Department of Otolaryngology Head and Neck Surgery, Johns Hopkins University, Baltimore, Maryland, United States of America.
J Neural Eng. 2019 Nov 11;16(6):064003. doi: 10.1088/1741-2552/ab486f.
Implantable neuromodulation devices that have cuff electrodes are known to exert mechanical pressure on the target nerves. The amount of pressure exerted by cuff enclosures is one of the key determinants of physiological safety of these devices since excess pressures can cause neural damage. Because direct measurements of pressure on a nerve are challenging, the current cuff design approaches rely heavily on theoretical models or numerical computations for pressure predictions. An experimental approach to test these devices for pressure can complement existing theoretical models and can also serve as a quality control step to screen cuff electrode designs before implantation.
We hypothesize that the pressure exerted on a nerve by a cuff can be estimated by measuring the resulting changes to the nerve's electrical impedance.
We investigated ten 1 cm-long explanted rat sciatic nerves: five that were used within an hour after surgery, and five after 50 h of storage in physiological saline. For each experiment we applied variable pressure on the nerve ex vivo and measured the resulting changes in its impedance. We found a strong correlation between the external pressure on the nerve and its impedance and generated a pressure-impedance calibration curve. At the upper limit of physiologically safe pressure, the nerve impedance increased by ~2 kΩ, whereas, a rise of ~3 kΩ corresponded to pressure value that onsets irreversible nerve damage.
As a proof-of-concept, we used this protocol to generate a pressure-impedance calibration curve for a monkey tibial nerve and estimated pressure exerted by a commercial silicone cuff electrode on the explanted nerve. This single-point measurement was in an agreement with an independent estimate of the pressure measured using a mechanical pull test within 3 mmHg.
已知带有袖套电极的可植入神经调节装置会对目标神经施加机械压力。袖套外壳施加的压力是这些装置生理安全性的关键决定因素之一，因为过大的压力会导致神经损伤。由于直接测量神经上的压力具有挑战性，因此当前的袖套设计方法主要依赖于理论模型或数值计算来进行压力预测。一种用于测试这些装置压力的实验方法可以补充现有的理论模型，并且还可以作为在植入前筛选袖套电极设计的质量控制步骤。
我们假设通过测量袖套对神经施加的压力引起的神经电导率的变化，可以估计神经上的压力。
我们研究了 10 根 1 厘米长的离体大鼠坐骨神经：5 根在手术后 1 小时内使用，5 根在生理盐水中储存 50 小时后使用。对于每个实验，我们在体外对神经施加可变压力，并测量其阻抗的相应变化。我们发现神经外部压力与其阻抗之间存在很强的相关性，并生成了压力-阻抗校准曲线。在生理安全压力的上限，神经阻抗增加了约 2 kΩ，而阻抗增加约 3 kΩ 对应于开始不可逆神经损伤的压力值。
作为概念验证，我们使用该方案为猴子胫神经生成了压力-阻抗校准曲线，并估计了商业硅酮袖套电极对离体神经施加的压力。单点测量结果与使用机械拉伸试验独立估计的压力值在 3mmHg 以内吻合。
