a Schematic illustration of human reflex. When a stimulus is received from a thigh muscle at the knee, the action potential in the sensory neuron is sent to the gray matter of the spinal cord. The sensory neuron in the spinal cord directly establishes a synaptic connection with the motor neuron. If the signal is strong enough, it can trigger action potentials in motor neuron, causing knee reflex. Moreover, knee reflex is a spinal reflex whose reflected nerve center is in the spinal cord, but is still modulated by the high central nervous system (brain). b Schematic illustration of the SPD. The external strain applied to the cantilever-based power device can emulate the logic process of human reflex to control the output power with the input mechanical stimulus.