Electric prosthetic hands should be lightweight to decrease the burden on the user, shaped like human hands for cosmetic purposes, and have motors inside to protect them from damage and dirt. In addition to the ability to perform daily activities, these features are essential for everyday use of the hand. In-hand manipulation is necessary to perform daily activities such as transitioning between different postures, particularly through rotational movements, such as reorienting cards before slot insertion and operating tools such as screwdrivers. However, currently used electric prosthetic hands only achieve static grasp postures, and existing manipulation approaches require either many motors, which makes the prosthesis heavy for daily use in the hand, or complex mechanisms that demand a large internal space and force external motor placement, complicating attachment and exposing the components to damage. Alternatively, we combine a single-axis thumb and optimized thumb positioning to achieve basic posture and in-hand manipulation, that is, the reorientation between precision and lateral grasps, using only four motors in a lightweight (311 g) prosthetic hand. Experimental validation using primitive objects of various widths (5-30 mm) and shapes (cylinders and prisms) resulted in success rates of 90-100% for reorientation tasks. The hand performed seal stamping and USB device insertion, as well as rotation to operate a screwdriver.

翻译：电动假肢手应具备轻量化以减轻使用者负担、仿人形外观以满足美观需求，并将电机内置以保护其免受损坏与污物侵蚀。除执行日常活动的能力外，这些特性对于假肢的日常使用至关重要。手内操作是实现日常活动的必要条件，例如在不同姿态间转换（特别是通过旋转运动），诸如在插卡前调整卡片方向、操作螺丝刀等工具。然而，当前常用的电动假肢手仅能实现静态抓握姿态，现有操作方法要么需要过多电机（导致假肢过重而难以日常使用），要么依赖复杂机构（需要较大内部空间并强制外置电机），从而增加装配难度并使组件易受损。本研究通过结合单轴拇指结构与优化拇指定位，在仅使用四个电机的轻量化（311克）假肢手中实现了基础姿态与手内操作——即精确抓握与侧向抓握间的重定向。采用不同宽度（5-30毫米）与形状（圆柱体与棱柱）的原始物体进行实验验证，重定向任务成功率可达90-100%。该假肢手成功完成了印章按压、USB设备插入以及操作螺丝刀所需的旋转动作。