Biosignals are signals from living beings that can be continually measured & monitored, and some common methods to measure those biosignals include electroencephalogram (EEG) to monitor the electrical activity in your brain, electromyography (EMG) for recording the electrical activity produced by skeletal muscles, and electrocardiogram (EKG or ECG) to measure electrical activity of your heartbeat.
Those can be used for brain interfaces which according to a recent Ericsson's report may become commonplace by 2030 with users just thinking about commands, prosthetic arms, health and disease monitoring, and so on.
Starcat has designed the HackEEG shield to experiment with