the skin patch shown in fig1 - 3 includes two integrated circuit chips 2 , 4 , two button cell batteries 6 connected to power supply terminals of the chip 2 , and an antenna 8 connected to an output terminal of the chip 2 . the integrated circuit chip 4 is connected to a thermistor 12 ( fig3 ) and several passive components ( not shown ). the electrical resistance of the thermistor 12 depends substantially on its temperature the button cells 6 are small and of relatively low profile and are of the type that are commonly used for hearing aids . the integrated circuit chips 2 , 4 and associated components , the button cell batteries 6 and the antenna 8 form a temperature sensor . the temperature sensor , when active , periodically measures the temperature of the thermistor and then transmits the measurement information via an rf link . a receiving unit receives the transmission and derives the temperature . the chips 2 , 4 , the batteries 6 and the antenna 8 are attached to an electrically insulating flexible circuit substrate 14 , as are all other electrical components of the sensor . the flexible circuit substrate is provided on its undersurface with a coating 16 of an adhesive material . a removable protective layer 20 of paper adheres to the adhesive coating 16 . the skin patch further includes a top or outer protective layer 22 of thermally insulating material over the temperature sensor and adhesively bonded to a peripheral margin of the flexible circuit substrate 14 . in use of the skin patch , the paper layer 20 is removed to expose the adhesive coating 16 and the patch is applied to the skin of a subject . the thermistor temperature equilibrates with skin temperature of the subject . the temperature sensor , if active , measures the temperature of the thermistor and transmits the temperature information . in order for the skin patch to be physiologically compatible with the subject , the material of the adhesive coating should be one that can remain in contact with the skin for an extended period of time , e . g . four to five days , without causing an unacceptable reaction . for a normal activity level , moisture transpires through the skin of a human subject at a moisture vapor transmission rate ( mvtr ) of about 425 g / m 2 per 24 hours . vigorous exercise produces a higher mvtr while more sedentary behavior results in a lower mvtr . moisture that transpires through the skin of a human subject may adversely affect the electrical components of the skin patch , i . e . the integrated circuit chips , the cells and the antenna , and accordingly it is desirable to protect these components from exposure to such moisture . the flexible circuit substrate generally has a very low mvtr capacity , so that if the flexible circuit substrate were imperforate , moisture would not permeate through the substrate and affect the electrical components . however , it is not desirable that moisture should be trapped in contact with the skin and therefore the skin patch must provide at least sufficient mvtr capacity for normal activity . this requirement necessitates that each layer of the skin patch have an mvtr capacity of at least 425 g / m 2 per 24 hours . moreover , the upper layer must in addition be resistant to liquid water exposure . a number of commercially available materials have sufficient mvtr capacity and are resistant to liquid water . referring to fig1 the patch is sufficiently large , and the electrical components are sufficiently small , that a substantial proportion of the area of the flexible circuit substrate 14 is not occupied by the electrical components . several holes 24 through the substrate allow moisture to pass through the substrate . the holes 24 may be spaced away from the more sensitive electrical components . moisture that passes through the holes 22 in the substrate and enters the upper layer 22 will tend to permeate the entire upper layer . a conformal coating 28 of a polyurethane or epoxy material is provided over the electrical components in order to protect them from corrosion and humidity effects due to moisture present in the upper layer . referring to fig3 the integrated circuit chip 4 implements a timer 32 , and the timer 32 , the thermistor 12 and passive components implement an astable multivibrator 30 . the astable multivibrator 30 generates a periodic output signal in the form of a square wave having a duty cycle that depends on the resistance of the thermistor 12 . the output signal of the multivibrator 30 is supplied to a microcontroller 34 implemented in the integrated circuit chip 2 . the microcontroller includes a counter 36 , which uses a clock signal generated by an oscillator 38 to measure the length of time in each cycle of the output signal of the multivibrator for which the output signal is in the logic high state and the length of time for which the output signal is in the logic low state , and calculates the ratio of these times . since the duty cycle of the output signal depends on the temperature of the thermistor 12 , this ratio also depends on the temperature of the thermistor . the ratio is encoded by an encoder 40 as a component of a digital transmission packet . the encoder supplies the digital transmission packet to a radio transmitter 42 which is also implemented in the chip 2 and uses the transmission packet to modulate a carrier and the modulated carrier drives the antenna 8 for radiating the signal . the skin patch is used in conjunction with a receiving unit ( not shown ) which includes an antenna for receiving the signal radiated by the transmitting antenna 8 , an amplifier for amplifying the received signal , a microcontroller for decoding the received signal and recovering the ratio value and calculating temperature based on the ratio value , a memory for storing calculated temperature values , and a readout device for displaying the calculated temperature values . in order to prolong shelf life and operating life of the skin patch , the microcontroller controls supply of power to the multivibrator 30 and the transmitter 42 . the microcontroller 34 further controls its own power consumption by use of low - power sleep and suspend modes . the microcontroller 34 is initially activated to its normal active mode by applying a specific signal sequence to contact pads 54 , which are exposed on the paper layer and are connected to the microcontroller through vias that pass through the flexible circuit substrate and the paper layer 20 . in the normal mode , the multivibrator and transmitter are powered . when the microcontroller has been activated , it periodically returns to and exits from the suspend mode . in the suspend mode of the microcontroller , the multivibrator and transmitter are not powered . when the skin patch is first assembled , the microcontroller enters a calibration mode and then enters the low - power sleep mode . in the sleep mode , the multivibrator and the transmitter are not powered . from this time until the skin patch is activated , the only power consumed is that which is required to maintain the microcontroller in the sleep mode . when the device is to be put to use , an activator circuit , which may be incorporated in the receiving unit , applies the wake - up signal sequence to the contact pads 54 . if the proper wake - up signal sequence is detected by the microcontroller , the microcontroller enters the normal operating mode , in which it supplies operating current to the multivibrator and the transmitter for measuring temperature and transmitting temperature information . the active mode alternates with the stand - by mode to conserve power when not measuring or transmitting . when the microcontroller detects the proper wake - up signal sequence , it also powers the multivibrator and the transmitter in order to measure the temperature , as sensed by the thermistor , and send a short repeated sequence of measurement data , which includes a unique identifier for the temperature sensor . this data , transmitted using the antenna 8 , is interpreted by the receiving unit , which provides the user with an indicator that successful activation has been achieved . the user can then disconnect the skin patch from the activation circuit . the paper layer is then removed , thereby also removing the contact pads 54 and effectively rendering the activation terminals of the microcontroller inaccessible . the patch is ready to be applied to the subject &# 39 ; s skin . after transmitting the measurement and identification data , the microcontroller removes power from the multivibrator and transmitter 42 and enters the standby mode . while in this standby mode , a timer 58 in the microcontroller continues timekeeping functions and after a predetermined interval wakes the microcontroller into active mode . the microcontroller activates the circuits when appropriate and performs the operations described above in connection with measuring the ratio and transmitting the encoded data . the microcontroller then removes power , re - enters the standby mode , and repeats the cycle . further details regarding the operation of the temperature sensor are disclosed in copending patent application ser . no . 10 / 017 , 098 filed dec . 12 , 2001 , the entire disclosure of which is hereby incorporated by reference herein for all purposes . [ 0028 ] fig4 illustrates a modification of the skin patch described with reference to fig1 - 3 . in accordance with fig4 the coating of physiologically compatible adhesive material is not provided on the undersurface of the flexible circuit substrate . the skin patch includes an additional layer 60 adhesively bonded to the undersurface of the flexible circuit substrate 14 and the adhesive coating 16 is provided on the undersurface of the layer 60 . the layer 60 is made of a material having an mvtr capacity of at least 425 g / m 2 per 24 hours . it will be appreciated that the invention is not restricted to the particular embodiment that has been described , and that variations may be made therein without departing from the scope of the invention as defined in the appended claims and equivalents thereof . for example , in one alternative embodiment the microcontroller may be activated by transmitting the wake - up signal sequence from the activation unit optically instead of electrically . further , it would be possible to provide sufficient memory in the temperature sensor to record temperature measurements over several days , in which case the stored data could be retrieved after the measurement period and it might not be necessary to include a transmitter and an antenna in the temperature sensor . although the invention has been described with reference to a temperature sensor , the invention is also applicable to other telesensors , for example telesensors that emit signals representative of heart rate , heart rate interbeat interval , activity level , including activity level at the sensor location , and blood oxygen level . unless the context indicates otherwise , a reference in a claim to the number of instances of an element , be it a reference to one instance or more than one instance , requires at least the stated number of instances of the element but is not intended to exclude from the scope of the claim a structure or method having more instances of that element than stated .