Enhancing electro-ionic modulation by using pedot:PSS

Neurological disorders adversely affect hundreds of millions of people around the world. Almost 15 million people suffer a stroke annually, and about 5 million of them die while approximately 5 million of them left permanently disabled. In literature, several methods were suggested to solve the problems caused by these negative effects. One of these methods is Functional Electrical Stimulation (FES) that is used to achieve muscle activity on paralyzed individuals by activating nerves. However, FES has several undesired side effects that obstruct continuous treatment. Application of high stimulation current can damage the nerve tissue and it can cause the stimulation of non-target nerves. In order to prevent the damage on nerve tissue, stimulation current should be reduced. In addition, nerve blocking should be used to prevent undesired stimulations and resultant pain feeling; however, application of high blocking current can damage the nerve tissue permanently. On the other hand, Electro-Ionic Modulation (EIM) is a neuromodulation method which is significantly successful in addressing the issues caused by FES. EIM achieves significant amount of decrease on the stimulation and blocking threshold by changing ion concentration of extracellular fluid around the nerve via interface while applying electrical current at the same time. Since it utilizes both electrical and chemical aspects simultaneously, EIM has potential to replace earlier electrical modulation methods being used in real-life applications. We used Ion Selective Membrane (ISM) as an interface in our previous study. We observed 20-25% decrease of stimulation threshold and 30-40% reduction of blocking current compared to electrical method in in-vivo experiments in frog's sciatic nerve as a result of depleting Ca2+ ions around the nerve. Depletion of Ca2+ ions around the nerve increases the number of open Na+ ion channels; hence, decreases the stimulation threshold. In this study, we utilized Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), which is a conductive polymer that is selective to positive ions, as an interface and obtained 55-60% reduction of stimulation threshold value. Underlying reason of reduced threshold can be explained as follows; while ISM only depletes Ca2+ ions around the nerve, PEDOT:PSS depletes higher amount of positive ions around the nerve compared to ISM. Although depleting Na+, K+ and Mg2+ ions around the nerve has an increasing effect on stimulation threshold, depletion of higher amount of Ca2+ ions around the nerve suppresses this adverse effect, therefore further reduces the stimulation threshold. Similarly, in-vivo blocking experiments conducted in frog's sciatic nerve resulted in 65-70% decrease of blocking current. Further reduction of blocking current can be explained with depletion of higher amount of Ca2+ ions around the nerve compared to ISM. In addition, findings of the experiments with PEDOT:PSS that dried for 24 and 48 hours showed that PEDOT:PSS is a suitable interface for the implanted device experiments for six months in rat's sciatic nerve. In conclusion, in-vivo stimulation experiments conducted in frog's sciatic nerve showed that using PEDOT:PSS as an interface exhibited almost 2.5 times better results than using ISM; therefore, applying reduced stimulation and blocking currents minimized the chance of damaging the nerve and increased the efficiency of the method.