The increasing popularity of smart phones and interactive netbooks are influencing other industries to incorporate a touch interface in their devices. Products that previously used keypads are being upgraded with touch panels to give products a leading-edge look and feel. Other benefits of touch panels as a replacement for buttons are the elimination of dirt and debris entering the device and increased durability.

Fig. 1 –The top layer of glass on the Projected Capacitive Touch Screen will not deform over time and provides a stronger, more robust interface.
Projected capacitive (PCAP) touch panels are now the top-selling form of touch panels. According to DisplaySearch’s 2011 Touch Panel Report, “Projected capacitive became the leading touch screen technology in terms of revenue in 2010… In 2011, projected capacitive shipments are expected to grow by more than 100% Y/Y and will account for 70% of all touch screen revenues.”

PCAPs can be constructed from different materials, the most common being PET films and glass. All-glass PCAPs are particularly well suited for use on medical equipment such as diagnostic imaging devices due to their increased durability, scratch resistance, and impermeability to the strong cleaning agents typically used in the medical environment. PCAP also offers an improved user experience with zero-bezel design, compatibility with medical gloves and true multitouch capability, resulting in more intuitive user interfaces and increased patient safety.

How PCAP Is Different From Resistive

Resistive touch panels work by bringing two conductive layers into contact when a finger pushes on the top surface of the touch panel (Fig. 1). The two conductive layers are formed from Indium Tin Oxide (ITO), a clear conductive material, which is sputtered onto a substrate such as PET film or glass. On a resistive touch panel, the top layer has to be a film so that it can be depressed and make contact with the bottom ITO layer.

PCAP touch panels also use two layers of ITO. The difference is that PCAP works by creating an electric field on one ITO layer and sensing the capacitive coupling to the other ITO layer. When an object like a finger is on the surface of the touch panel, it affects the capacitive coupling between the two layers (Fig. 1). The touch controller triangulates the location of this disturbance and calculates a pair of coordinates for the touch location.

Since the two ITO layers don’t need to make physical contact, the top ITO substrate can be a piece of glass instead of film. Using glass for the top layer provides all the durability benefits mentioned above, and also allows the entire enclosure to be completely sealed, preventing contaminants from getting inside the device. The device enclosure can be built in such a way that the top piece of glass is also the faceplate of the device. No additional bezel is required, which is why this type of design is referred to as a zerobezel design (Fig. 2).

The Benefits of PCAP

Fig. 2 – The strength of the top layer of glass in a PCAP panel can also serve as the top layer. A faceplate is not required.
Anyone who has used a resistive touch panel has probably had an experience where the system registered touch at a different location than where the finger was actually placed. This is caused by physical changes in the top film layer due to temperature, humidity, and normal wear-and-tear. As the film layer changes shape, the ITO pattern changes position, causing the reported position of the finger to drift over time. This is a serious drawback for medical devices, where touching the screen multiple times to activate the desired function during a crisis situation could jeopardize patient safety.

PCAP touch panels that use glass for the ITO substrate do not suffer from this drift problem. The glass ITO substrate does not change over time, which means the position of the ITO does not change. The reported finger position is always the same, and the PCAP touch panel never requires manual recalibration. PCAPs sometimes execute a background calibration cycle to compensate for changes in the electric field caused by temperature and humidity variations. These cycles are completely automated and occur in the background when the touch panel is not in use.

The all-glass construction of a PCAP touch panel also increases the temperature range in which a touch panel can accurately function. Recent advances in PCAP are allowing touch panels to be used in environments ranging from –30 °C to +70 °C. The ability to withstand extreme temperatures allows these robust touch panels to be used in industrial applications as well as medical environments where different temperatures are required in multiple settings.

Additional robust features, including the ability to withstand harsh chemicals used for cleaning and sanitizing surfaces, benefit medical applications. The surface layer of a medical device will accumulate dirt, debris, and germs from everyday use, and these contaminants must be effectively removed. PCAP touch panels can also be tuned to work with surgical gloves so the user does not need to remove his or her gloves before interacting with the device.

Fig. 3 – Optical adhesives increase durability, sunlight readability and the overall lifespan of the device.
As medical devices advance, so should their touch interface technology. PCAP is now available in single and true multitouch functionalities. Some applications may only need a single touch interaction and/or gestures for their device. The increased ruggedness of a PCAP touch panel makes it suitable for these applications even when the multi-touch capability isn’t being used. As the medical industry continues to expand the use of touch panels in its devices, true multitouch will enhance not only the interaction with the device, but also the interaction medical professionals have with their colleagues and patients, resulting in better user experiences and improved quality of care.

Another benefit of PCAP is the ability to build larger-size panels. Everyone is familiar with a smartphone’s small screen size, but PCAP has entered new industries and is being used for laptops and interactive gaming machines with screen sizes up to 20 inches. The larger area opens doors for workplace collaboration and multiple people benefiting from the same PCAP screen. Showing patients’ test results, X-ray feedback, and being able to zoom in or out will enhance the overall experience.

Medicine and the PCAP Interface

Medical devices benefit from all of the above characteristics listed, creating a more interactive and accurate user experience. Not only do patients benefit from the ability to see results up close, but it also encourages collaboration that strengthens the relationship between the doctor and patient.

Doctors and medical professionals are able to receive real-time results for their patients and don’t have to remove their surgical gloves to give accurate feedback.

The increased transmissivity, clarity, and visibility allow access to crystal-clear feedback. Previous medical displays had extreme contrast ratios and items of concern, such as tumors, may have gone undetected. Increased clarity of the interface also clarifies the test results, due to high transmissivity levels.

Taking PCAP to the Next Level

Component testing and the increased demand for PCAP prove that the allglass construction of a PCAP touch panel is steps ahead of previous touch technologies. There are options available to enhance PCAP even more.

As shown in Fig. 2, the flat, glass surface of a PCAP allows for a zero-bezel look. For additional robustness, many applications require a coverplate bonded to the top layer. There are different optical adhesives available in the market, including double-sided adhesives and liquid bonding agents. Liquid agents, typically cured with a UV light source, eliminate air gaps and bubbles that may occur with other adhesives. The optical bonding is done with machinery that applies a uniform layer of adhesive and then cures it.

Optical bonding also increases visibility by eliminating reflective images and increasing sunlight readability. The bonding adhesive creates an additional layer of durability, by increasing and strengthening the glass, which also extends the lifespan of the device. The optical performance can be further enhanced by using the same liquid bonding agent to bond the touch panel to the TFT, eliminating another source of reflections and refractions (Fig. 3).

Current PCAP panels allow for more robust designs, longer lifespans, the ability to withstand dirt, debris, and harsh cleaners, and also enhance the look and feel of devices. Applications that benefit from PCAP range from Point of Sale, to white goods, to medical devices. With new PCAP-sensing technologies emerging daily, PCAP can only get better. Sizes currently range up to 20 in., but in the very near future even larger PCAP touch interfaces will become a part of daily life.

This article was written by David Nolte, Sr. VP of Engineering, and Tony Gray, Principal Engineer for Ocular LCD, Dallas, TX. For more information, Click Here 

Medical Design Briefs Magazine

This article first appeared in the November, 2011 issue of Medical Design Briefs Magazine.

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