Flexible Electronic “Tattoo” is Versatile, Dual-Signal Heart-Monitor Patch

Modern materials science and fabrication techniques are enabling diverse avenues for innovation in the area of medical sensors that go beyond established basics. A multidisciplinary team based at the University of Texas (Austin) Cockrell School of Engineering has developed and tested a flexible, comfortable, long-lasting, piezo-driven wearable electronics tattoo (e-tattoo) that can be placed on the skin, and it captures cardiac health-related data in two ways.

First, it captures the well-known electrocardiograph readings (ECG or EKG). Simultaneously and time-correlated, it also provides less-known seismocardiograph (SCG) readings. SCG is a measure of chest vibrations associated with heartbeats and provides additional information. When combined with ECG data, it helps assess the accuracy of the ECG readings. “We can get much greater insight into heart health by the synchronous collection of data from both sources,” noted project leader Nanshu Lu, an associate professor in the departments of Aerospace Engineering and Engineering Mechanics and Biomedical Engineering.

The e-tattoo made of polyvinylidene fluoride, a piezoelectric polymer, is remotely powered by a smartphone. The UT team claims it’s the first ultra-thin and stretchable technology to measure both ECG and SCG. Furthermore, although it’s not the first ECG-sensing tattoo, the others are fabricated from non-stretchable materials, making them bulky and uncomfortable for wear beyond short periods. This sensor can be worn for several days while providing constant heart monitoring, a major advantage over brief, “at the doctor” hookups and readings.

1. Shown are various aspects of the stretchable PVDF vibration sensor (i.e. PVDF e-tattoo) and 3D method for SCG measurement: A schematic of the stretchable PVDF e-tattoo (a); photographs of an undeformed and stretched PVDF e-tattoo (b); the PVDF e-tattoo (red boxed) and a commercial accelerometer (green arrowed) attached on human chest (c); SCG signals measured by the PVDF e-tattoo and the accelerometer (d); a photograph of the 3D setup for mapping human chest deformation (e); a photograph of a human chest mounted with three PVDF e-tattoos and painted with a random speckle pattern (positions of the three e-tattoos are denoted as Top, Mid., and Bot. (f); the out-of-plane displacement map averaged at S1 and S2 peak times, respectively (g,h); measured signals by 3D DIC method (raw, filtered) and the accelerometer (Acc.) from the chest (i); SCG signals at three different positions (Top, Mid., and Bot.) captured by 3D method and PVDF e-tattoos, respectively (j,k); the correlation of SCG peak times (S1 and S2) measured by the 3D method, PVDF e-tattoo, and the accelerometer (l). (Source: University of Texas)

The sensing patch consists of a 28-μm-thick polyvinylidene fluoride (PVDF) film with 80-nm thick Cu-Ni electrodes on the top and bottom surfaces. This piezoelectric polymer was patterned into a serpentine mesh by a mechanical cutter plotter for a final mesh size of 38.1 mm × 18.1 mm (Fig. 1). To test the patch itself and establish baseline specifications for applied stress versus resultant strain output, they used a tensile-testing load frame and instrumentation (Figs. 2 and 3).

2. Electromechanical characterization of the PVDF e-tattoo: A schematic of a PVDF e-tattoo subjected to tensile test with in-situ electrical measurements (a); photographs of a PVDF e-tattoo under tensile test, from 0% to 30% (b); Stress-strain curves of three different PVDF e-tattoos, whose slope suggests the effective modulus to be 8.5 MPa (c); the measured electrical resistance versus tensile strain curves indicate the stretchability of a straight PVDF ribbon to be 5.8% (black) and a filamentary serpentine (FS) PVDF network to be 112.9% (red) (d); voltage output from the FS PVDF e-tattoo under 5 Hz sinusoidal strain amplitude from 0.01% to 1% (e); experimental (red) and FEM (black) results of compensated peak-to-peak voltage output with respect to peak-to-peak applied strains (f). (Source: University of Texas)

3. Shown are a representative stress-strain curve of a straight PVDF ribbon (a); a voltage-follower circuit for piezoelectric sensors (b); the pattern of FS PVDF sensor with respect to the stretched direction and the transverse direction of PVDF sheet (c); a photograph of a serpentine-shaped PVDF ribbon under the tensile test and an illustration of the voltage compensation factor f (d). (Source: University of Texas)

The project involved more than just fabricating a sensor and attaching it to a subject’s chest area. Piezoelectric sensors are subject to unwanted motion artifacts (which appear as noise), so the team minimized those by using a pair of identical tattoos placed adjacent to each other and then looking at their difference, in effect filtering out the common-mode signal (Fig. 4).

4. Dual-sensor-based motion artifact cancellation scheme: Raw and filtered SCG signals captured by one PET-covered PVDF sensor under rest (blue shaded) and motion (yellow shaded) conditions (a); periodograms of the filtered SCG signals under rest and motion conditions (b); a photograph of the dual sensor system attached on a human chest (c); filtered SCG signals (d) and corresponding envelopes (e) recorded by upper and lower PVDF sensors and their subtraction result under normal and motion conditions; signal-to-noise ratios (SNR) of single and dual sensor systems with different substrates (PET vs Tegaderm) under normal and motion conditions (f). (Source: University of Texas)

Even placement of the e-tattoo is an issue, so they painted the chest of an adult male with a random speckle pattern (using a pet brush!) and used a pair of cameras set at a 30-degree angle to capture the surface of the chest for 16 seconds at a 500-Hz frame rate. Three independent PVDF e-tattoos were attached at different locations on the chest to determine the location dependency and optimum placement of the ECG/SCG sensor.

Full details on the project, including rationale, principles, materials, fabrication, electrical schematic diagrams, test arrangements, captured waveforms, processing algorithms, and analysis of results are in their paper “A Chest-Laminated Ultrathin and Stretchable E-Tattoo for the Measurement of Electrocardiogram, Seismocardiogram, and Cardiac Time Intervals” published in Advanced Science (Wiley), along with a Supporting Information posting. The research was funded by the Office of Naval Research, the Air Force Office of Scientific Research, the National Science Foundation, the National Institutes of Health, and Texas Health Catalyst at Dell Medical School.