Smart Glass for Healthcare:
Shielding Medical Devices from Radio Waves

Introduction

In our previous article on the applications of smart glass for healthcare, we discussed the advantages of non-porous glass surfaces for improved hygiene.

In this article, we are going to look at the inherent capability of some smart glass technologies to shield medical devices from electromagnetic interference (EMI), such as radio waves from nearby mobile phone transmitters.

This protection is applicable to smart glass window facades on healthcare facilities, commercial buildings and private residences.

At-Risk Medical Products

Medical devices are defined by the European Union as solutions which provide “diagnosis, prevention, monitoring, prediction, prognosis, treatment or alleviation of disease”. There are currently over 500,000 types of medical device on the EU market.

Examples include CAT scanners, magnetic resonance imaging (MRI) scanners, infrared thermometers, contact lenses, breast implants, pacemakers, blood gas analysers, brain wave machines and software apps, amongst many more.

In Vitro Diagnostic medical devices (IVDs) are a special case, with examples such as HIV blood tests, pregnancy tests and blood sugar monitoring systems for diabetics.

Those which incorporate electronics are notoriously difficult to shield from EMI as manufacturers need to comply with both the FDA and the FCC in the US, and in Europe with the requirements of the CE mark and the European Medicines Agency.

Real-World Examples

Here are some examples of real-world EM interference suffered by medical devices:-

• Electrosurgery units which transmitted noise onto real-time video images from an endoscope being used during an operation;
• A cell phone base station on the roof of a hospital which was causing interference to their MRI scanner, despite having ‘Switch off your cell phone’ warnings throughout the building;
• The US FDA has reported cases of failures in apnoea monitors (used to monitor the breathing patterns of newborns during sleep) due to nearby RF broadcasters. The device is supposed to trigger an alarm if the newborn stops breathing;
• Invasive blood pressure monitors which were reported as jumping from 3 to 10 mm Hg whenever a paging transmitter on the hospital roof was activated;
• Radio frequency interference from police walkie-talkies causing powered wheelchairs to drive themselves.

The list goes on, and is comprehensively captured in the ‘Banana Skins’ articles featured on Incompliance Magazine. I would definitely recommend this resource to anyone involved in the design of healthcare installations.

The New EU Medical Devices Regulation

The current version of the EU Electromagnetic Compatibility (EMC) Directive (2014/30/EU) came into force in April 2016 and requires that products ‘must not emit unwanted electromagnetic pollution and must be immune to a normal level of interference’. 

There are in fact specialised EU directives or regulations for radio equipment, aircraft components, road vehicles and – yes, you guessed it – medical devices. 

In fact, two new Medical Devices Regulations on medical devices and in vitro diagnostic medical devices (2017/745 and 2017/746, respectively) entered into force in May 2017, replacing the old Medical Devices Directive of 1993. 

What are we shielding from?

EM interference can come from lightning strikes, powerful solar events (a.k.a ‘space weather’), mobile phone transmitters and nearby airport surveillance radars.

Interference can cause failure in any electronic device, but the effect is exacerbated for medical devices as this can lead to severe health risks or even loss of life.

How does the shielding work?

Any closed shell of conductive material placed on a building envelope has some capability to shield the interior from EM interference by either reflecting or absorbing the EM waves. 

When formed in a mesh or continuous plate structure and connected to electrical ground, we often call this a ‘Faraday Cage’, after the British scientist Michael Faraday who invented them in 1836. 

The Faraday cage ‘deflects’ the incoming EMI to electrical ground, stopping the waves from entering the cage. Common-place examples of Faraday Cages are found on aircraft, automobiles and microwave ovens. In hospitals, MRI scanners are normally located in specially shielded rooms, designed to function as a Faraday Cage.

The degree of EMI reflected or absorbed will depend on the frequency of the incident waves and the physical properties of the building materials.

Examples of materials that are commonly used to reflect EM interference include:

• surface coatings on the glass, e.g. low emissivity coatings
• steel and aluminium which forms the window frame, and
• foil-coated insulating foam, which is often installed inside wall cavities.

Shielding against EMI using smart glass

We can now add to this list smart glass technologies such as SPD and PDLC, since they are composed of a transparent conductive material (usually indium tin oxide, or ITO) which forms a sandwich around the switchable layer.

The ITO layer in smart glass covers most of the glass area and so will act just like the foil in the wall cavity and deflect incident EM waves, protecting electronic products inside the building. 

The only difference is that the ITO within smart glass is not connected to ground but to the output stage of a smart glass electronic controller or isolating transformer, and will thus need surge suppression devices (like varistors) to absorb any EM interference. 

ITO may not be as efficient as a specially-designed Faraday Cage, but it should serve to reduce EMI.

We are not the first to suggest this. The UK company Kemtron already provides ITO-based window shielding as a vapour-deposited conductive coating on acrylic, polycarbonate or glass. 

Various thicknesses of ITO can achieve resistances of 5, 10 or 25 ohms/square, which is not as effective as an aluminium wire mesh, but the ITO does offer better optical clarity, preserving exterior views for patients.

Conclusion

Smart glass technologies which incorporate transparent conductors such as indium tin oxide can provide some shielding against EM interference, protecting sensitive electronics in medical devices within healthcare installations as well as in private and commercial buildings.

The issue of EM interference is difficult to solve, even for large medical device manufacturers, and can result in risks to health and loss of life. Smart glass technologies can help to reduce these risks. 

References

1. Electromagnetic Shielding, Pilkington Glass, URL
2. Medical Device EMI Shielding, Surface Treatment Experts, URL
3. Banana Skins, Feb 2020, Incompliance Magazine, URL
4. EU Medical Devices Regulation 2017/745, European Commission, URL
5. Indium Tin Oxide, MIT, URL
6. Indium Tin Oxide, Quora, URL
7. “What determines whether a material is transparent?”, Scientific American, URL
8. EMP Attacks, Washington State Health Department, URL
9. EMI Shielded Windows, Kemtron, URL