Wearable human activity sensors developed in the past decade show a distinct trend of becoming thinner and more imperceptible while retaining their electrical qualities, with graphene e-tattoos, as the ultimate example. A persistent challenge in modern wearables, however, is signal degradation due to the distance between the sensor's recording site and the signal transmission medium. To address this, we propose here to directly utilize human skin as a signal transmission medium as well as using low-cost gel electrodes for rapid probing of 2D transistor-based wearables. We demonstrate that the hypodermis layer of the skin can effectively serve as an electrolyte, enabling electrical potential application to semiconducting films made from graphene and other 2D materials placed on top of the skin. Graphene transistor tattoos, when biased through the body, exhibit high charge carrier mobility (up to 6500 2V-1s-1), with MoS2 and PtSe2 transistors showing mobilities up to 30 cm2V-1s-1 and 1 cm2V-1s-1, respectively. Finally, by introducing a layer of Nafion to the device structure, we observed neuromorphic functionality, transforming these e-tattoos into neuromorphic bioelectronic devices controlled through the skin itself. The neuromorphic bioelectronic tattoos have the potential for developing self-aware and stand-alone smart wearables, crucial for understanding and improving overall human performance.

翻译：过去十年开发的穿戴式人体活动传感器呈现出日益纤薄且不易察觉，同时保持其电气性能的明显趋势，石墨烯电子纹身便是其终极范例。然而，现代可穿戴设备面临的一个持续挑战是：由于传感器记录部位与信号传输介质之间的距离导致的信号衰减。为解决此问题，本文提出直接利用人体皮肤作为信号传输介质，并采用低成本凝胶电极快速探测基于二维晶体管的可穿戴设备。我们证明，皮肤的真皮层可有效充当电解质，使得施加于皮肤表面的石墨烯及其他二维材料半导体薄膜的电势成为可能。通过人体偏置的石墨烯晶体管纹身展现出高电荷载流子迁移率（高达6500 cm²V⁻¹s⁻¹），而MoS₂与PtSe₂晶体管的迁移率分别可达30 cm²V⁻¹s⁻¹与1 cm²V⁻¹s⁻¹。最后，通过在器件结构中引入Nafion层，我们观察到神经形态功能，从而将这些电子纹身转化为可通过皮肤自身控制的神经形态生物电子器件。此类神经形态生物电子纹身有望开发具有自我感知能力的独立智能可穿戴设备，这对于理解和提升人体整体性能至关重要。