A one piece self contained disposable patch having multiple layers including an electronics layer adapted to be attached to a patient for measuring at least the oxygen saturation level of blood (SpO 2) of the patient, comprising: a light emitter mounted to said patch; a light detector mounted to said patch to detect the light from said light emitter passing through or reflected from the patient so as to acquire data relating to at least the oxygen saturation level of blood of the patient; an electronic circuit mounted to the electronics layer of said patch for effecting operation of said light emitter and said light detector, and to calculate from the data acquired at least the oxygen saturation level of blood of the patient, said light emitter and said light detector each extending from said electronics layer; one moisture resistant and electrically insulated barrier layer on the lower surface of the electronic layer and another moisture resistant and electrically insulated barrier layer on the upper surface of the electronics layer sandwiching and protecting the electronics layer from the environment: power source providing means (28; 40, 42) mounted to said patch for supplying power to said electronic circuit and said light emitter; a peel off sheet attached to an adhesive layer of the patch that comes into contact with the patient to protect the adhesive layer that comes into contact with the patient and the respective patient contact surfaces of said light emitter and said light detector; and the adhesive layer provided at the patch to enable said patch to be attachable to the patient wherein the patch is activated when the peel off sheet is removed from the patch. a light emitter mounted to said patch; a light detector mounted to said patch to detect the light from said light emitter passing through or reflected from the patient so as to acquire data relating to at least the oxygen saturation level of blood of the patient; an electronic circuit mounted to the electronics layer of said patch for effecting operation of said light emitter and said light detector, and to calculate from the data acquired at least the oxygen saturation level of blood of the patient, said light emitter and said light detector each extending from said electronics layer; one moisture resistant and electrically insulated barrier layer on the lower surface of the electronic layer and another moisture resistant and electrically insulated barrier layer on the upper surface of the electronics layer sandwiching and protecting the electronics layer from the environment: power source providing means (28; 40, 42) mounted to said patch for supplying power to said electronic circuit and said light emitter; a peel off sheet attached to an adhesive layer of the patch that comes into contact with the patient to protect the adhesive layer that comes into contact with the patient and the respective patient contact surfaces of said light emitter and said light detector; and the adhesive layer provided at the patch to enable said patch to be attachable to the patient wherein the patch is activated when the peel off sheet is removed from the patch. power source providing means (28; 40, 42) mounted to said patch for supplying power to said electronic circuit and said light emitter; a peel off sheet attached to an adhesive layer of the patch that comes into contact with the patient to protect the adhesive layer that comes into contact with the patient and the respective patient contact surfaces of said light emitter and said light detector; and the adhesive layer provided at the patch to enable said patch to be attachable to the patient wherein the patch is activated when the peel off sheet is removed from the patch.