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POLYMERIC EMI SHIELDING: PROVIDING A MORE SECURE FUTURE FOR PATIENTS

Citation: Vaes M, Deurloo L-P, Sas M, “Polymeric EMI Shielding: Providing a More Secure Future for Patients”. ONdrugDelivery Magazine, Issue 108 (June 2020), pp 24-28.

Marnik Vaes, Leen-Pieter Deurloo and Martin Sas discuss the role of polymeric electromagnetic interference shielding in providing a more secure future for patients in a connected world.

Driven by the need to reduce costs, healthcare systems around the world are currently undergoing a paradigm shift – from treating acute and chronic conditions in hospital and intensive care settings to a remote point-of-care approach. This transformation requires medical device manufacturers to integrate data recording functions into their products that enable remote patient monitoring and ultimately improve patient outcomes at lower treatment costs. This is the promise of connected devices.

“Shielding can be a complex issue to solve, as the majority of connected medical devices interact directly with wireless infrastructure or via a consumer device.”

Of course, medical devices must also be demonstrably safe when used as designed, in combination with other therapies and in a range of environments – from home to hospital. Devices that require direct contact with patients – even simple skin contact – must also be biocompatible and function without interfering with, or impairing, basic immunological functions or causing injurious, negative physiological, allergic or toxic reactions.

Connected devices are challenged with an additional safety consideration – interference with the main content stream of the radio signals produced can potentially adversely affect device performance. This is caused by the phenomenon known as electromagnetic interference (EMI) or radio frequency interference (RFI). EMI is caused by the tendency of electronic devices to generate strong electromagnetic “noise” during operations. EMI has been a challenge in radio-based communications since the work of Guglielmo Marconi approximately 150 years ago. It remains a challenge for electronics, packaging and compliance engineers to this day.

The principal area of concern – and the focus of this article – is EMI caused by non-ionising radiation. Electromagnetic Compatibility (EMC) standards and testing ensure that electrical devices are able to operate safely in close proximity with a minimum level of RFI. The solution to managing EMI is shielding, which can isolate the devices from their surroundings and from the signals of other devices. In simple terms, shielding involves creating a form of Faraday cage around sensitive components within the device, usually using a metal encasement or similar solution.

However, shielding can be a complex issue to solve, as the majority of connected medical devices interact directly with wireless infrastructure or via a consumer device – for example, a smartphone or other handheld device. They rely on a range of radio frequency (RF) bands with differing of signal power levels and operate in a range of communication modes. These include short-range wireless communication technologies such as near-field communications (NFC), Bluetooth, WiFi, ZigBee and the low-power version of these wireless communication protocols from industrial, scientific and medical (ISM) and short-range device (SRD) licence-free bands. All create a need for shielding against EMI. Figure 1 identifies the power performance for common wireless devices.

Figure 1: Typical RF power performance for common wireless devices expected to interfere with healthcare environments.

In general, RFI becomes significant at frequencies above 30 MHz, with typical levels of radiated emissions in units of electric field strength. Consumer electronics and healthcare-related EMC standards classify corresponding devices into a number of categories, according to their intended use environment. They also define immunity levels and limits to radiated RFI across a wide frequency range. Figure 2 summarises some of these limits in comparison with the radiated power levels of certain wireless technologies.

Figure 2: EMC-radiated emissions limits in terms of SE in dB based on EN 55011:2016, EN 55032:2015 and IEC EN 60601-1-2:2015 with examples of wireless technologies radiating RF power levels. Range of LNP FARADEX compound shielding effectiveness is highlighted.

Shielding effectiveness (SE) indicates the capacity of the material to act as a shield against internal or external EMIs, providing protection from damaging electrical devices. It is determined by the material’s overall conductivity level, wall thickness and target frequency range.

“Increasing miniaturisation and the growing engineering complexity of connected devices, along with the demand to make them lighter and less intrusive, is posing challenges and highlighting design limits.”

Conventional approaches to providing EMI shielding have relied on metal enclosures, usually using aluminium alloys; this method currently accounts for more than half the market. However, increasing miniaturisation and the growing engineering complexity of connected devices, along with the demand to make them lighter and less intrusive, is posing challenges and highlighting design limits. Weight becomes a greater consideration, with even the lightest aluminium alloys likely to be unsuitable – not to mention costly. In addition, the increasing complexity and sensitivity of these devices, combined with reductions in design space, could render them more susceptible to interference. Clearly, other solutions are needed to meet these evolving demands.

Some manufacturers have explored alternative approaches for providing shielding, such as metal coatings, vacuum metallisation and conductive paints on plastic enclosures. While these methods can be effective, they are less so than metal enclosures and rely on secondary processes to the plastics following initial production. These steps add to the system cost and complexity – and increase the overall environmental footprint of the products. In addition, not all thermoplastics may be suitable for such secondary treatments.

The ideal solution, therefore, is to use a polymer that can provide EMI shielding as an integral property of the resin. This ensures a high degree of shielding and, by reducing the need to accommodate secondary treatments, allows greater flexibility in design. LNP FARADEX compounds, developed by SABIC’s Specialties business, provide EMI shielding performance as an embedded, intrinsic property of the resin and the moulded part.

The optimal dispersion of conductive fibres in the moulded part is critical to achieving maximum shielding performance. SABIC has conducted extensive moulding studies to optimise fibre dispersion by selecting appropriate injection moulding conditions. Figures 3a and 3b demonstrate how SABIC’s processing expertise can provide insights into material performance. Figure 3a shows poor dispersal of the fibres; the left-hand side of both images show a resin-rich part. Figure 3b shows the optimised fibre versus resin concentration after taking advantage of SABIC’s processing knowledge to maximise the Faraday cage effect.

 

“The optimal dispersion of conductive fibres in the moulded part is critical to achieving maximum shielding performance.”

As well as simplifying the process of providing shielding, LNP FARADEX compounds offer a number of additional benefits, as summarised in Figure 4. Without the need for secondary processes, the material offers manufacturers considerably wider design freedom. Medical devices can be manufactured using more complex 3D shapes, offering greater comfort and convenience for patients. The material performance properties may also help to improve device development efficiency,by enabling the design freedom associated with the use of plastics versus other materials. Additionally, LNP FARADEX compounds provide the opportunity to reduce both the weight of the final device and its assembly costs.

Figure 4: Comparison of key features for the different shielding methods.

In healthcare applications, patient safety remains a primary concern. Therefore, all plastics intended for use in medical devices that come into contact with the skin must be certified as biocompatible and undergo a range of tests – most notably the Biological Reactivity Testing (USP Class VI) and ISO 10993 “Biological Evaluation of Medical Devices” – to ensure that they fulfil the correct parameters.

“The demand for connected medical devices will continue to grow
rapidly for the foreseeable future, driven by the pressing need to constrain spiralling healthcare costs.”

To address demand for a biocompatible material that also provides EMI shielding, SABIC has developed a new healthcare grade LNP FARADEX NS003XXW compound, which has been pre-assessed for biocompatibility according to ISO 10933. For those manufacturers considering switching from a metal, or metallised approach, to a polymer, the LNP FARADEX NS003XXW compound also offers the benefit of SABIC’s management of change policy, which provides medical device manufacturers with surety of supply and formula lock.

The demand for connected medical devices will continue to grow rapidly for the foreseeable future, driven by the pressing need to constrain spiralling healthcare costs. It will also be driven by advances in technology, both in the health parameters measured and the technology that can perform the monitoring.

In such a dynamic environment, a biocompatible plastic with inherent EMI shielding properties, such as SABIC’s LNP FARADEX NS003XXW compound, can provide device manufacturers with a cost-effective material solution, enabling improved accessibility for a greater number of patients and – because the devices can be lighter and more convenient to use – can increase their acceptance. In addition to product development, SABIC also brings its in-depth materials and processing expertise to assist manufacturers throughout all stages of the product lifecycle, including design, prototyping, moulding techniques and post-production quality control.

For those device manufacturers seeking to position themselves at the forefront of the expanding connected medical device market, SABIC is interested in collaborating to address the requirements for both EMI shielding and biocompatibility in a single material – the LNP FARADEX NS003XXW compound.

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