As ageing and increasingly healthy populations proliferate developed and developing regions, engineering decisions such as choosing resistors for diverse medical diagnostic and therapeutic applications become more crucial. Stephen Oxley, Senior Applications Engineer, TT Electronics, explores further in this ES Design magazine article.
Medical devices are leaving the confines of hospitals and are serving the growing community-and home-based healthcare markets. Growth in emerging economies is making them more affluent; populations are growing healthier and living longer. Yet ever-increasing demand for healthcare equipment is paralleled by growing pressure on costs.
In particular, analysts cite the need for faster, lower power, higher precision and more intelligent electronic devices. Specific sub-segments with higher growth in electronics content include diagnostics, patient monitoring and therapeutic equipment, predicted to expand at a rate of 9% p.a. between 2010 and 2017. (IHS iSupply Market Tracker, Industrial Electronics, Q3 2011). Portable and consumer medical devices are showing the highest growth rates. In 2016, the global health care equipment & supplies market is forecast to have a value of $442.8 billion, an increase of 29.1% since 2011 (MarketLine Industry Profile Global Health Care Equipment & Supplies, June 2012)
Much of this development is stimulated by ever-improving computing power of digital systems. However, as the human body is intrinsically analogue, there will always be an important role for high reliability passive components in systems including diagnostic, imaging, patient monitoring, instrumentation, and pharmaceutical delivery and dispensing applications. It is estimated that around 20 passive components are needed for each active integrated circuit in a design.
Regulation, and the unique characteristics of the healthcare environment, impose a particular burden of care on designers choosing passive components such as resistors. It is useful to classify medical applications into three representative areas, each illustrating a different set of constraints for the electronic designer.
The first area, contact, includes all devices with electrical connection to the body. Examples include the delivery of high-energy pulses for defibrillation, the detection of biologically generated signals for ECG or EEG and the measurement of body impedance for respiratory or plethysmographic monitoring.
Imaging encompasses X-Ray, MRI and ultrasound technologies, all with their own special demands on resistive components, in particular the ability to work with very high voltages and magnetic fields.
Finally, the medical instrumentation and analysis category covers the area of IVD and laboratory instruments where accuracy, repeatability and stability are paramount.
In an effort to reduce the time-to-defibrillation delay and improve cardiac-arrest survival, health-service providers have increasingly turned to a strategy of wider access to defibrillation to augment emergency medical services. In some countries, this is being implemented by providing automatic external defibrillators (AEDs) for use by police, first aid volunteers and even ordinary members of the public.
Whilst AEDs pose the additional challenge of size- and cost-reduced components, all defibrillators need stable and repeatable measurement of the charging voltage, as this determines the amount of electrical energy delivered to the patient.
In essence, the defibrillator-charging circuit uses high-voltage resistors, with a high-value resistor, normally in the range 5MΩ to 50MΩ, and a low-value resistor providing a potential divider for voltage feedback.
Critical features of such high voltage resistors are linearity (expressed as voltage coefficient or VCR), temperature coefficient (TCR) and long-term stability under voltage stress. Thick film resistors are best suited to this application. Their temperature characteristic is typically ‘U’ shaped with limits expressed by the TCR, normally in the range ±25 to ±100ppm/°C. The TCR error can be reduced by choosing the highest-possible ohmic value, which lowers self-heating, and by designing layouts that avoid proximity to heat-generating components.
The voltage characteristic, by contrast, only ever has a negative gradient, with a limit expressed by the VCR, typically between -1 and -5ppm/V. High-voltage resistors use special design techniques to minimise VCR, but this needs to be traded-off against product size. As the gradient increases at high voltage, VCR error can be reduced by only operating the resistor at up to 75% of the full rated voltage. Designers need to choose resistors with both a low VCR and a high voltage rating. Furthermore, if the nominal VCR is known, compensation is relatively simple.
Environmental stability describes the limits of non-reversible resistance change under given loading and environmental conditions. The most-demanding condition is high humidity, but devices are available which use a specially formulated high-density epoxy material to achieve typical resistance changes less than 0.25% after 56 days at 95% RH and 40ºC.
Exposure to defibrillation pulses is an issue for any directly connected monitors, such as ECG, respiratory and plethysmographic monitors. In particular, this necessitates guarding against damage to sensitive input stages of such equipment.
Moreover, it is even more important to avoid diverting the defibrillation energy from the patient, achieved by adding resistance to the monitor input circuit. This normally takes the form of a pulse-withstanding resistor built into the lead set. Secondary protection may be provided within the monitor itself.
The percentage of the total defibrillation energy received by a protection resistor depends on its ohmic value, so designers need to select the highest value consistent with that required by the monitor function. Designers also need to take account of the actual test circuit chosen for the application. In addition, specific EMC standards, IEC601, are defined for medical equipment. IEC601 requires designers to factor in the number of leads comprising the lead set.
Energy ratings for the protection resistor should be in the region of 25J at 1K falling to 2.5J at 10K. This level of performance is achieved by today’s carbon composition and high surge metal glaze devices. For PCB mounted resistors providing secondary protection, pulse withstanding thick-film products are used. Guaranteed pulse performance comes from resistors utilising special materials and adjustment techniques: double-sided resistors, providing two parallel resistance elements in a single chip, offer twice the energy capacity without compromising the size benefits.
A third area, in which resistor innovations are coming to the aid of contact medical applications, is where ECG monitors and analytical instruments require sensitive first stages to amplify small signals. Here, high ohmic values are required in the feedback resistor – ratings outside the range normally available. To meet these needs, off-the-shelf flat chips provide resistance values up to 50GΩ, whilst glass-sealed resistors extend capability right out to 100TΩ (1014Ω).
Imaging applications include X-Ray, ultrasound and MRI systems, and each of these applications has its own requirements for resistive components. One key area is that X-Ray systems require stable and accurate high-voltage supplies to provide the accelerating voltage for X-Ray generation - typically in the 50kV to 100kV range. Circuits are often assembled in an oil-filled chamber to reduce the clearance constraints on the layout, thereby enabling a compact X-Ray head design.
An effective design approach is to use an ultra-high voltage thick-film resistor, selected from the specialised devices available that provide up to 100kV in a single element in an oil-filled assembly. Wire or screw termination options allow stacking into multiple resistor assemblies, whilst unsleeved versions eliminate the possibility of air pockets. Availability in matched sets ensures accurate ratio tolerance and facilitates designs with very low TCR through cancellation.
In contrast with X-Ray imaging designs, ultrasound transducers require termination networks that can operate at high frequency and provide multiple channels of resistive termination. Standard or custom BGA thin-film resistor networks can be produced to meet application requirements up to 15MHz with 128 channels.
MRI scanners pose further challenges to designers, in particular the need for control circuits to operate inside extremely strong magnetic fields. Components must be free of ferrous alloys and nickel: the materials commonly used in the termination caps of most types of axial resistor or as anti-leaching barriers in chip resistors. Capless through-hole resistors and nickel-free chip resistors should be sought for such requirements.
Nonmagnetic solutions are also available for circuit protection in MRI scanners. For example, a recently developed non-magnetic fuse provides typical cut-off current of 995mA for a rated DC current of 570mA at 250V DC. With residual magnetism of less than 1.0008, the fuses are provided in surface-mount flip-chip packaging and can be tailored for different fusing characteristics
Precision resistors deliver the tight tolerances, low temperature sensitivity and high stability demanded across a broad range of laboratory analysis equipment.
The input stage of an instrument with a resistive sensor, such as a thermistor in a precision temperature-monitoring circuit, consists of a bridge of resistors, which must be closely matched in value. Since it is the ratio between values that matters rather than absolute values themselves, the maximum difference between TCRs, (that is, the tracking TCR) is more important than the absolute TCR.
Most through-hole precision resistors are available in matched sets with specified ratio tolerance and tracking TCR, and this solution gives the best available precision. Alternatively, where space saving is a major consideration, SMD thin-film products with multiple elements can provide high precision in a compact single-component solution.
Options for the precision through-hole resistor approach range from semi-precision devices like the PR Series, through the popular precision RC Series, to ultra-precision devices capable of equalling the performance of costly metal foil technology using advanced metal film techniques.
The precision SMD offering includes conventional thin-film chip resistors using nichrome elements. However, ultra high stability requirements can be met by devices that exploit the self-passivating properties of tantalum nitride film.
When evaluating the long-term stability of resistors, designers should consider several environmental tests. Some of these are early-life factors, such as exposure to solder heat. Others, like TCR are reversible. Nevertheless, most are long-term factors. Best practise is to design based on only one of these figures: the one that most closely reflects operating conditions.
Shelf life metrics apply where loading is negligible and the environment is benign, but where power dissipation is the main factor, the load figure should be applied. For humid environments, designers should focus on measuring the Long Term Damp Heat figure. In all these tests, most of the value change happens within the period of the test, as the value will tend to stabilise.
The figure of most value to designers is the maximum total error in resistance value at the end of product life, or before scheduled re-calibration if this is applicable. This is termed the total excursion, and is calculated from the root of the sum of the squares (RSS) of applicable, statistically independent short-term and long-term factors.
Applying this calculation to an ultra-precision MAR series resistor demonstrates an order-of-magnitude improvement in total excursion compared with standard precision resistors like the RC series.
Thick film technology
Thick film technology developed for resistors has more recently come to the fore in new medical instrumentation applications, measuring impedance of test samples rather than current or voltage. One example uses impedance measurements to determine the rate at which cancerous cells are killed during chemotherapy. A key requirement is biocompatibility of the materials: in this case, using a silver-silver chloride contacts, thick-film printed onto ceramics enables researchers to determine the effectiveness of different chemotherapy treatments on individual patient tumours. The results will help them develop bespoke treatments targeting specific cancers.
Similar technologies, also based on Ag/AgCl contacts atop dimensionally stable ceramic substrates, are being developed for measuring the impedance of tissue to provide faster and more reliable early detection of cancer and pre-cancerous cells. This promises better cervical cancer screening compared with existing methods.
Every year brings new applications like this, which are harnessing electronics to diagnose illness, monitor medical interventions, and keep both patients and healthcare workers safe. Innovations in resistors are providing designers with the solutions they need to meet all the challenges of performance, cost and availability.