Streamlining the design of portable medical electronics
Quality healthcare aimed at prevention is a trend that is rapidly gaining momentum. Medical devices that capture vital parameters and possess multiple connectivity features enable a seamless flow of information between caregivers and patients. Patient vitals can be located in a centralized repository, which can be accessed and processed by authorized personnel. Home healthcare is booming because of an increase in the aging population, rising healthcare costs, and demand of quality healthcare from remote locations.
Technological innovations in the field of medical electronics and communication can drive the cost of healthcare. Applications for health at home include chronic disease management, post operative care, fitness, general wellness etc.
An example of a device is a Patient Vital Signs monitor which measures the basic parameters from the patient, including electro-cardiogram (ECG/EKG), pulse oximeter, blood pressure, glucose monitor, respiratory measurement, and temperature measurements. Medical devices designed for home use have different requirements in terms of performance, connectivity features, cost, and ease of use compared to equipment for professionals. With the use of highly integrated system-on-chip (SoC) architectures, the cost and size of medical devices is going down.
Some of the functional blocks of medical devices are common to many medical devices. These include bio-sensors, signal conditioning circuitry, analog-to-digital converters (ADC), and power management circuitry, among others. Today’s SoC devices offer configurable blocks which, along with the capability to integrate analog and digital components, allow the same circuitry to be reused among different functional blocks to significantly the reduce cost of a system compared to architectures designed using discrete components.
Figure 1 gives an overview of a patient monitor designed for home use. The device will also have a stable operating system to handle various peripherals such as USB, capacitive touch sensing solutions, image sensors for high speed video, memory to store data, and integrated analog front-end design with in-built micro-controllers for robust and effective design.
Ease of use plays a critical role in medical devices at home. Features like an intuitive user interface, graphical interface, ease of connecting probes to measure parameters, touch interface, gesture based recognition are very important. The device can also have interfaces which will enable video and audio interaction with caregivers.
Figure 1: Main block diagram
The most important and critical factor for a medical device at home is connectivity. Once the vital parameters are captured in the device, it can be connected to a computer through USB to upload medical information to data centers. Complete medical data related to patients can be stored in a centralized repository. The database of the patient along with the all the essential medical parameters (i.e., the time data was captured, patient’s previous medical history, etc.) can be logged onto the server.
Other connectivity options in the module include LAN, WiFi, 3G, and WiMax, among others. Depending on the scenario where the medical device is to be used, particular connectivity features can be present. Care has to be taken such that the user is not required to perform confusing put configuring of the device for the various protocols.
Standard formats like DICOM (Digital Imaging and Communication in Medicine) can be used to communicate and ensure criticality and safety of patient data. The data has to be encrypted and transmitted over the channel to prevent potential compromise of medical data, Figure 2.
Figure 2: Top-level block diagram
The authorized personnel (caregivers) would have a secure login to access the medical data from the data centers. The comments provided by the experts are also logged on the server which forms a repository of patient data along with expert’s comments. The servers can run complex algorithms to aid the caregivers in making a diagnosis.
The algorithms can automatically diagnose certain conditions and display this to the patients and care providers. The interface to the healthcare givers should also be intuitive and easy to use. The software must be robust and error free to ensure there are no issues involving erroneous medical data.
Further, the feedback from the caregivers must be communicated to the patients. Use of equipment must provide ample benefits to patients as well as drive the cost of healthcare down. This also increases the extent of quality healthcare by breaking the geographical barriers.
The following parameters which are necessary for a patient monitor are discussed in detail:
- Electrocardiogram (ECG)
- Pulse oximeter
- Glucose monitor
- Respiratory monitor (Spirometer)
- Blood pressure monitor
Electrocardiogram (ECG/EKG)
ECG is an interpretation of the electrical activity of the heart over time, captured and externally recorded by skin electrodes. It is measured non-invasively by capturing small voltages that are generated on the surface of the skin every time the heart beats.
Two leads are connected to the patient’s body for a single channel ECG and the voltage generated is the differential voltage of two connected leads. The range of the voltage is less than a millivolt in presence of large offsets and high noise from various sources of electrical interference.
The signal has to be amplified by an instrumentation amplifier, filtered, digitized, analyzed and stored for further processing and communication, Figure 3. The frequency range of interest in an ECG is typically between 0.05Hz and 150Hz. The microcontroller used to manage the signal chain can also be used to analyze the data captured.
Figure 3: Block diagram of electrocardiogram
Pulse oximeter
Pulse oximetry is a non-invasive method allowing the monitoring of the oxygenation of a patient’s hemoglobin. The level of oxygenation of blood can be found (SaO2) based on the intensity of light attenuated by the body tissue. Oxygen saturation is the ratio of oxygenated hemoglobin to the total hemoglobin. Body tissue absorbs different amounts of light that is passing through it depending on the oxygenation level of the blood.
The system consists of a LED driver and a photo-diode, Figure 4. The output of the photo-diode is conditioned by programmable gain amplifiers and then digitized. This component will have an AC component and DC component. The AC component is a result of absorption by the arteries while the DC component is a result of absorption by tissues and veins. Analog components on PSoC can be used for this application. The external components include LED emitter, detector. The digital data obtained is stored in a non-volatile memory.
Figure 4: Block diagram of pulse oximeter
Glucose monitor
A glucose meter (or glucometer) is a medical device used to determine the concentration of glucose in the blood. A small drop of blood, obtained by pricking the skin with a lancet, is placed on a disposable test strip that the meter reads and uses to calculate the blood glucose level. The two types of glucometers are electrochemical based and optical reflection based.
Electrochemical-based glucometers are the most popular type, Figure 5. Electrochemical test strips have electrodes where a precise bias voltage is applied with a digital-to-analog converter and a current proportional to the glucose in the blood is measured as a result of the electrochemical reaction on the test strip.
There can be one or more channels, and the current is usually converted to a voltage by a trans-impedance amplifier for measurement with an analog-to-digital converter. The chemical strips have to be used externally and temperature correction has to be implemented for accurate measurement.
Figure 5: Block diagram of glucose monitor
Respiratory measurement
Respiratory rate is measured using a device known as a spirometer which measures the volume and speed of air that is inhaled and exhaled by the lungs. A spirometer provides a first-level diagnostic test for some pulmonary diseases.
Spirometers typically use turbine transducers or pressure sensors to measure the respiratory measurement, Figure 6. Turbine encoders measure the rate of flow based on the number of rotations which in-turn depends on the airflow rate and volume.
Figure 6: Block diagram of a spirometer
Blood pressure
Blood pressure (BP) is the pressure exerted by circulating blood upon the walls of blood vessels, and is one of the principal vital signs. During each heartbeat, BP varies between a maximum (systolic) and a minimum (diastolic) pressure.
In an automatic blood pressure monitor, an arm cuff is inflated to prevent the flow of blood into the local main artery, Figure 7. This pressure is gradually reduced until blood starts to flow in the artery. This is a measure of systolic pressure. Pulse rate is sensed at this time. The pulse rate when there is no restriction determines the diastolic pressure. An external pressure sensor has to be used.
The microcontroller monitoring the sensor can also be used to drive the motor pump to increase and decrease pressure accordingly. The signal from the pressure sensor is conditioned with an instrumentation amplifier and is digitized by an ADC. The systolic, diastolic pressure and pulse rate are then calculated in the digital domain.
Figure 7: Block diagram of blood pressure monitor
Design of portable and affordable medical devices
Various other medical parameters such as weight, temperature, and ultra-sound imaging can be added to this device, and complete monitoring solution can be provided at home. Microcontrollers with dynamic reconfiguration and programmable analog capabilities enable developers to re-use component blocks for different parameters which are common across various medical devices. This helps to reduce the cost, size, and power consumption of devices.
Power-efficient design is critical for these kinds of designs. Use of a highly integrated SOC reduces the number of components required in the system, thereby reducing cost and power consumption. Safety and reliability are also important considerations in the design of medical devices, which must meet or exceed all norms provided by regulatory bodies in different geographical locations (e.g. CE in Europe, FDA in USA). Electrostatic discharge (ESD) protection is a must for all devices. The design of power circuitry for medical devices is also a challenge since the device could be operated from the AC-line mains.
A lot of challenges will surface in making this model of healthcare a reality. An entire eco-system needs to be created consisting of medical device manufacturers, service providers in maintaining the technology infrastructure, insurance companies, patients, and healthcare providers who embrace this model of healthcare which offers substantial benefits for all of them. We can come across challenges in security and safety aspects, as well. Improper operation can cause security and safety concerns while using these devices. The risks are multi-fold in a home environment where the users are not professionally trained.
Fingerprint recognition could be used, such that the medical data of one patient is not mixed with other patient’s data. Given that a lot of consumables are used for the various measurements, care also has to be taken to ensure that counterfeit products cannot be used. Standards for healthcare ecosystems are being developed by the Continua Health Alliance, which is a collaboration of healthcare and technology companies who have joined together to improve the quality of healthcare. These initiatives allow companies to develop interoperable systems and peripherals. The Alliance has already defined standards for technologies like Bluetooth and USB in the sphere of healthcare.
Healthcare is changing as technology drives the industry forward to make tele-health more affordable and easy to use. Patient safety, security of patient data, affordability, ease of use, and connectivity are the key factors that needs to be considered when designing portable medical electronics for home use.
About the authors
Ajay Bharadwaj is currently working with Cypress Semiconductor Corp. as a Senior Applications Engineer. He holds a Bachelor’s degree in Electronics and Communication engineering. He was a co-founder of a medical device startup. His interests include analog design, digital design and entrepreneurship. He can be reached at ajai@cypress.com.
Pavan Srikanth is currently working with Cypress Semiconductor Corp.as a Senior Applications Engineer. He holds Bachelor’s degree in Electrical and Electronics engineering from BITS, Pilani. His interests include embedded design and market research. He can be reached at pavs@cypress.com.
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