Wearable medical devices are biosensors attached to the human body to detect and monitor changes and capture biological/physiological data. These devices are usually non-invasive devices that function autonomously. Monitoring older adults and subjects with chronic conditions in the home and community are a growing focus of these applications.

Common types of physiological functions

This is a growing list of types of the body’s biological/physiological functions commonly measured by medical devices.

• Body temperature
• Heart rate (pulse) rhythm, and volume
• Blood oxygen
• Blood pressure- diastolic / systolic
• Blood sugar (glucose)
• EKG / ECG (electrocardiogram) – measures the electrical activity of the heart muscle, providing information on the heart’s response to physical exertion
• EEG (electroencephalography) – measures electrical activity in the brain
• EMG (electromyography) – measures electrical activity of muscles
• Accelerometer measures the movement of the body and muscle activity
• Respiration- count, rhythm or regularity, character/type
• Other blood tests, urine tests, sputum, etc

Why are accurate measurements needed?

The information collected from these devices is used not only to assess the general physical health of a person but also to give insights into possible diseases and indicate progress toward recovery after diagnosis. Medical devices often regulate these devices. The measurements taken must be accurate. Some are relatively simple numbers, while others are more complex measurements like counts or ratios. For some values, the “normal” ranges vary based on the patient’s age, weight, gender, and overall health.

Standardized measurement techniques

Recent advances in sensor technology, microelectronics, telecommunication, and data analysis techniques have enabled the development and deployment of these wearable systems for patients’ remote monitoring. Healthcare professionals often use the data from these devices to make medical diagnoses or track patients’ health as they go about their daily activities. It is critical that in capturing this physiological data, these new devices conform to standardized measurement techniques and parameters.

Three main building blocks

Wearable systems for patients’ remote monitoring consist of three main building blocks:

1. Sensors and data collection hardware to collect physiological and movement data
2. Wireless hardware and software to relay data to a remote center
3. Data analysis to extract clinically relevant information from physiological and movement data

Major challenges in making these measurements

Voler System’s design experts specialize in ultra-low power device technology and can build small, low-cost devices to serve this market. We have helped many startups, as well as established medical device companies, develop such wearable products. The two major challenges with this type of technology are determining where on the body the measurement will be taken and, for some types of measurement, ensuring that good contact is achieved between the device and the patient’s skin so that the necessary signals can be picked up?

Location of measurement

Many of these measurements are location-dependent. In particular, blood pressure and body temperature vary depending on where they are taken. Below are some examples of the effect of location on measurement acquisition:

• EMG (Electromyography) measures electrical activity associated with skeletal muscle contractions. Placed on the arm, it can measure individual fingers being moved. Placement is quite tricky because it must be placed accurately (within a few millimeters), or it will measure the wrong muscle.
• Heart rate can be measured and recorded by a wrist-worn device with pulse oximeter (infra-red) technology but cannot yet be obtained using current EKG capabilities.
• EKG presents some other challenges for wearable medical devices. It is simple to implement in theory, but the measurement points have to be rather far apart (at least one and a half inches). If the device is too small, this can be a significant issue. One device with this capability is a cell phone application that measures EKG via two electrodes attached to the phone. One thumb is placed on each electrode and held there until the measurement has been obtained. This works because the two thumbs are far apart on the body.

• The placement of the electrodes used for an EEG is critical. Most laboratory systems have 21-leads. Wearable devices typically have far fewer. The most accurate data will be obtained by attaching the electrodes directly on the scalp without hair in the way, which is very inconvenient for a wearable device.

• Skin temperature can vary greatly, so it is not a good indicator of core temperature (to detect fever, for example) except at a few locations. The forehead is one of the best locations, but one of the worst for a wearable device’s convenience.

• It is currently impossible to make an accurate blood pressure measurement except with a blood pressure cuff that is compressed and released. This is not convenient for a wearable device.
  
  

Challenges with electrode contact with the skin

To obtain precise and consistent measurements when collecting EKG, ECG, or EMG data, the device must be designed to make good contact with the skin. Not only that, but the condition of the skin itself can affect measurements – wet skin transmits electrical impulses more readily and reliably than dehydrated skin. Electrode pads are used for laboratory EKG measurements, which improve the skin-to-device connection, but they are not very convenient for wearable technology.

About Voler Systems

We provide full-service R&D consulting from concept and design to production of medical devices for human use. Since 1979, clients have turned to us for reliable new products and test systems involving sensors and measurement electronics. Our multidisciplinary team knows the regulatory process and procedures and delivers high-quality products on-time and on-budget.