Hair, sweat, and body movement can make conventional wearable sensors less reliable, especially when electrodes cannot sit closely against the skin. Researchers at Pennsylvania State University have developed a conductive liquid ink designed to address those limitations by conforming directly to the body.
The colorful ink can record signals from the heart, muscles, and brain in human laboratory tests. It also worked on the scalp through hair, an area where many external sensor systems face difficulties.
A Sensor Applied Like Face Paint
The research team calls the material a WE-PPD sensor tattoo because it is applied in a way that resembles face paint. The liquid is nearly transparent when first applied, but users can mix in food coloring to create personalized colors and designs.
Unlike factory-made sensors that are placed on top of the skin, the liquid ink can settle more closely onto the surface. It can also reach small gaps in the skin, which may help improve biosignal recording.
The primary material is PEDOT:PSS, a polymer that conducts electricity. The formulation also includes DBSA as a softening agent, along with an ethanol- and water-based solution.
According to Liputan6.com, the ink can be combined with porous silver material. That combination is intended to create sensors that remain flexible while the body moves.
Results From Human Tests
The conductive tattoo was tested while participants used a treadmill and lifted weights. It detected heartbeats during movement, when rigid sensors can be more likely to shift or detach.
Researchers also used recorded body signals to control a robot. In scalp measurements, the material recorded brain activity through hair.
| Test | Demonstrated Capability |
|---|---|
| Treadmill and weightlifting | Detected heartbeats while the body moved |
| Robot control | Read body signals used to control a robot |
| Scalp measurement | Recorded brain activity through hair |
Why Existing Sensors Can Struggle
Epidermal electronic devices have been developed for more than a decade to measure electrical body signals, temperature, and strain. These ultrathin polymer devices contain embedded circuit components and can adhere without adhesive.
However, earlier electronic tattoos do not always perform well on curved or hairy body surfaces. Electrode placement can also require adjustment across larger areas because biosignals may be distributed across different points of the body.
Sweat and hair create further obstacles for externally applied devices. Air gaps between the skin and a manufactured sensor can reduce the accuracy of biosignal readings.
A conductive ink for scalp use was also developed in 2024 to measure brain waves in people with hair. Such approaches could support EEG electroencephalogram recording outside hospital settings.
Reusable Design, With Work Still Needed
Larry Cheng, a Penn State mechanical engineer and one of the paper’s authors, has researched body sensors for more than 10 years. His work has included recording signals from the brain, heart, and muscles.
Rigid materials such as iron or other metals can provide more stable recording, but they can shift or come off more easily during exercise. Hydrogels can absorb water, expand, and stretch with the skin, yet they tend to degrade more quickly during long-term use.
The WE-PPD sensor tattoo is designed to be cleaned and then redrawn on the skin for reuse. The approach is considered more practical than relying on the main sensor devices, which cost considerably more.
The work was presented in a scientific paper published by PNAS, and the technology has filed a provisional patent application. Further development is needed to address heat risks that could injure a patient’s skin during MRI scanning.
The researchers also see potential uses beyond human bodies. Conductive ink could be applied to plants to monitor plant health and agricultural needs.
