RF Shielding with Smart Glass for Medical Devices

RF shielding (against radio frequencies) using smart glass building facades can protect medical devices from catastrophic failures.

In a Nutshell

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 capability of some smart glass technologies to shield medical devices from radio frequencies (RF). These high energy bursts can arise due to nearby mobile phone transmitters, solar flares or even lightning strikes.

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

Medical Devices

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

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 RF, as manufacturers need to comply with several regulatory bodies.

Examples include the FDA and the FCC in the US. In Europe the requirements are stipulated by the CE mark and the European Medicines Agency.

RF Interference

Here are some examples of real-world interference suffered by medical devices through a lack of RF shielding:-

  • 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.

EU Medical Devices Regulation

The current version of the EU Electromagnetic Compatibility (EMC) Directive (2014/30/EU) came into force in April 2016. 

It 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 (including both medical devices and in vitro diagnostic medical devices) entered into force in May 2017, replacing the old Medical Devices Directive of 1993. 

What are we shielding from?

RF 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 RF shielding work?

Any closed shell of conductive material placed on a building envelope has some capability to shield the interior from RF interference. It achieves this by either reflecting or absorbing the radiation.

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

The Faraday cage ‘deflects’ the incoming RF radiation to electrical ground, stopping the waves from entering the cage. 

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 RF 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 window frames
  • foil-coated insulation foam, often installed inside wall cavities.

RF Shielding using Smart Glass

Smart glass technologies such as SPD and PDLC can also help in this regard, 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 in smart glass covers most of the glass area and acts just like the foil in the wall cavity, deflecting incident RF and protecting electronic devices 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 an isolating transformer. 

Hence, the conductors need surge suppression devices (like varistors) to absorb any RF interference. 

ITO may not be as efficient as a specially-designed Faraday Cage, but it should serve to shield against RF interference.

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 per square. This is not as effective as an aluminium wire mesh, but the ITO does offer better optical clarity, by preserving exterior views for patients and medical staff.

Conclusion

Smart glass technologies which incorporate transparent conductors can provide RF shielding against high-frequency interference. 

This protects sensitive electronics in medical devices within healthcare installations as well as in private and commercial buildings.

The issue of RF shielding is difficult to solve, even for large medical device manufacturers. 

However, the difficulties and costs are clearly outweighed by the benefits of minimising the risks to health and avoiding the loss of life.

Smart glass technologies on building facades in the healthcare sector can help to significantly 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
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