the present invention is a membrane capsule structure that encapsulates a living cell mass , e . g ., islets of langerhans . cells of this type are sensitive to the presence of particular constituents or conditions , and generate an electrical signal in the presence of the constituent or condition to which the cells are sensitive . the constituent may be , for example , be glucose in the blood stream of a patient . the condition may , for example , be blood pressure or bodily temperature . the electrical signal is generally a voltage or potential difference , but may be any other type of electrical signal , such as a current or a signal characterized by a particular frequency . the electrical signal is generated between electrical poles on the cell mass , such as the + and - poles shown in fig2 - 6 . while each cell mass is shown having two poles , a cell mass may have more than two poles , i . e ., the cell mass may generate multiple voltages , currents , or other electrical signals between multiple poles on the surface of the cell mass . fig5 a depicts the electric field between the poles of the cell mass inside and in the vicinity of a capsule with a membrane of a relatively high electric conductivity , i . e ., low resistance . the electric field spreads to the volume conductor which partially short circuits the field . as shown in fig5 b , this short circuiting effect is smaller when the membrane conductivity is lower . when the capsule is electrically insulated there is no external short circuiting , as shown in fig5 c . under these conditions the potential difference that the two internal electrodes see is maximized and a smaller mass of cells is necessary to generate a detectable electrical signal . as shown in fig2 - 4 , a capsule 1 encapsulates a mass of cells 6 in a manner which enables the cell mass 6 to receive adequate nutrients for survival , but which minimizes or eliminates short circuiting between at least one pair of the + and - poles of the cell mass 6 . the cell mass 6 is immersed in an electrolytic medium 8 , other physiological media or in a tissue . membrane capsule 1 includes a low - conductivity membrane 2 and a semi - permeable membrane 4 . the low - conductivity membrane 2 has an inner diameter closely sized to the diameter of the cell mass 6 for sealingly engaging the cell mass 6 along a circumference of the cell mass between the + and - poles for increasing the electric resistance between the + and - poles . the low - conductivity membrane has a length l substantially co - extending with a polar axis c between the + and - poles of the cell mass for increasing the electrical resistance therebetween . the longer the length l , the higher the electrical resistance between the poles . the cell mass 6 may have non - uniform shapes , and the poles may be positioned on different positions on the cell mass . accordingly , the polar axis c may be any axis which runs between the poles such that a low - conductivity membrane 2 may be positioned on the cell mass 6 as described herein for increasing the electrical resistance between the poles . in a preferred embodiment , the low - conductivity membrane 2 is a cylindrical portion of the capsule having an inner diameter of 150 - 300μ , and extending from the point of contact with the cell mass for a total width of about 100μ - 500μ . since the low - conductivity membrane 2 increases the resistance between the poles , the lower the conductivity of the low - conductivity membrane , the more the electrical resistance will be increased , and the better the system will perform . a bio - compatible low - conductivity material , such as a fluoroelastomer , e . g ., polytetrafluoroethylene ( teflon ™), a polymethylsiloxane , nylon or other polymeric material is preferred for the low - conductivity membrane 2 . it will be appreciated that the low - conductivity membrane may be permeable to certain materials , provided that it provides the requisite increase in electrical resistance . the increased resistance to short - circuiting of the present invention is achieved by reducing or preventing flow through the membrane 1 along the likely short - circuit pathways between the + and - poles of the cell mass shown in detail in fig5 . in a preferred embodiment , as shown in fig2 the diameter of the low - conductivity membrane 2 and the circumference of the cell mass 6 between the + and - poles are substantially equal so that there remains little or no electrolytic solution 8 between the cells 6 and an inner surface of the low - conductivity membrane 2 . such intervening electrolytic solution would offer a relatively low resistance electric current flow pathway that would short - circuit the poles of the cell mass . the tight coupling between the capsule membrane 2 and cell mass 6 limits the permissible value of the inner diameter of the capsule 1 to slightly less than the diameter of cell mass 6 . the capsule inner diameter is preferably on the order of 150 - 300 microns . the diameter provides a good ratio of the surface - area of the capsule to the volume of material within the capsule to allow maximum diffusion of nutrients and other materials to and from the cell mass . also , the ratio is sufficient to enable the use of membranes with lower permeability . it also makes it possible to introduce the capsules subcutaneously , when necessary by means of a small diameter hypodermic needle . the inner surface of the membrane 2 is preferably processed or made from a material so that cells 6 tend to adhere to it . for example , the inner surface may be coated with a material which cells tend to adhere to , e . g ., con a or polylysine l , or a high voltage electric current may be discharged through the membrane to increase the adhesion of the cell mass . while the inner portion of the low - conductivity membrane may be porous , to it is preferable to keep this layer very thin , preferably about 1 micron , since a thicker layer might enable a short circuit layer to form . obviously , all of the materials used in the capsule are bio - compatible . the semi - permeable membrane 4 includes pores for enabling nutrients and waste materials to diffuse to and from the cell mass 6 through a cavity 22 filled with the medium . the semi - permeable membrane 4 should be permeable to relatively small molecules , up to molecular weights of 30 , 000 - 50 , 000 , and impermeable to larger molecules , such as proteins and cell anti - bodies . the porosity of the semi - permeable membrane 4 is preferably the minimum necessary for the maintenance of the cell mass 6 . in other words , the inward diffusion of nutrients and o 2 , and the outward diffusion of metabolites and co 2 and excretions should be sufficient to support long term cell survival , while maximizing the electrical resistance of the membrane . the semi - permeable membrane 4 must also be bio - compatible compatible and may be , for example , psf ( polysulfone ) and pvc / pan ( polyvinylchloride / polyacrylonitrile ) polymers such as a polyvinyl chloride acrylic copolymer . the semi - permeable membrane preferably has a thickness on the order of 50 microns , with the discriminating semi - permeable portion of the membrane having a thickness on the order of 1 - 2 microns . such membranes are commercially available as hollow fibers . as shown in fig2 the center of the cell mass 6 is positioned at the inner surface of the low - conductivity ring 2 . thus , the electric resistance between the cells along opposite ends of the cell mass 6 , for example cells at points a and b , is high . in the embodiment shown in fig3 the cell mass 6 is positioned near the junction 20 between the semi - permeable membrane 4 and the low - conductivity membrane 2 , but within the low - conductivity portion 2 . a cavity 12 filled with the medium is defined between the low - conductivity membrane 2 and the cell mass . in either case , the electric potential across the cells will not be short - circuited and the recordable electric signal will be significantly improved . fig6 a and 6b show the difference in the electric field around a cell mass 6 in a capsule without a low - conductivity membrane 2 ( fig6 a ) and a capsule with a low - conductivity membrane 2 according to the present invention ( fig6 b ). the further the low - conductivity ring 2 extends beyond the cell mass 6 , the larger the electric dipole moment and the larger the measurable signal from the cell mass 6 will be . however , the length l of low - conductivity ring 2 may not be increased without limit . as the ring length is increased , the diffusion distance for nutrients , etc . through the semi - permeable membrane 10 and up to the cell mass 6 would become too large for survival or optimal functionality of the cell mass 6 . a length l of 3 - 10 times the diameter of the cell mass is preferred . as shown in fig4 in an alternative embodiment , a low - conductivity cylinder 2 , about 0 . 2 - 0 . 5 mm in length , may be inserted into a semi - permeable capsule 4 . the low - conductivity cylinder 2 should completely seal with the outer diameter of the cell mass 6 which is disposed in the center of the low - conductivity cylinder 2 . an estimate of the improvement in the detectable output voltage from a sub - or intra - cutaneous implant using the capsule discussed herein is shown in table 1 and fig7 - 10 . these calculations are based upon an electric equivalent circuit which models the cell mass as a voltage generator within the capsule which is placed in a volume conductor with properties similar to those around a sub - or intra - cutaneous implant . the equivalent circuit is shown in fig7 and the terms therein defined in table 1 . table 1 also shown baseline values for each of the modelled components of fig7 as measured , or best estimates . table 1______________________________________symbol resistance kohms description______________________________________r . sub . s -- 100 source internal resistance . r . sub . sh -- 500 shunt - leak between cell mass & amp ; membrane . r . sub . i1 , r . sub . i2 . . . 500 capsule axial resistancesr . sub . m1 , r . sub . m2 . . . 100 capsule radial resistances . r . sub . tl , r . sub . t2 . . . 10 tissue resistances near capsule surface . r . sub . ac -- 10 tissue resistance between implanted electrodes & amp ; external electrodes . r . sub . e -- 100 tissue resistance between external electrodes . v . sub . s -- signal source in cell mass . vout -- externally measured signal . ______________________________________ using the equivalent circuit , the signal amplitude at the skin surface v out relative to the voltage generated by the cell mass v s is compared . in fig8 the shunt resistance between the poles across the inner surface of the non - conductive membrane is varied while all other values are left unchanged . the signal attenuation is decreased by up to a factor of 100 when the shunt resistance is increased . fig9 and 10 show equivalent graphs for changes in the capsule axial and radial resistances as well . the low - conductivity ring can be formed , for example , by any of the several methods : the entire capsule can be made of the low - conductivity membrane material . the low - conductivity ring may then be masked while pores are made in the semi - permeable portion of the capsule by any conventionally known process . the low - conductivity membrane may also be obtained by providing a much thicker membrane over the low - conductivity portion of the capsule . the much thicker membrane could be of the same material or a different material as the semi - permeable portion of the capsule . the low - conductivity ring area could also be coated , plated or wrapped by a low - conductivity material , such as the material from which the semi - permeable membrane is constructed , after the pores are made in the semi - permeable membrane . the highest resistance to short circuiting would be obtained when the internal face of the membrane ring 2 is made low - conductivity . finally , the capsule can be constructed from cylinders made out of two different materials , one of which is a regular semi - permeable membrane and the other is a permeable or non - permeable membrane with very low conductivity . the parts may be assembled by adhesive , heat , etc . while most commonly used semi permeable membranes for cell implantation are not electrically charged so that they are permeable to both non electrolytes and electrolytes , the electric resistance of the membrane can be further raised by using charged membranes which interfere with the penetration of ions and charged molecules . however , the membrane may only be mildly charged , since some permeation into the capsule by charged particles is required for essential materials such as certain amino acids , etc . since the regular ions of the medium ( sodium , potassium , etc .) are not consumed by the cells , the membrane need not be permeable to these elements . electric potential measurements are preferably made by electrodes 10 ( fig2 ) positioned either inside the capsule near each end , in contact with the cell mass , or in the external conducting volume . measurements can also be performed between one internal electrode and one external electrode . in any case , the semi - permeable membrane 4 encloses the capsule at least one of the + and - poles of the cell mass 6 . the end of the capsule toward the other pole may be enclosed by a semi - permeable membrane , as shown in fig2 or by a low - conductivity membrane , as shown in fig3 . the electrodes are preferably constructed from a bio - compatible material such as platinum or gold . the wire leads connecting the electrodes with the measurement system are electrically insulated . the electrodes are preferably connected either directly or through an amplifier to a potential detector . in the alternative , the electrodes may terminate below the skin surface at a distance from the capsule and generate a large dipole moment in the tissue . this dipole moment may be detected from electrodes on the skin surface and processed accordingly . although the present invention has been described in detail with respect to certain embodiments and examples , variations and modifications exist which are within the scope of the present invention as defined in the following claims .
a capsule for encapsulating implantable cells for improving the detectability of electrical signals generated by the cells is provided . the capsule includes a low - conductivity membrane and a semi - permeable membrane . the low - conductivity membrane seals around the circumference of the cell mass between the electrical poles of the capsule , and further extends for increasing the electrical resistance between the poles . the semi - permeable membrane enables nutrients and waste materials to flow to and from the cell mass . the semi - permeable membrane encloses at least one of the poles of the cell mass , and cooperates with the low - conductivity membrane to completely enclose the cell mass . the low - conductivity membrane may enclose one of the poles , if desired . electrodes are used to detect the electrical signals from the cell mass . various methods of making the capsule are disclosed .