“A kick in the chest” is how patients living with an implantable cardiac defibrillator describe the pain caused by the shocks the device delivers (1). Ventricular arrhythmias interrupt the heart's regular electrical flow, causing fast, irregular heartbeats that can hinder its ability to circulate blood. Although effective at resetting the heart’s rhythm, defibrillators cause a great deal of pain and may cause anxiety and depression that severely alter patients’ quality of life (2). “You don't get any sort of warning whenever [the device is about to go off],” said  Allison Post, a biomedical engineer at The Texas Heart Institute. “It's either you reset the rhythm, or you die.”

We developed this hydrogel tool to create this long line of pacing that could go over areas of scar and normalize conduction before and after the scar area. There's just nothing else on the market or in development that propagates along a line rather than a point. 
- Allison Post, Texas Heart Institute

Researchers led by cardiac electrophysiologist Mehdi Razavi at The Texas Heart Institute and their collaborators at the University of Texas at Austin led by biomedical engineer Elizabeth Cosgriff-Hernandez developed an injectable hydrogel that turns the blood vessels into flexible wires that can send electrical signals to myocardial tissues (3). This new technology spreads activation over a large area and allows for lower-energy, painless defibrillation. It also offers a new way to directly control the heart's rhythm after an arrythmia episode, especially in parts of the heart that are damaged or scarred. 

“I had not seen anything like this previously in terms of a biomaterials design for the heart,” said Karen Christman, a biomedical engineer at the University of California, San Diego, who did not participate in the study. “People have been looking at hydrogels to treat the heart for myocardial infarction to address the underlying issues with fibrosis and inflammation, but not to address the arrhythmia issue.”

The hydrogel involves two precursor solutions that are kept separate in a dual syringe system. Once injected, they travel towards a mixing head where they blend and trigger a redox reaction that initiates polymerization. This reaction transforms the liquid mixture into a gel-like substance with a sturdy Jell-O consistency that takes the shape of the vein into which it is injected, filling even the smallest branches. The solidified hydrogel is then connected to a pacemaker, providing a wide area of contact with the heart muscle. “It uses the existing hardware that's already out on the market that's already well vetted and proven effective,” said Post, coauthor of the study. “We're just going to be extending its power and reach using this hydrogel.”

Developing a material that solidified properly inside the vein was one of the challenges the researchers had to overcome. “If the solution isn't viscous enough, it will be cleared before it can gel inside the vein. If it's too viscous, it'll form a plug rather than fill the whole thing,” said Post. 

Once the formula was optimized, the researchers injected the hydrogel into the anterior intraventricular veins of pig hearts, noting that the gel spread uniformly across the length of the veins. Occluding the vein did not have adverse effects on the animals. “It was surprising that that amount of hydrogel was okay,’ said Christman. The hydrogel stayed in place for up to four weeks in the pigs without causing significant damage or interfering with heart function.

Current technologies for restoring the heart’s rhythm rely on point pacing, where a single contact point delivers stimulating energy, affecting only the surrounding tissue. This method can be problematic, especially when the electrical signal encounters scarred areas in the heart. Scars can disrupt the signal flow, creating eddy currents that may lead to pro-arrhythmic conditions and the formation of reentrant arrhythmia circuits, or loops of continuous, uncoordinated electrical activity, which contribute to poor myocardial function, reduced contractility, and impaired conduction. “We developed this hydrogel tool to create this long line of pacing that could go over areas of scar and normalize conduction before and after the scar area,” said Post. This eliminates the reentrant circuits and therefore eliminates and prevents arrythmias. “There's just nothing else on the market or in development that propagates along a line rather than a point,” Post said.

Lastly, the researchers performed ablation on the pig hearts to mimic a heart with scarred tissue. After ablation, pacing from a single point showed delayed and uneven activation of the heart muscle. In contrast, the hydrogel reached the middle and inner layers of the heart muscle much earlier, which had a positive effect on normalizing heart rhythm.

Ongoing work involves a long-term evaluation of the hydrogel, with a focus on finetuning the delivery system and exploring different conductivity ranges. Other next steps include conducting chronic pacing studies using a diseased porcine model. Although the initial focus has been on cardiovascular applications, there is growing interest in expanding the use of this technology to neural applications, where the properties of soft materials like hydrogels are particularly advantageous. The potential for this technology extends to musculoskeletal stimulation and other areas requiring electrical conduction enhancement. “This could have wide ranging potential in the neurostimulation space and any place in the body that needs electrical conduction,” Post said. 

References

1. Implantable Cardioverter Defibrillator (ICD) Insertion. (2024). at <https://www.hopkinsmedicine.org/health/treatment-tests-and-therapies/implantable-cardioverter-defibrillator-icd-insertion>
2. Ghezzi, E. S. et al. Burden of mood symptoms and disorders in implantable cardioverter defibrillator patients: a systematic review and meta-analysis of 39 954 patients. Europace  25, euad130 (2023).
3. Rodriguez-Rivera, G. J. et al. Injectable hydrogel electrodes as conduction highways to restore native pacing. Nat Commun 15, 64 (2024).