Developing the Next Generation of Neural Stimulation Devices
Creators: name: Abdul Azees, Ajmal
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400303
subject: Biomechanical engineering

description: The cochlear implant is a one of the best bionic devices ever developed and is a life-changing technology for many people, yet it has limitations. Electrical current spread within the cochlea hinders precise neural stimulation, affecting speech understanding in noisy environments and music. If two or more electrodes (‘channels’) are simultaneously stimulated, the broad current spread causes summation of electric fields, leading to high interaction between channels. Consequently, commercial cochlear implants use sequential rather than simultaneous stimulation. However, simultaneous stimulation would be more beneficial, as it would be more representative of the sound. Stimulating with greater precision would enable the delivery of spectral and fine temporal information to the auditory system, ultimately improving cochlear implant performance.

This study explored novel optogenetic stimulation methods, whereby light is to control neurons that are genetically modified with opsins, to improve the spectral and temporal resolution of the stimulus, which may lead to enhanced cochlear implant performance compared to traditional monopolar electrical stimulation. A hybrid cochlear implant was fabricated for the mouse cochlea that consisted of micro-LEDs and electrodes and suitable coatings to safeguard the electrical components. In vitro tests were conducted to test and characterise the hybrid array. Then this hybrid array was used to test the hypotheses in vivo using a transgenic mouse model expressing an excitatory opsin (the H134R variant of channelrhodopsin-2). Responses to the stimuli were recorded through multi-channel recordings in the inferior colliculus of the midbrain.

A series of acoustic, electrical and optogenetic stimulation acute experiments were conducted to evaluate the spread of activation and interactions between channels during multi-channel simultaneous stimulation in mice. Results demonstrated that the spread of activation from optical stimulation was approximately half that of monopolar electrical stimulation (measured at two set levels above threshold, normalised between stimulation modalities), and similar to that during acoustic stimulation. Channel interactions during simultaneous optogenetic stimulation were significantly lower, with 13-fold less influence on threshold of adjacent channels compared to electrical stimulation alone.

Additionally, a series of hybrid stimulation was performed by combining sub-threshold electrical with the sub-threshold optogenetics stimulation to address the temporal limitations of optogenetic-only stimulation. The spread of activation, measured at the same levels above the threshold, was significantly lower compared to electrical stimulation. Interactions between channels were significantly lower compared to electrical stimulation at same levels. Moreover, the electrical current required to achieve the threshold of activation (i.e. activity that is 30% greater than normalised spontaneous activity) during hybrid stimulation was reduced by 35% compared to electrical-only stimulation.

Finally, optical stimulation was combined with the electrical current steering across two channels to determine if the reduction in activation threshold over the hybrid channel could be enhanced. Hybrid current steering demonstrated the ability to form virtual channels between physical electrodes. The study also explored which current steering ratio resulted in the highest hybrid interaction (i.e. highest interaction with the optical channel) by examining the threshold change. Although the reduction in activation threshold varied with different current ratios, there was no significant difference between these reductions. This suggests that current steering was too coarse to have an impact on the hybrid channel. Having an alternating array of LEDs and electrodes would be as effective as channels that occupy the same position. 

Overall, this study provides significant advancements in precise neuromodulation that could be applied to cochlear implant technology. This study explores a paradigm-shifting approach for cochlear implants, addressing both the temporal limitations of optical stimulation and the spatial limitations of electrical stimulation through the combination of optogenetic and hybrid stimulation. The research also contributes insights into critical design considerations such as electrode/LED positioning and coating technologies.