As one of the most mature tools for neural probes, implantable microelectrodes can accurately record electrical signals at the neuron level. For this reason, they have been widely applied to the basis of neurobiology, and have greatly promoted the development of BCI. According to the different electrode materials, microelectrodes can be divided into glass micropipette electrodes [23,24,25], metal microwire electrodes [26,27,28], and semiconductor substrate electrodes [29,30,31]. Glass micropipette electrodes, also known as patch clamps, are used to record the electrical activity of ion channels on biological membranes [32]. Compared with micropipette electrodes made of high-temperature drawn capillary glass tubes, microwire electrodes have lower high-frequency impedance, higher signal-to-noise ratio, and better mechanical properties, which can detect the fluctuation of voltage value without damaging cell activity. Microwire electrodes are the earliest implantable microelectrodes used for long-term recording of brain activity [33]. However, when the number of channels increases, the distance between the microwire electrodes cannot be precisely controlled during arrangement so that the consistency of electrode performance cannot be guaranteed, and electrode arrays assembly is also not easy to achieve [34].

With the development of photolithography and silicon etching technology, metal microwire microelectrodes are gradually replaced by silicon microelectrodes with good mechanical properties and biocompatibility. The Utah electrode [35,36] and Michigan electrode [37] are the two most representative types of silicon-based microelectrode arrays. For the Utah electrode array (UEA), the electrode recording point is exposed at the tip of each microneedle by mechanical cutting combined with chemical etching, and then metal is deposited [36]. The rest of the needle is insulated with polyimide to obtain a microelectrode array with precise size and spacing [38]. UEA-based BCI systems have been approved by the United States Food and Drug Administration (FDA) for some clinical trials. In 2006, Hochberg et al. [39] used implantable BCI for the first time to enable quadriplegic patients to drive computer screen cursors and activate simple robotic devices just by thinking. Over the years, the number of clinical studies and the leaps they have made in the clinical field have increased significantly. In March of this year, Chaudhary et al. [40] implanted BCI into the amyotrophic lateral sclerosis (ALS) patient who lost all muscle-based communication pathways and he selected a letter to form words and phrases to communicate his needs and experiences via auditory neurofeedback training, which also proved that brain-based volitional communication is possible even in a completely locked-in state. The UEA is not only used for recording, but also for stimulation purposes. For example, inducing tactile feedback in the hand region of the somatosensory cortex may help improve the accuracy of BCI devices [41], as well as the dexterity of prosthetics [42].

密歇根电极阵列是一种类似于UEA的针状电极。不同的是，它在针上有多个平面记录点[43，44，45]，可以实现高密度立体声录音。2017年，Barz等人[46]将组装好的3D密歇根电极阵列长期植入非人类灵长类动物中，这些灵长类动物经过训练可以执行伸展和抓握运动任务。这一结果支持神经科学研究中特定应用神经接口的设计。

基于MEMS技术，由刚性硅制成的微电极阵列可以有效获取脑神经元的高密度活动信息。然而，硅的刚性使其无法匹配生物组织的物理性质。同时，由于其不变形的特性，在组织运动时会对细胞造成损害，因此不适合长期植入人体。因此，希望通过电极柔韧性来优化植入式电极的性能。

神经电极的灵活性包括选择柔性聚合物作为基板，并用各种新的电极材料代替传统的金属电极。首先，具有柔性探针的电极是电极柔韧性的典型方法，通过表现出更多的适用性，提供更适合神经组织的界面;所选的柔性基板材料应具有良好的生物相容性、柔韧性以及与微细加工工艺的相容性，例如聚二甲基硅氧烷（PDMS）[47，48，49]、聚酰亚胺（PI）[50，51]、聚对二甲苯[52，53]和SU-8 [54]。此外，探针的几何形状也会影响电极附近局部组织的排斥。Wu等人设计了一种鱼骨形聚酰亚胺神经电极，通过增加电极和探针之间的距离，有效地减少了组织反应[55]。

另一方面，通过具有高电荷注入能力和优异电化学性能的各种类型的有机电活性电极材料可以进一步提高电极性能，其中包括碳基纳米材料，即碳纳米管（CNT）[56，57，58]，石墨烯[59，60，61]和导电聚合物（CPs）[62，63，64].此外，上述材料的纳米复合材料也已成为流行的选择。例如，将导电聚合物聚（3，4-乙烯二氧噻吩）（PEDOT）与具有高机械硬度的碳基纳米材料相结合，可以防止PEDOT薄膜在长期操作后变形和开裂[65]。最近，Vajrala等[66]通过简单且可重现的电沉积方法制备了高孔且坚固的PEDOT-CNF的新型纳米复合材料，实验结果表明其性能优于纯PEDOT材料。

一些研究证实，柔性电极确实可以减少对周围脑组织的影响[67]。然而，一个重要的问题是柔性神经探针可能太软而无法穿透脑膜并到达目标部位，因此需要使用额外的加固结构。刚性骨架层[68]和插入梭[69，70，71]的方法可以提高柔性探头的刚性，但前者会限制装置的灵活性，后者会暂时增加植入物的占地面积，在植入过程中对神经组织造成额外的损伤。一种更可接受的方法是用生物可吸收涂层暂时加固探针，这在涂层溶解后恢复探针的柔韧性。常用的生物可吸收涂层包括聚乙二醇（PEG）[72]、聚乳酸共乙醇酸（PLGA）[73]、丝素蛋白[74]、蔗糖[75]、麦芽糖[76]、葡聚糖[77]及其双层结构[78，79]。这些聚合物不仅仅是一种插入助剂，还可以作为生物友好型涂层来减少剔除。在实际使用中，应结合聚合物的刚度、降解速率和生物吸收性，根据应用场景选择合适的生物涂层[80]。