic systems in certain levels to specify where silicones can be used and where the gap between use in electronics and use in medical device applications can be bridged. The author chose to employ a hierarchy proposed in 2006 by Joseph Fjelstad.¹ This hierarchy suggests that six separate levels exist in electronics systems beginning with Level 0, which is intellectual property, and ending at Level 5, which is the complete electronic system or device. This article focuses on two of the six levels because silicones are most likely to be found in those areas: Levels 2 and 5. Level 2, the electronic package, is the first instance where silicones are used and includes applications such as silicon based chip packaging, piezoelectric and optical sensor packaging and microelectromechanical systems (MEMS) packages. Level 5 applications include electromagnetic interference (EMI) shielding and general device encapsulation applications.

Silicone materials are supplied in a variety of forms, including fluids, adhesives, gels and elastomers. The basic formulation component of each material is the silicone polymer. Silicone polymers are a repeating chain of bonded silicon and oxygen atoms that easily rotate about those bonds. It is the flexible nature of the siloxane bond that provides benefits to virtually all silicone based materials. Curable silicones, adhesives, gels and elastomers also contain reinforcing fillers, reactive crosslinker siloxane polymers, catalysts and inhibitors that control the rate of reaction. The benefits of silicones for electronic applications are twofold: they resist thermal degradation and can incorporate large amounts of functional fillers while retaining elastomeric properties. By virtue of how these materials are manufactured, silicones contain low levels of ionic species and their physical properties tend towards high dielectric strength (typically 500 to 900 volts per mil). Silicones, unless modified to do otherwise, do not effectively conduct thermal energy or electricity.

Level 2 Packaging

Electronic chip packaging and temperature. Although the operating temperatures of silicon based electronic packages may not pose a challenge to materials traditionally used in electronic packaging applications, the recent trend in soldering options may cause materials to crack or degrade. Lead free solder reflow, in which temperatures can reach 185 °C for a brief time, creates stress between dissimilar substrates that comprise the chip package. This stress is created by mismatches in each material’s coefficient of thermal expansion (CTE). Silicones have high CTE values (200 to 500 parts per million per degrees Celcius), but their low modulus (<350 megapascals) imparts a low stress on components in thermally challenging environments. Silicones can serve in several functions within a chip package. Some incorporate functional thermal fillers and can be used in a heat management capacity, and others are unfilled and protect the whole package from environmental concerns such as contaminants, vibration and shock.

Optical and piezoelectric sensor packaging and MEMS. Sensors are essentially a subset of electronic packaging; many use a silicon based die that is mounted or attached to a substrate (the package). Optical and piezoelectric sensors are specific examples of packaging applications where the chip must have protected access to environmental conditions. Piezoelectric sensors are required to detect slight changes in pressure and silicone gels have been a mainstay in encapsulating these types of sensors. Optical sensors most often employ complementary symmetry metal oxide semiconductor (CMOS) types of chips that require uninhibited access to light sources. The optical clarity of certain formulations is essential to the efficient transmission of light through the silicone to the chip. Silicones can be formulated for resistance to ultraviolet light degradation for applications that require it. MEMS that employ micro fluidic valves or pumps typically use machined elastomers. More than 50 patents describe the use of silicones or related materials in the manufacture of MEMS devices. US Patent number 7216671 California Institute of Technology (www.uspto.gov) describes the use of silicones in flexible valve and pump systems. The patent also alludes to the biocompatibility that silicones offer in health care applications.

Level 5 Electronic systems

EMI shielding and general encapsulation applications. Wireless devices can have significant problems with EMI. Reducing or eliminating EMI requires careful consideration of device sealing. Electrically conductive fillers can be added to silicone formulations to create mouldable gasketing for shielding applications. Ultimately, electronic systems or parts of those systems need protection from aqueous environments that are common to medical device operating conditions. Silicones are generally permeable to gaseous water, but can be formulated to reduce permeation. They provide a biocompatible barrier in many device configurations.

Integration

The evolution of medical electronics will inevitably bring semiconductors within extremely close physical proximity to the environment. In other words, future developments in medical electronics can be expected to be outside a hermetically sealed electronics environment such as a pacemaker. This change in proximity will challenge engineers to select the most efficient materials with which to build their medical devices. However, there are benefits and drawbacks in all the materials used in a medical device and the use of materials other than silicones in medical devices is unavoidable, but utilising silicone when possible can yield some significant advantages. These are outlined below.

Regulatory history

The first benefit is the regulatory history of silicone materials. The use of silicones in the medical device industry is well known and this history and the continued development of medical devices with silicones benefit the industry by providing a standard. Even new and novel medical devices tend to utilise components with a history of use in the industry. The integration of electronics into the medical device arena will be no exception to this rule.

Elimination of negative interactions

The second benefit is material compatibility. Many silicones are formulated with platinum catalysts that can have their cure inhibited by other materials. The effect of cure inhibition can range from parts that fail to achieve the appropriate hardness to a complete lack of cure. The frustrating aspect of cure inhibition is that it is rarely consistent, sometimes the part is moulded or extruded perfectly and then suddenly parts are completely defective, even using a single lot of material. Using silicone materials in Level 2 and Level 5 applications can eliminate some of the variables associated with cure inhibition.

Adhesion and bonding

The third benefit is bonding. Most devices typically require bonding in the assembly of the device. Adhesion and bond strength is driven by two primary factors: surface energy and chemical compatibility.

Adequate surface wetting. Surface energy is the thermodynamic effect related to a material’s intramolecular forces and is a determining factor in how an adhesive spreads across a substrate surface. It is commonly accepted that the surface energy of the substrate must exceed that of the adhesive for adequate contact. The better an adhesive spreads out, the more intimate is the contact between adhesive and substrate molecules, which allows more reactive groups to interact or bond. The interactions between the adhesive and substrate make a stronger bond.

Chemical compatibility. The second critical factor to consider is how the adhesive forms a mechanical or chemical bond to the substrate surface. Adhesion can be achieved through a few mechanisms, but for the purposes of this discussion the focus is on chemical adhesion. Chemical adhesion is defined as the chemical bonding of two substrates by covalent bonding, hydrogen bonding or other Van der Wals forces. Substrates with reactive groups available for bonding such as reactive hydroxyl groups on glass and oxide layers on metals such as aluminum make this chemical bond easier to achieve. Substrates with inert surfaces such as graphite and polytetrafluoroethylene can make adhesion difficult.

Silicones bond well to silicones for the reasons described above, the surface energy is the same and the reactivity of bonding sites is ideal. Engineers seeking to utilise a silicone adhesive to seal CMOS sensors will find a good bond between the adhesive and any silicones used in that configuration.

Evolving use

Silicones can provide benefits to engineers who are integrating medical devices and electronics that are not only limited to the function of the silicone in the electronic component. Considering the timing and costs associated with medical device design and development, engineers constantly seek the most efficient solution to challenges. Evolution of devices and more specifically the change in how semiconductors are configured in medical devices may point to a need for materials to serve in both an electronic and bioinert function. Using silicones in Level 2 and Level 5 types of applications can benefit engineers by reducing or eliminating the regulatory concerns, negative interactions and the bonding problems that can occur when using other materials.

Stephen Bruner is Marketing Director at NuSil Technology, 1050 Cindy Lane, Carpinteria, California 93013 USA, tel. +1 805 684 8780, e-mail: steveb@nusil.com, www.nusil.com.