Apple Invents the use of Thermally Actuated Electrodes in wearable devices to improve skin-contact for biological measurements (ECG, EMG+)
In June 2021 Patently Apple posted a report titled "Apple's VP of Technology talked about Health Sensors on Apple Watch and possibly AirPods in the future in new interview." Then in August 2021 we covered a more in-depth report titled "Apple's Machine Learning Research Team have Published a Paper on using Specialized Health Sensors in Future AirPods."
Yesterday, the US Patent & Trademark Office published a patent application from Apple that supports the revelations of specialized health sensors in AirPods, AirPods Max along with other devices such as Apple Watch and a body-composition instrument such as a smart scale as presented in Apple patent FIGS. 3A-C.
Apple's patent reveals sensor technology and, more particularly, to thermally actuated electrodes for improved skin-contact physiological measurements. as presented in Apple patent FIGS. 3A-D below, or other wearable.
In Apple's patent background they note that physiological parameters are commonly measured using electronic devices with analog or digital displays that utilize skin-contact electrodes to provide detectable electrical signals. The electrodes are made of a conductive material such as a metal. Examples of biological measurements that rely on contact electrodes are electromyography (EMG), electrooculography (EOG), electroencephalogram (EEG), electrocardiogram (ECG), body temperature, blood pressure, heart rate measurement and the like.
In a skin-contact physiological measurement, the quality of the skin contact can significantly affect the accuracy of the results. Currently, in some skin-contact physiological measurements, an electric conductive gel is applied to the electrode to control the impedance between the skin and the electrode, also known as the skin-electrode contact impedance. The contact impedance may be modeled with electrical elements such as a capacitor and one or more resistors.
Thermally Actuated Electrodes for Improved Skin-Contact Physiological Measurements
The subject technology in Apple's patent application titled "Thermally Actuated Electrodes for Improved Skin-Contact Physiological Measurements," is generally directed to a new device for skin-contact biological measurement.
In some implementations, the device includes one or more electrodes to enable signal transmission through a skin contact and a control mechanism that is coupled to the electrodes to adjust an electrode-to-skin impedance (ESI). The control mechanism can apply electrical activation to implement the ESI adjustment.
In one or more implementations, an apparatus of the subject technology includes a processor and one or more electrodes mounted on a device and coupled to a control mechanism. The control mechanism is activatable by the processor and is able to adjust the ESI by maintaining a desired electrode-to-skin pressure with a varied contour of a local skin surface.
In some implementations, a system according to the subject technology includes a portable communication device and a device communicatively coupled to the portable communication device. The device includes one or more electrodes and a control mechanism coupled to the one or more electrodes to adjust the ESI by using a thermal actuator to maintain a desired electrode-to-skin pressure with a varied contour of users skin surface.
In some aspects, the thermal actuator can convert thermal energy into mechanical energy via thermal expansion and contraction of solid material. Examples of thermal actuator include shape memory alloy (SMA) actuators, hot-and-cold-arm actuators, and bimorph type actuators.
The shape memory alloys can, for example, be made of a nickel-titanium alloy. The hot-and-cold-arm actuators are based on the asymmetric thermal expansion in the micro-structure of a material.
The bimorph type actuators may consist of two or more layers of dissimilar materials and operate based on the difference in the coefficient of thermal expansion (CTE) of the dissimilar materials.
Apple's patent FIGS. 1A and 1B below are diagrams illustrating a structure and operational states of an example of a device with an activated electrode for improved skin contact.
More specifically, Apple's patent FIGS. 1A and 1B are diagrams illustrating a structure and operational states of an example of a device #100 with an activated electrode for improved skin contact.
The device #100, as shown in FIG. 1A, includes an electrode #102, a spring #104, two rigid bars #106 (106-1 and 106-2) coupled together via a pivot P, and a thermal actuator #110 (hereinafter, actuator 110). The actuator is coupled to the rigid bars at points A and B, and the spring is coupled to the rigid bar 106-1 at a point C and to a fixed location D.
The length of the actuator can be changed with the application of heat. The change in the length of the actuator can cause movement of the electrode. For example, contraction of the actuator, as shown in FIG. 1B, can pull the electrode in the direction of axis #112. This can maintain a desired pressure between the electrode and an adjacent surface, such as a skin surface when the electrode is part of a biological measurement system.
Biological measurement such as measurement of electroencephalogram (EEG) is an area of growing interest in wearable technology spaces. To take measurements using dry electrodes (without a conductive gel) presents multiple challenges that compromise the signal-to-noise ratio (SNR) of the reading. An important objective in these measurements is to reduce the ESI as much as possible.
Good initial contact between the skin and the electrode is required for low ESI. In addition, the good electrode-to-skin contact should be maintained to avoid degradation of signal quality due to motion artifacts. According to some implementations of the subject technology, in the device the actuator can be realized using a Shape Memory Alloy (SMA) or other actuators to increase electrode-to-skin contact force (or pressure). The increased contact force reduces initial ESI between the complex and varied skin contours that can be presented across the distribution of human population.
Additionally, increased contact force allows for the contact to be maintained during the measurement due to increased retention of friction force. This also allows the electrode to be triggered when needed, avoiding high pressure points when measurements are not being taken.
The SMA can be a nickel-titanium composite and acts like a contracting muscle when heated to its transition temperature. This material property of the SMA makes it an excellent candidate as a micro-actuator. The SMA actuator has advantageous features, for example, it is flexible (can be mounted on complex surfaces) and simple to use and has a low volume.
Recent research results show that the addition of hafnium (hf) and Zirconium (Zr) can offer a broader transformation temperature range and greater dimension stability to the SMA.
Apple's patent FIGS. 2A and 2B below are diagrams illustrating a structure and operational states of an example of a device #200 with an activated electrode for improved skin contact.
FIG. 2A shows the device #200 trying to be in contact with a skin #220. The device has an activated electrode #202 coupled to a solid arm #206, which has a pivot P and is coupled to a thermal actuator #214 (hereinafter, actuator 214) at a point B. The actuator is coupled, at points A and C, to wires #212 that connect the actuator to a supply voltage V+ and ground potential G. The actuator can be implemented using an SMA or other thermal expansion-contraction-based materials and is embedded into a support material #210 that forms part of the structure of the device. Examples of thermal actuator include hot-and-cold-arm actuators and bimorph type actuators.
The supply voltage V+ can provide an electrical current that runs through the actuator 214 and causes contraction of the actuator 214, which results in insertion of a force to the solid arm 206 that in turn results in pushing the electrode 202 to the skin 220, as shown in FIG. 2B, in order to maintain a good electrode-to-skin contact.
For additional details, review Apple's patent application number US 20220409137 A1.
- Viki Powell: Health Tech Hardware Engineer. Powell is also part of the Oxford Artificial Intelligence Society leadership team while studying my MBA at Oxford.
- Ali Moin: AI/ML Researcher