Choosing Touchscreen LCDs for your Medical Device Design

There are many features that drive LCD and touchscreen selection for a device in any industry, but these decisions become more restricted in the medical device space.

Features that Drive LCD Selection

First, and most importantly, a display must be selected that has the appropriate supply chain to support the medical device throughout development and use. Choosing between industrial and consumer displays is an important decision. Industrial displays have long term support and are generally developed for factories or aviation in scenarios where the supply chain needs to remain intact for the lifetime of the product. Because of this long-term support and development lag, industrial displays tend to cost more and utilize older technology than consumer displays. Consumer displays utilize newer technology, but generally are not supported beyond a few years (a common goal for medical device supply chain stability is 7 years).

The next touchscreen selection decision is type of screen: resistive or capacitive. Capacitive screens are more familiar to users from consumer technology and can accommodate multi-touch user interface features such as zooming and rotating (although these types of gestures are rarely incorporated into medical device GUIs) but can have issues with different glove types. Capacitive screens also can accommodate a thick layer of cover glass to protect the screen without a decrease in screen responsiveness. Although capacitive screens used to be restricted for use in the medical field due to glove incompatibility, most capacitive screens are now tunable to different glove types. Manufacturers can tweak aspects of the touchscreen design to make capacitive screens work better for a specific use environment, whether that be a physician’s office, an operating room, or at the patient’s bedside.

Conversely, resistive touch screens don’t have a need for this fine-tuning; because resistive screens rely on pressure, it doesn’t matter with what the screen is pressed (finger, gloved finger, pen/stylus, etc.). However, resistive screens require a much higher contact pressure and are limited in their multi-touch functionality. This is a downside for resistive screens due to users’ expectations of high quality, responsive touch screens from their personal smartphones, tablets, and laptops. There also exists a trade-off with resistive touch screens between glass thickness and responsiveness. The thicker the glass layer that is placed over the LCD display, the harder the user must push on the screen to get the glass to deflect enough for the system to register a touch. However, if the glass is too thin, the touchscreen will be prone to breaking at a lower pressure.

Finally, touchscreen-LCD integration is an important consideration for a medical product display. Because the touchscreen needs to fit within the active area of the display, data sheets and specifications of touch screens can be hard to find or inaccurate, and it is rare that a touchscreen exists from the manufacturer intended to go with a given display, selection of the touchscreen once the display is selected presents challenges. Additionally, besides mere selection of a set of a display and a touchscreen that are compatible, there is the physical interfacing between the display and the touchscreen. The two main methods for touchscreen-LCD attachment are optical bonding and tape bonding. Optical bonding fills the gap between the display and the touchscreen with a shock absorbent polymer glue and offers the display better reliability and better viewing properties but comes with a higher price tag. Tape bonding of the touchscreen to the display leaves a gap of air between the two components, worsening the viewing properties of the screen but providing a cost advantage over optical bonding.

Screen Issues in Medical Devices

Medical devices are produced in lower volumes than devices with displays in the automotive, manufacturing, or aviation industries. This limits the number of LCD suppliers that specifically cater to medical devices and leads to increased lead times. Because displays are not designed explicitly for the issues that medical devices face, considerations as to if displays are appropriate to use in the design of a medical instrument are crucial.

As discussed above, screens must be compatible with their use environment which for medical use, this usually means that gloved hands must be able to successfully use the touchscreen. Also, medical devices and IVD systems are subject to more rigorous cleaning procedures than devices in other industries. Screens must be cleanable by an array of solvents, including 70% isopropyl alcohol, mild bleach dilutions, Cidex®, and CaviCide™. Most anti-reflective coatings are not tolerant to these cleaning solutions and would need to be validated against their use if selected for a product.

Tips for Success in Selecting Touchscreen LCDs for your Medical Device

Ensure required specifications for both the display and touchscreen are well understood and agreed upon. Sometimes, relatively arbitrary specific aspect ratios, sizes, or resolutions of screens are called out in the device requirements without consideration for the many other factors affecting display selection.

Start selection early. Long lead times and back-and-forth with screen manufacturers can bottleneck product design. Get ahead by starting LCD and touchscreen selection early in the design and development process.

Plan for obsolescence. Especially if using consumer displays, keep device design flexible to new displays so that a minimum set of parts need to be revised to accommodate a change in display.

Remember that ultimately, screen selection is about usability and customer perception of the instrument and therefore can make or break a medical device. Ensure tradeoffs of cost, availability, and performance of candidate displays are well characterized.

Tensentric is a team of highly experienced engineers developing a wide range of medical devices and in vitro diagnostic systems. Tensentric has completed over 200 development projects for clients in the medical device and IVD space since the company’s inception in 2009 and is ISO 13485:2016 certified.

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