Patent Application
20060173261
The invention relates to medical sensors, which are applied to a patient's skin for monitoring biopotentials and in particular to sensors that penetrate the patient's skin to make the skin more electrically permeable.
Diagnostic tests, treatments and the presence of illness require measuring and monitoring electrical signals generated by the physiological functioning of a patient. Typical electrical signals or biopotentials that are commonly monitored are those producing electrocardiograms (ECG) from the heart, electroencephalograms (EEG) from the brain and electromylograms (EMG) from the muscles. Such signals are of relatively low level and may be weak, such as 100 microvolt or less signals present in an electroencephalogram (EEG). The frequency range of the signals extends from 0.05 for electro-cardiograms to 3000 Hz for brain stem evoked potentials.
Skin mounted monitoring sensors are typically used to obtain the biopotentials. The human skin is made of three distinct layers; stratum corneum, viable epidermis, and dermis. The outer 10-15 micrometers of skin, called the stratum corneum, is dead tissue that forms the primary barrier for the body. The stratum corneum is the major contributor to skin impedance and to a reduction in biopotential signal magnitudes as well as a major factor in the signal to noise ratio characteristics of skin-mounted sensors. Below the stratum corneum lies the viable epidermis (50-100 micrometers). The viable epidermis is comprised of living cells, but contains few nerves and is devoid of blood vessels. Penetration of the skin to the viable epidermis is painless since the nerves are found in deeper tissues. Below the viable epidermis is the dermis. The dermis forms the bulk of skin volume and contains living cells, nerves and blood vessels.
Difficulties often arise when measuring weak biopotentials with skin mounted electrodes or sensors. One problem is that the outermost layer of skin has high electrical impedance. High electrical impedance reduces signal magnitude so that a data signal may be difficult to obtain when electrical noise is present.
When measuring biopotentials with a sensor placed on the patient's skin, it is of primary interest to obtain as low contact impedance as possible, i.e. it is a common practice to remove a part of the stratum corneum prior to applying the sensor so as to lessen the skin impedance. Traditionally this has involved rigorous preparation of the skin at the location where the biopotential sensor is applied. This rigorous preparation includes removal of grease and oils by means wiping with e.g. alcohol as well as removal of dead, insulating skin cells by means of abrasion. After the biopotential sensor is placed on the prepared area there is typically a stabilizing period before a reliable signal is obtained.
The procedure described above is however time consuming, particularly when several sensors are to be applied and is inconvenient in any clinical situations such as preparing the patient for surgery.
In order to eliminate the disadvantages described above piercing of the skin to reduce its impedance has been suggested. A device, i.e a sensor for doing this comprises spikes penetrating the dead, insulating outer layer of the skin into the conductive part of the skin, however not penetrating as deep as to come in contact with blood circulation and nerve cells. This allows for quick application without any rigorous skin preparation typically required with traditional biopotential sensors to obtain a good skin contact. This sensor also gives a good contact at initial application of the sensor and does not need as long a stabilization time as a traditional biopotential sensor. As an example of these skin-piercing biopotential sensors can be mentioned the one described in U.S. Pat. No. 6,622,035.
The object of the invention is to obtain a simple and practical structure for a biopotential sensor. An advantage of the present invention is that the sensor is extremely simple and practical when compared to the prior art. Simple structure means in practice that the invention can be materialized very simply, whereby costs can be kept at reasonably low level.
In the following the invention will be described by means of the examples shown in the drawing, in which
Preferably the spikes 9 projecting from the carrier 8 penetrate skin 4 so that spike tips lie within viable epidermis 6. This provides impedance reducing, electrical signal pathways across stratum corneum 5 without causing pain or contact to blood circulation of the patient. Reference number 10 shows the layer of electrically conducting material, for example Ag, applied onto the top surface of the carrier 8 and onto the surfaces of the spikes 9. Reference number 11 shows the layer of electrically conducting material, for example Ag, applied onto the bottom surface of the carrier 8.
A biopotential sensor transforms an ionic biopotential into an electronic potential, which is transferred to measuring electronics. In order to transfer the signal, the conductive coating on the sensor must be electrically connected to a conductor, which in turn transfers the signal to the measurement electronics. This can be achieved by creating an electrical connection from the skin 4 to the bottom surface of the carrier 8. As told above in the present invention the carrier and the spikes are made of silicon bulk material. The gist in the present invention is to make the otherwise non-conductive silicon bulk material conductive. According to the first embodiment of the invention silicon bulk material can be made conductive by doping the silicon material with impurities such as antimony, arsenic, phosphorus, boron, aluminium or gallium.
The process of doping introduces an atom of another element into the silicon crystal to alter its electrical properties. The “dopant”, which is the introduced element, has either three or five valence electrons, which is one less or one more that silicon's four. There are several methods of introducing impurities into the silicon. One method is Ion Implantation. This method bombards the wafer with the desired impurities by selecting the required impurities and speeding them up by giving them energy. Another method is to coat a layer of silicon material with impurities and then heat the surface. This allows the impurity atoms to diffuse into the silicon. The amount of impurities added is in the range of 0,1 . . . 100 parts per million atoms.
Creating this way electrical connection from the skin to the electrically conductive material layer 11 applied onto the bottom surface of the carrier offers a straightforward and cost-effective way to materialize a practical biopotential sensor. In this connection it is important to realize that signal can be transferred very simply from the conductive material layer 11 to the measurement electronics. The measuring electronics can be integrated into an assembly as schematically shown in
The connection between the assembly and the electrically conductive layer 11 can be formed in the same way as described in
The embodiments described above are not intended to restrict the invention but only to clarify the basic idea of the invention. It is quite clear that details can be varied within the scope of the claims. The invention has been described above with an embodiment using silicon as a bulk material. The invention is however not restricted only to silicon but also other suitable materials, for example appropriate plastic materials can be used.
