Medical equipment and devices must be designed with rigorous standards to function in environments where patient safety and product reliability are critical. Today, there is great demand for innovative, feature-rich, and affordable electronic equipment to be used in diagnostic labs, hospitals, and clinics, as well as for consumer self-care. Engineers are constantly challenged to deploy differentiating leading-edge electronics that demonstrably improve patient care in a cost-effective manner for in-home patient care and portable monitoring and therapeutic devices.
Mechanical membrane switches and increasingly feature-rich capacitive user interfaces are used in a wide range of medical equipment and devices — defibrillators, EKG leads, electronic sensors, glucose meters, infusion pumps, patient monitors, and portable oxygen units, as well as disposable dental, medical, and surgical equipment. Therefore, it is important to closely compare and understand the respective technologies behind user interface and switch options.
MEMBRANE SWITCH TECHNOLOGY
Membrane switches provide a lightweight and lower-cost option for integrating user interfaces and electronic components into medical devices. Adapted from the white goods industry, membrane switches not only provide an excellent value-to-performance ratio, but are also rugged, easy to clean, and intuitive to use.
Low-profile control panel assemblies are ideal for touchpad applications, combining both the switch contact system and custom electronics into one complete package, which eliminates the need for an additional motherboard or display board. Standard membranes are simple designs comprised of thin, micro-motion assemblies, with one or more layers of silver or carbon conductors printed on polyester substrate layers. Pressure-sensitive adhesives are used to bond the layers together. Some companies, such as Molex (Lisle, IL), offer advanced manufacturing capabilities, such as printed double-sided silver circuits that produce the equivalent of multi-layer PCBs or that eliminate printed “crossovers” for environmentally exposed applications. These advances are customized in a variety of configurations to fit smaller, more demanding, cost-competitive applications. Assemb lies can be designed as thin as 0.70 mm (0.028").
Available in many standard styles, membrane switches can be customized in a variety of configurations to fit specific applications. The flexible switch substrate allows for the addition of simple electronic components, eliminating the need for additional LED or display-board mounted electronics below the switch substrate. Custom integrated membrane switch assemblies can incorporate LEDs and other electronic components — such as photo diodes, resistors, and capacitors — connected to the membrane-switch substrate using conductive epoxy technology. Depending on the equipment and functionalities, more sophisticated membrane applications can integrate copper flex circuits and PCB assemblies, LCDs, plastic housings or metal backers, and lenses. Membrane options include non-tactile smooth surfaces (i.e., microwave ovens, treadmills), poly-domes, and silicone rubber keypads with carbon or gold contacts.
Tactile metal domes are frequently used to provide the “snap” or tactile feedback and audible sound when a switch is user-actuated. A more economical option than rubber, stainless steel snapdome arrays for membrane switches are produced through a high-speed stamping process, with auto-placement on the membrane to ensure reliability and a consistent user response. The mechanical contact system is extremely reliable in rugged environments and offers a superior tactile response that is consistent from switch-to-switch. Metal dome or embossed arrays offer easily integrated low-cost contact systems that still allow for custom contact configurations.
Rubber membrane keypad assemblies increase switch travel and tactile feedback for a more desirable user experience. Rubber provides a more discrete key appearance with multiple surface finishes for enhanced aesthetics. Cosmetically appealing 3D rubber keypads offer the reliability of a membrane or PCB-substrate switch, with the three-dimensional look and tactile feel of a silicone rubber keypad. A rubber keypad utilizes a variety of contact systems ranging from carbon pills to gold pills, and to actuating tactile metal domes in a membrane-switch assembly. Mounting methods used to apply the rubber actuator to the switch may vary. One simple method of mounting a rubber actuator is to use a double-sided, pressure-sensitive silicone rubber adhesive. Switch options can include patented Molex rocker switches, hard keycaps, and IMD (in-mold decorating). A recent innovative mounting method includes a simple pull-through protrusion molded into the rubber itself. The protrusion is then pulled through a printed circuit board substrate, where an interference fit secures the rubber component to the assembly.
Tactile and non-tactile membrane switches can be converted to bonded LED assemblies by adding light-emitting diodes. Membrane switches with embedded LEDs for backlighting and status indication may feature embossed windows for enhanced viewing angles and fully automated component bonding. LED/display flex assemblies allow for flexible mounting configurations and multiple circuit substrate options. Flexcircuit substrates offer a three-dimensional packaging alternative to the combination of printed circuit board, connectors, and wires. This allows for a single interconnect solution that is smaller, thinner, lighter, and highly reliable.
LED-display assemblies are ideal in a variety of medical equipment applications and can improve manufacturing assembly processes with reduced EMI (electromagnetic interference) and cosmetic options. Placing an LED assembly on the top surface of a user interface produces a wider viewing angle, increased visual brightness, decreased light bleed, improved assembly, and easier use of curved surfaces. Value can be found by creating a single LED assembly to replace light pipes or bulky and costly wire harnesses. Display assemblies may be designed with a large number of indicators, and combined with switches, backers, seven-segment LED-displays, LCDs, and other components to enhance or expand the performance and functionality of the displays.
Applying LEDs to flex circuits can be accomplished using high-speed SMT processes and bonding technology. These two innovative methods of bonding and encapsulating components provide customers with greater value and a lower cost interface. Silver printed circuits on polyester cannot handle soldering temperatures, so Molex began using anisotropic (Z-axis) conductive adhesives, subsequently developing a fully conductive epoxy to accommodate the high-speed SMT lines.
Membrane switches are mechanical, with moving parts that reduce product longevity and increase maintenance requirements. However, initial capital investment is generally lower. Membrane panels require only simple circuits without complex electronic components or design elements. The pressure needed to depress keys is consistent and firm enough to necessitate that depression be intentional. So, accidental activation is less likely than with capacitive switches. Membrane switches are extremely resistant to shock and easy to shield, offering good protection against static discharge and EMI/RFI (radio frequency interference). An internally vented sealed design offers resistance against dust and moisture. The compact size of membrane-switch technology can make it the best fit for smaller, more demanding, and cost-competitive applications.
CAPACITIVE TOUCH PAD AND SWITCH TECHNOLOGY
Capacitance is the property that enables a device to store an electric charge. Capacitive sensing technology uses embedded circuitry to create an electrical field able to detect the presence of a human finger or other conducting object entering its field. The product recognizes a change in capacitance, and actuation occurs in response to user input; more specifically, the body’s innate conductivity.
Advanced capacitive-switch configurations incorporate both membrane and capacitive circuits, effectively providing the best of both technologies. A remarkably efficient user interface incorporating solid-state construction and circuitry, advanced capacitive technology is more reliable than mechanical counterparts. Embedded technology enables the capacitive touch screen. There are no moving parts that can wear out or detract from product performance. A single capacitive assembly can replace dozens of mechanical switches and controls. Touch screen controllers reduce power consumption, improve accuracy, and require less frequent recalibration.
Touch pad user interfaces can be built on printed circuit boards, with substrate options such as rigid FR4, flexible polyimide, and flexible polyester for multidimensional designs. Sealed capacitive switches are highly resistant to moisture, dust, harsh chemical exposure, contaminants and EMI, making them exceptionally durable for a range of demanding applications — including industrial, commercial, and medical equipment. Touch screens feature an easy-to-read, light-enabled surface that’s easy to clean. Overlays are available in a variety of cosmetic surfaces, such as glass, polycarbonate, polyester, leather, wood, or acrylic. Virtually any non-conductive material can be used.
Capacitive touch pads are a versatile medium for creating elegant or otherwise aesthetic visual effects. The capacitive interface supports extensive layout customization with discrete switch buttons, rotary wheels, linear sliders, and track pads, as well as combinations of tactile and non-tactile input styles with adjustable input for light touch or heavy touch. Integrated motion and fluid sensing can add application functionality. There are virtually unlimited cosmetic and LED backlighting options for any icon or display shape, size, and color. Haptic, or tactile, vibration, feedback, and proximity sensing are optional functional elements available in integrated designs.
Capacitive touch pads allow more creative and functional use of light than membrane switches. The capacitive field can sense at a distance great enough to accommodate the insertion of lighting elements and a light guide, which allows more design flexibility. Active option keys may remain lit constantly. Keys can light on depression. Different color indicators can be used to alert users. Accidental activation of capacitive switches can be addressed by changing key sensitivity or time required to depress a key before actuation occurs.
The solid-state design improves reliability. In designs that don’t require lighting elements, capacitive panels can be constructed with an overlay that is a continuous sheet of metal. Sheet metal requires more pressure to activate keys but affords greater protection against accidental activation as well as to water spills or other unintentional contact. The technology allows use with any type of glove. It can also incorporate Braille or number keypad locators.
Many users simply prefer the tactile appeal of a capacitive touch screen to a membrane surface. It is important for healthcare providers and patients to feel comfortable with medical apparatus. Enhanced equipment aesthetics and functionality can increase patient confidence. Capacitive circuitry design options can lend appeal or an air of warmth and quality to complement the desired ambience of a medical setting.
In select cases, the choice of switch technology may be a foregone conclusion based on application requirements. Membrane switches are a budget-wise option for high-volume applications. Capacitive switch products offer even greater design flexibility and valueadded features. For products sold in the global market, capacitive technology can also more readily accommodate multiple languages and upgrades.
PRELIMINARY TECHNICAL AND DESIGN CONSIDERATIONS
Membrane and capacitive technologies warrant addressing a number of technical considerations:
Merits and trade-offs associated with circuit types (PC boards, flexible circuits, etc.);
Matching the most appropriate substrate to the application demands;
Optimal trace routing for ESD (electrostatic discharge) protection;
Power requirements and thermal management for circuitry, LEDs, and other options.
Some factors medical device designers must consider to ensure use of the best available user-interface technology, in addition to product safety, performance, and reliability, include: the product’s operating environment (e.g., hospital, diagnostic lab, or home), interconnect sealing requirements, biocompatibility, intuitive operation, resistance to sterilization, and cycle-life durability.
Other considerations in switch design engineering for medical equipment: cost; safety, reliability, and risk mitigation; state of current equipment architecture; and return-on-investment by injecting new technology (product differentiation, addressing marketplace demands, etc.).
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