Future needs of injectable preparations
The conductive hydrogel preparation in our study represented an initial evaluation of a carbon nanotube complex that was made to be expressible through a syringe and needle. However, materials properties need to be considered in the context of the specific use case in which they are applied. In the case of injectable materials, they should liquify upon applying a specific pressure for easy flow out of a container (eg, a syringe) and then immediately solidify upon expression out into whatever material they will be contained in (eg, within myocardial tissue). In the context of our study, while some conductive hydrogel was retained in the myocardium as seen on gross examination, some loss of hydrogel also occurred during injection. We performed the experiments ex vivoin a still beating heart with an epicardial injection approach. However, any such injectable material, if delivered endocardially, will have to be optimized to prevent any material loss that could then embolize. Furthermore, the amount injected and retained needs to be predictable and thus further research to evaluate the minimum quantity of material needed to impact conduction will be necessary.
Prior studies have looked at complexing material with alginate or other substances to optimize properties for injection and retention in tissue(16,17). However, most of these studies focused on alginate as a material scaffold that could optimize structural integrity of the tissue or as a delivery tool for stem cells, drugs, or other materials. The impact of complexing other substances with carbon nanotubes and similar conductive materials will need to be considered in the context of their impact on conductivity of the material itself as well as the impact on the material-myocardial interface in terms of cell-to-material-to-cell interaction. Also, whether commercially available trans-endocardial injection catheters such as the MyoStar catheter (Biosense Webster, Diamond Bar, CA, USA) will be able to accommodate the conductive hydrogel, will need to be further evaluated. Thus, while our work demonstrates proof of concept, these limitations need to be considered prior to in vivo studies, particularly if endocardial injections are performed.