Piezoelectric films are unique materials that excel as dynamic strain sensors, making them ideal for monitoring life signals on the human skin surface or even when implanted inside the body. These thin and flexible films can detect subtle physiological changes, such as a heartbeat or breath, through external layers. This article explores several key applications of piezoelectric films in vital signs monitoring.
**Working Principle**
When a piezoelectric polyvinylidene fluoride (PVDF) film is stretched or bent, it generates an electrical signal—either charge or voltage—between its top and bottom electrode surfaces. The magnitude of this signal is proportional to the degree of deformation. Unlike traditional piezoelectric materials that respond primarily to pressure, PVDF films are particularly sensitive to stress applied in different directions. A small force applied along the length of the film can create significant lateral stress, while the same force spread over a larger area results in less stress. This makes PVDF highly responsive to dynamic stress. For example, a 28-micron thick PVDF film typically produces 10 to 15 mV per microstrain.
The term "dynamic stress" refers to the fact that the charge generated by mechanical deformation is lost through the connected circuit, which means the film cannot detect static stress. This characteristic becomes advantageous when measuring varying levels of pre-stress, as the film only responds to changes in stress. Its minimum response frequency is around 0.1 Hz, allowing it to capture low-frequency physiological signals effectively.
**Introduction to Piezoelectric Thin Film Sensors**
As a dynamic strain sensor, the piezoelectric film is well-suited for detecting life signals on the skin or within the body. Some membrane elements are sensitive enough to pick up a pulse through the outer casing. This article will focus on the typical applications of various piezoelectric films in the field of vital signs monitoring.
**How Piezoelectric Film Sensors Work**
When a piezoelectric PVDF film is stretched or bent, it generates an electrical signal between its upper and lower electrodes. The amount of signal produced is directly proportional to the deformation. While most piezoelectric materials are sensitive to pressure, PVDF films exhibit a unique property: a small force applied along the length of the film creates a large stress across its width. This sensitivity to dynamic stress makes PVDF ideal for detecting subtle movements. A 28-micron-thick PVDF film has a sensitivity of about 10 to 15 mV/microstrain.
Because the charge from deformation is lost through the circuit, these films cannot measure static stress. However, this limitation becomes an advantage when monitoring changing stresses, as the film only detects variations. With a minimum response frequency of 0.1 Hz, they are capable of capturing low-frequency physiological signals like heartbeats and breathing patterns.
**Key Features of Piezoelectric Film Sensors**
Piezoelectric films are thin, lightweight, and highly flexible, making them suitable for passive use in medical applications. They are especially effective at detecting minute signals, even in environments with limited power. In some cases, the energy generated by the film itself can be sufficient for operation. Additionally, these films are extremely durable, able to withstand millions of bends and vibrations without losing performance.
**Applications of Piezoelectric Film Sensors**
**Contact Sensor**
Piezoelectric films can be directly attached to the skin, such as on the wrist, to monitor pulses. The MEAS Sensor Model 1001777 is a versatile option, though its adhesive may not be biocompatible. Instead, 3M9842 tape can be used for short-term applications. These sensors are capable of detecting very small physical signals and can operate over a wide dynamic range, making them ideal for filtering out noise and capturing subtle changes.
**Accelerometer**
The Minisense 100, designed by Precision Electronics, uses a cantilever beam structure to detect movement. When subjected to vibration, the mass causes the film to deform, generating high voltage output. These sensors are used in smart badges that track the position and activity of workers or patients, waking up when movement or muscle tremors are detected.
**Auscultation**
Electronic stethoscopes often use piezoelectric films due to their durability, high sensitivity, and wide bandwidth. These sensors are typically housed in a conventional metal chest piece and convert sound waves into electrical signals, which can then be amplified, filtered, or transmitted for further analysis.
**Sensor Group**
Composite acoustic sensors with multiple piezoelectric elements can monitor various points simultaneously. For instance, DeepBreeze uses nearly 100 sensors to map airflow during respiration, creating an image of lung function that is safer and more reliable than X-rays.
**Bed Monitoring**
Piezoelectric films and cables can be embedded in mattresses to monitor a patient’s heartbeat, breathing, and movement. Hoana Medical’s system detects these signals through clothing and sheets, triggering alarms if abnormalities occur.
**Baby Respiratory Monitor**
Respisense, developed by Infantrust, uses PVDF to monitor a baby’s breathing by placing the sensor near the waist. If no movement is detected, a buzzer alerts caregivers, helping prevent SIDS.
**Respiratory Thermoelectric Monitoring**
PVDF is also sensitive to temperature changes, making it useful for monitoring breathing rate. C-Lect Medical’s PIPPA monitor uses a small film element placed near the mouth and nose to detect temperature fluctuations from exhaled air, providing accurate readings even with oxygen.
**Recommended Products**
- **US MEAS Piezo Film Sensor - SDT1-028K**
This shielded sensor features a rectangular PVDF film connected to a plastic housing and coaxial cable. It is ideal for high-EMI environments.
- **US MEAS DT Series**
Available in various sizes and thicknesses, the DT series offers high voltage output and is commonly used in strain sensing applications.
- **US MEAS LDT Series Vibration Sensor**
The LDT0 model is a flexible sensor that generates high voltage when bent, making it suitable for detecting motion and vibrations.
These sensors are widely used in healthcare, offering non-invasive, accurate, and reliable solutions for monitoring vital signs.
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