In this complex landscape of medical systems, highly reliable passive components will always be essential. As many as 20 passive components are required for each active integrated circuit (IC) in a single design. Specialist components are typically in demand, necessary to achieve the application-specific values that fall outside the realm of commodity, mass-produced resistor products. Designers need access to options such as energy ratings, pulse-withstanding thick films, and double-sided resistors that safely offer twice the energy capacity of critical value to contact medical devices with direct electrical connection to the body.
Controlling Electrical Contact with Patients and Equipment
Defibrillators provide an excellent example of the design and performance challenges faced by device developers. Automatic external defibrillators (AEDs) are broadly needed by police, first responders, and even average citizens—considered essential in improving time-to-defibrillation and patient survival.
Designers of AEDs face the additional challenge of size- and cost-reduced components that require the steady and repeatable measurement of the charging voltage. The result determines the amount of electrical energy the cardiac patient receives. The defibrillator-charging circuit uses high-voltage resistors, with a high-value resistor ranging from 5 MΩ to 50 MΩ, and a low-value resistor as a possible barrier for voltage feedback.
Accuracy of value is critical to the setting of the correct energy dosage (Figure 1), and, apart from the tolerance, this depends on the temperature coefficient of resistance and voltage coefficient of resistance (VCR). The former may be positive or negative and is generally non-linear. In contrast, the voltage characteristic only has a negative gradient, with a limit expressed by the VCR, typically between –1 and –5 ppm/V.
Designing for Patient Care and Equipment Protection
Defibrillation pulse exposure presents a concern for any directly connected monitors, such as ECG, respiratory, and plethysmographic displays. Electrical damage to the sensitive input stages of these machines wreaks havoc on performance and must be prevented.
Even more critically, the device itself must balance performance and safety. Defibrillation energy cannot be diverted from the patient; this is managed by adding resistance to the monitor input circuit which takes the form of a pulse-withstanding resistor built directly into the lead set. Secondary protection within the monitor itself is also an option.
The total defibrillation energy collected by a protection resistor is reliant on its ohmic value. For this reason, designers must opt for the highest value consistent with what is required by the monitor function. Designers must also consider the actual test circuit selected for the application, and factor in the number of leads in the lead set based on electromagnetic compatibility standards such as IEC601.
A protection resistor in this type of application should offer energy ratings in the range of 25 J at 1kΩ down to 2.5J at 10kΩ, and be supported by today’s carbon composition and high-surge metal glaze devices. For example, pulse-withstanding thick-film products provide secondary protection for printed circuit board-mounted resistors. Resistors utilizing special materials and adjustment techniques offer guaranteed pulse performance. Double-sided resistors maintain the benefits of size, but double the energy capacity via two parallel resistance elements on a single chip.
Pre-Qualified Pulse Performance
Specialist resistors are necessary to both generate and protect against defibrillator surges. When a defibrillator pulse is applied to a patient, the surge energy delivered is intended to re-establish a normal heartbeat. The specialist resistor must simultaneously prevent surge energy being diverted from the patient, and ensure it does not damage monitors and sensitive electronic equipment (Figure 2). Pre-qualification is key—for example, testing to IEC 60601/61000 standards, reducing design risk, and offering developers a faster route to a successful product.
Protection resistors are either designed within a monitor cable set or within the monitor itself on a printed circuit board. As they block defibrillation energy from entering the monitor, they absorb some portion of that energy themselves. Consider the total energy in a defibrillation pulse can be up to 350 joules. While surface mount chip resistors dissipate only a small amount, perhaps 1 percent of that energy, it still equates to a large amount of energy for a small electronic component.
Design options span a range of capabilities in terms of energy management. Pulse withstanding chips and high pulse withstanding chips (HPWC), for example, have gained ground in medical applications based on their proven performance in industrial settings. As an untrimmed resistor, an HPWC offers a 5 percent tolerance.
The HDSC is an untrimmed, double-sided version. This design doubles surge capacity via its thick film material on the bottom and top of the chip. The defibrillator pulse chip resistor (DPCR) is the latest advancement on this front (Figure 3). Ideal for use at monitor inputs, this unique option provides a guaranteed defibrillator pulse withstand performance in a compact footprint. Surge protection is maximized across two categories—surges that are deliberate, for example from a defibrillator, and surges that are random such as a lightning strike or surge in a power line. The DPCR is qualified against defibrillator surge standards ensuring maximum energy and voltage performance, and protection of medical equipment with absolute certainty. The DPCR has also been uniquely characterized to electro-static discharge (ESD) standards, reducing development risk and time to market for medical equipment designers.
Thick-Film Technology Expands Application Options
Thick-film technology has become viable in new medical instrumentation applications. Instead of measuring current or voltage, it can gauge impedance of test samples, for example during chemotherapy to verify the rate at which cancerous cells are destroyed. Illustrating material biocompatibility, this resistor can leverage silver/silver-chloride (Ag/AgCl) contacts, thick-film printed onto ceramics. As a result, researchers have a powerful tool in determining the effectiveness of different chemotherapy treatments and advancing treatments tailored to particular types of cancers. Comparable technologies, also incorporating Ag/AgCl contacts and ceramic substrates, show promise in improving cervical cancer screening. By calculating the impedance of tissue, scientists are empowered with quicker and more dependable early detection of cancer and pre-cancerous cells.
Advances in environmentally sound thick-film technologies add further value for medical designers working to pioneer new products, get to market quickly, and maintain product lifecycle for long-term medical deployments; as illustrated by TT’s Green High Voltage Chip resistor which contains no lead at all and meets standards for lead-free design without exemption. This extends longevity of the design itself as developers avoid the complexity and cost of replacing components as legislation for lead-free design continues to expand in the medical arena.
In the competitive and evolving medical market, there is a continuous flood of new applications that enable healthcare anywhere. Innovative resistor advances provide designers with the technologies necessary to meet the ever-increasing challenges of implementation, budget, and accessibility to safe, reliable performance.
Stephen Oxley is a business development engineer at TT Electronics. With a Master of Engineering in Electrical and Electronic Engineering and deep expertise in development and test engineering, Oxley has spent more than two decades providing pre-sales technical assistance and product education as well as after-sales support to clients, distributors, and sales engineers. Contact him at firstname.lastname@example.org or via LinkedIn.