Questions for Discussion Panels

Panel on Packaging challenge: Designing and evaluating methods for packaging of miniature (< 50 cubic mm) implantable active electronic devices

  1. Are the standards applied to consumer electronics appropriate for implantable medical devices in the context of the formation of liquid water?
  2. How small can an implant package become and still be considered appropriately safe from a “measured” hermeticity perspective, and are non-hermetic devices a viable alternative for chronic implants beyond experimental devices?
  3. What failure modes are anticipated from a breakdown in hermetic encapsulation (humidity-induced cross-talk and corrosion?), and do these failure modes present a threat to the well-being of the patient?
  4. Is there a “magic number” for the leak rate of a hermetic capsule below which the effects of moisture ingress are not observed?  (Is it limited by material wettability and water dipole diffusion?)

 

Panel on Stimulation challenge: Electrode modifications to increase the charge injection density

  1. What are the charge/phase and charge density requirements for neural stimulation? What do we know so far?
  2. What limits charge injection in vivo: electrode damage or tissue damage? What are the safe limits of stimulation in vivo, and whether the “water window” is identical for different materials and tissues? What are the best methods to assess the damage (CV, SEM, histology)?
  3. In vivo measurements of stimulation charge capacities are substantially below measurements in physiological saline. Why? Is this due to electrode-tissue interaction or are we using inappropriate measures of charge capacity in vivo?
  4. Are there limits on how small we should make stimulation electrodes? Is there a size below which functional responses are size independent i.e. the electrodes become “point sources” even to the closest neurons?
  5. In selecting the electrode coatings – Porous Pt, fractal TiN, graphene, carbon nanotubes, PEDOT, iridium oxide – what other factors are important beside the charge density and biocompatibility?
  6. Where are we going with bioactive coatings, e.g. PEG-PLA, or steroid eluting degradable coatings? What problems do these coatings address for stimulation electrodes? Is this the new interface, i.e are we transitioning from “electrode coating-tissue” to “biofunctional coating- tissue” interfaces?
  7. How advantageous is isolating the metal electrode interface from neural tissue via “ionic bridges” in providing safe stimulation?

 

Panel on Electrode Assessment challenge: Relevance of impedance-based analytical techniques, in-vitro and in-vivo, for determining stability of chronic electrode-tissue interface

  1. What is the rationale behind EIS frequency range selection for in vitro and in vivo electrode assessment?
  2. Does encapsulation of counter (or “reference”) electrodes pose a potential problem for experimental artifacts?
  3. Do we have tractable lumped parameter models for the electrode tissue interface? How relevant are these models?
  4. CVs as a function of sweep rate can be informative. It is similar to EIS if a wide sweep rate range is covered, with the major difference being that the currents are large and the electrode is greatly perturbed from equilibrium. Should CVs be used more routinely? One particular issue of interest is the relationship between EIS and injectable charge density.

 

Panel on Intra-cortical Implantation challenge: Reducing the tissue response with novel electrode designs

  1. Ranking of key problems in chronic recordings among abiotic failure modes (insulation delamination, corrosion, lead failures, etc) and biotic failure modes (dendritic pruning, neuronal loss, micromotion-induced inflammation, neo-vascularization, etc) ?
  2. How reliable does the recording capability of intra-cortical implants needs to be (e.g. ability to record single-unit, multi-unit, or LFP activity after 10 years of implantation) in order for them to adequately address various neuro-tech market needs?

 

Panel on Retinal Prosthesis challenge: Improving spatial and temporal resolution with novel electrode designs and stimulation patterns

  1. Improving the electrode-retinal interface: How do we bring and maintain the electrode closer to the neural tissue (surgical approach, electrode fixation)? Can we make an electrode with mechanical properties that better match biological tissue? Can electrode coatings (e.g neurotrophin and other biologic eluting conducting polymers or transfecting cells with mRNA using electroporation) improve the neural interface and promote neurite ingrowth? Are penetrating electrodes preferable to planar electrodes for localizing responses?
  2. Shaping the stimulus waveform: What is the best way to reduce the complexity of a perceived phosphene? Are very short- or long-phase time useful in selectively exciting different parts of the retinal network ((e.g. rod bipolar cells)? What is the optimal frequency of stimulation to prevent response fading? Is low frequency sinusoidal stimulation safe and efficacious? Can the strategy of stochastic resonance be applied to retinal stimulation?
  3. Current steering: How does the placement of the return electrode affects response and how do we assess this response (e.g. testing more clinical-sized/scaled electrodes in a variety of retinal surface positions around a variety of ganglion cell types to assess robustness of selective stimulation phenomena)? Is current sharing between a distant monopolar and a local guard electrode efficacious? Does the position of one or multiple return electrodes shape the area of retinal activation (and/or the perceived phosphene)? What is the tradeoff between inter-electrode distance and distance from the retinal tissue in terms of being able to localize punctate phosphenes?

 

Panel on Intra-neural Recording and Stimulation challenge: Improving selectivity and bandwidth for limb prosthesis control and sensory feedback

  1. Is intraneural better than epineural? Why?
  2. How should we go intraneurally? (slanted Utah, polyimide, regeneration,…)
  3. Which are the current main technological bottlenecks to address to have intraneural recording and stimulation really working?
  4. Which is the main clinical application for this kind of technology?