Tough & Conductive: 3D Printed Polymer Hydrogels for Flexible Sensors

International researchers came together recently to study more about complex 3D printing materials, releasing their findings in ‘Tough and conductive polymer hydrogel based on double network for photo-curing 3D printing.’

Both conductive properties and hydrogels are popular for use in 3D printing today, and in this study, they are combined for greater innovation in flexible sensors. Made of polymerics, hydrogels—as their name would suggest—are able to hold water and are often used in research projects centered around bioprinting, cross-linking, soft tissue engineering, and more. While deemed suitable for a range of applications, up until recently their uses have often been limited due to inferior mechanical properties.

Conductive hydrogels, however, are attractive for use in research and development due to the following:

• Strong adhesion
• High porosity
• Good swelling
• Biocompatibility

All of the above-mentioned features make CHs excellent choices for use in human motion sensors, and even wound dressings, bolstering infected skin for regeneration.

“Currently, printable CHs are mostly ionic conductive with a free-moving ion,” stated the researchers. “Due to the simple preparation process and large deformation, it is very suitable for 3D printing.”

Challenges arise in using conductive hydrogels, however, as flexible chains deform but a single network structure is easily broken down, causing low mechanical strength; however, combinations such as conductive polymer (polypyrrole (PPy), polythiophene (PTh), polyaniline (PANI), and hydrogel may come together to create interpenetrating polymer network (IPN) or double network (DN) structures.

Serving as a chain to manage stress within the structure, conductive polymer hydrogels (CPHs) are polymerized, immersed, and then integrated into the network, forming the IPN. The authors report, however, that molding time is too long, at one to two days, and thus ‘far from meeting the requirements of 3D printing…”

In this study, hydrogel synthesis was performed via DLP 3D printing.

Electrical conductivity of sample hydrogels was evaluated under EDOT content of 12 wt%.

“The electrical conductivity of dry gel was ascribed to π-electron conjugated polymer of PEDOT. For ruling out the influence of ions, the evaluation of PEDOT was based on the conductivity of the dry sample,” stated the authors.

“Compared to only one oxidizer APS, Fe³⁺ as electron acceptor took electrons from π-electron conjugated polymer of PEDOT to form carrier in the chains [41]. When the FeCl₃ content was low, few carriers led to a poor electrical conductivity. When FeCl₃ was excessive, too much oxidizing points led to a decrease in the molecular weight of PEDOT, which weakened the contact of electrically conductive phases. At the same time, peroxidation also destroyed the conjugated structure of PEDOT [42], which caused the electrical conductivity decline. When the ratio of EDOT, APS and FeCl₃ was 1:1:1, the electrical conductivity of dry gel is the largest (0.5 S cm⁻¹).”

The hydrogel exhibited swelling as EDOT was increased; however, a constant state was attained as it decreased.  In testing the sensor application, the researchers created a sample ‘finger,’ attaching the hydrogels and noting that as the finger bent, the hydrogels were able to transform fast mechanical signals into electrical signals. Varying pressures were noted, and the fingers were able to deform and stay without breaking.

“It was demonstrated that hydrogels could be used as pressure sensors to monitor human activity,” concluded the researchers. “This study provided a new idea for 3D printing conductive hydrogel, which could be used in stretchable electrical devices and soft robotics applications.”

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