Measure EMG signal and use this to control a robotic gripper.
The median frequency of the EMG (electromyogram; the electrical signal from muscle) is related to fatigue (frequency decreases as fatigue increases). Build hardware/software to measure frequency. This can be done with either analog or digital circuitry.
Build a cardiotachometer – a device that measures pulse rate. The signal from the heart can either be from an ECG (electrical) source, or from a light source (measuring light transmission through skin)