Application of chaotic vibrations to the development of a bedsore-prevention bed

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Recently, the number of bedridden old people is increasing. One of the most serious problems of these people is a bedsore, also called pressure ulcers, which is caused by inadequate blood supply due to prolonged pressure by bony areas resulting in necrosis of tissues. To prevent the bedsores, it is necessary to relax the pressure on one's body, which requires large manpower for nursing. Application of mechanical vibrations, especially chaotic vibrations with broadband spectra, is a potential way to prevent the bedsores. In this research, biological signals of a human body subjected to time-varying pressure are measured and analyzed as a fundamental study aiming at developing a bedsore-prevention bed. Sinusoidal pressure and chaotic pressure are applied to the loins, in reclined position on the bed. Electrical signals on the skin are then measured : an electrocardiogram (ECG) is detected from the chest of the body, while an electromyogram (EMG) is detected from the muscles of both arms. Pulse sound diagram (PSD) is recorded with a microphone. Based on time-series of ECG, EMG and PSD, the signals were then analyzed by power spectra and by calculation of their Maximum Lyapunov exponents. It is found that waveforms of EMG of a human body are sensitive to sinusoidal pressure, while waveforms of ECG are not. The maximum Lyapunov exponents of EMG after the excitation whether by sinusoidal or chaotic pressure take larger values than those before the excitation, which suggests that signals from the muscles are activated by the time-varying pressures. In contrast, chaotic pressure gives the ECG lower Maximum Lyapunov exponent in both normal and blood-stagnated condition. Same trend is found for the PSD under blood-stagnated condition. These results suggest that time series of heartbeat tends to become periodically by the chaotic pressure.