Pressure switch / Medical: safe operation in difficult conditions

A product is only as good as the sum of its parts. This is especially true for the construction of medical devices. They must be compact, robust and user-friendly, from sensors to switches.

Medical devices must not only be reliable, but also safe to use. To ensure both, they are built to strict standards. The demand for multifunctional, compact and economical electronics for devices in diagnostic laboratories, hospitals and clinics, as well as for self-treatment by the home patient is increasing. Developers are constantly challenged to differentiate themselves from other products without increasing the cost of medical care – both for home care and for portable surveillance and therapeutic systems.

To be robust enough to withstand harsh medical conditions, designers need to examine how each component works in each medical device – from tiny sensors to complex measurement systems. This does not apply only to the electronics in the interior. For how long a device is in use, decide at the end of the user. It must also be able to rely on the components, especially on the switches with which it operates the device.

Resistant to liquids

In medical devices, switches are exposed to many sources of danger, such as cleaning solutions, introduced contaminants by humans or the environment, water vapor, electrostatic discharge or pressure differences. In order to prevent contaminant penetration and damage switches, Light-Touch (LTSW) switches in medical applications must increasingly conform to IP67 protection. This international standard specifies the resistance of a product to foreign bodies and liquids. The two figures indicate what protection it is: against the penetration of dust (6) and water (7).

Pressure switches for hearing aids and other portable medical devices must withstand sunscreens, lotions and water. The switches are often covered with a silicone membrane. The plastic, however, causes the operation of the switch is less accurate, partly feeling a little spongy. These disadvantages at the level of the haptic are generally compensated by the increase in the life of the product. And yet, manufacturers must be aware of this feature when they choose silicone membrane pressure switches.

© Panasonic

Photo 2: Safely packed. With the help of a laser, a thin sheet of nylon is welded via the actuator.

Panasonic IP67 pressure switches are designed differently (Photo 1). With its small footprint (2.6 mm × 1.6 mm), the CMS pressure switch is one of the thinnest of its kind and is ideal for portable devices such as smart watches, fitness wristbands, and headphones and hearing aids. , The peculiarity: instead of sticking a silicone membrane under the actuator, Panasonic uses a new laser method in which a thin plastic film made of nylon (polyamide) is welded to the actuator (Photo 2). The nylon film hardly affects the operating force and the click-through rate.

Quality assessment by the user

Another factor that determines the life of a medical device is the perceived quality by the end user. In laparoscopic surgery, interventions are performed inside the abdominal cavity with the help of an optical instrument. Surgeons must be able to rely on the tactile feedback of their electrosurgical aids when cutting or suturing tissue.

The haptic feedback is also important for diabetics. Over time, in many patients, progressive nerve damage affects the sense of touch. To avoid misuse and dosing errors, a consistent and reliable tactile feedback is essential.

Tactile metal shots give – as their name indicates – a "slamming" back and audible feedback when you press the switch. They are part of the contactor system housed in the housing of the switch assembly. This ensures that the timing of the tactile feedback corresponds almost exactly to the moment of actuation.

Fig. 3: Pressure switch characteristics: Evolution of the actuating force (thrust force) via the stroke of actuation (switch stroke).

© Panasonic

Fig. 3: Pressure switch characteristics: Evolution of the actuating force (thrust force) via the stroke of actuation (switch stroke).

how Photo 3 shows that all pressure switches are designed for a characteristic operating force, which can be expressed as a click rate, which is the relationship between switch operation and contact forces. A higher ratio means a precise shift feeling. The operation path or the deflection of a depressed switch can also affect the feel of a switch. Metal clamshell discs or the corresponding embossed layers are easy-to-integrate and cost-effective contact systems, yet offer sufficient possibilities for customer-specific configurations.

Side pressure switch

The new features and enhancements should not affect the size of the devices, as the trend is towards ever more compact products for years. Take the example of hearing aids: they are more convenient and compatible than ever, can now be connected wirelessly to the smartphone and are equipped with lithium-ion batteries. However, it should be remembered that the device should be as unobtrusive as possible, primarily a question of dimensions, and that it should be used with one hand.

Figure 4: Side mounted pressure switch: The switch can be mounted directly on the printed circuit board.

© Panasonic

Figure 4: Side mounted pressure switch: The switch can be mounted directly on the printed circuit board.

The requirements can be satisfied for example by a pressure switch arranged laterally. As the switch is not higher than the circuit board, designers can reduce the product or have more space. The central arrangement also means that the printed circuit board provides support for the switch and thereby improves the resistance of the structure to the direction in which the switch is operated (Photo 4). Mounting the switch directly on a printed circuit board eliminates the need for a flexible or additional printed circuit board to connect the switch and the circuit board, which reduces costs.