Batteries and other electrical power sources are used in a wide range of applications, from consumer products to devices for medical purposes in the body. Improvements in such power sources are continually being sought and I have accordingly devised an improved such electrical power source and a method of making the same.

According to a first aspect of the present invention, there is provided a method of producing an electrical power source, which comprises providing at least one well having at least two opposed primary electrodes, and at least two opposed secondary electrodes; filling said well with an aqueous medium including a polymerisable system containing at least one polymerisable monomer capable of providing at least one mobile ionic group and at least one fixed ionic group, and being polymerisable to form a viscous hydrogel; covering said well so as to substantially exclude air therefrom; and permitting said polymerisable system to polymerise in said covered well in the presence of an applied field extending between said primary electrodes, such that the resulting hydrogel substantially fills the space in said well.

An example of such a polymerisable monomer is an acrylamide or an acrylamide derivative. The polymerisable system then preferably includes a cross-linking co-monomer such as a bisacrylamide (for example, N, N-methylene bisacrylamide).

The polymerisation of such a polymerisable system may be initiated by a free radical initiator. A preferred free radical initiator includes a persulfate, such as, for example, ammonium persulfate. The polymerisable system may further contain a polymerisation controller such as tetramethyl ethylenediamine (known as TEMED), which accelerates the rate of formation of free radicals from persulfate, which in turn initiates polymerisation.

It is preferred that the initiator and/or the cross-linking co-monomer should be present in an amount more than that required to form a cross-linked copolymer so that there is an enhanced amount of mobile ionic groups. It is known that increasing the amount of initiation results in decreased average polymer chain length, increased gel turbidity and decreased elasticity.

The polymerisation reaction of acrylamides or the like is a vinyl addition polymerisation reaction, initiated by the free radical initiator just described. The persulfate free radicals convert an acrylamide or acrylamide derivative to free radicals, which in turn react with inactivated monomers to begin and propagate the polymerisation chain reaction. The elongated polymer chains are cross-linked by the cross-linking agent (typically N, N-methylene bisacrylamide) which is generally randomly distributed along the polymer chain, resulting in a reticulated polymer with substantially linear chains and links between the chains.

Further examples of such polymerisable systems include at least one monomer which undergoes polymerisation via a condensation reaction. Examples of such condensation monomer systems are those including at least one polymerisable amino acid (such as glutamic acid or the like); alternatively, at least one hydroxy functional fatty acid can be used, especially such an acid which can polymerise to a lipid-like polymer hydrogel. Such polymers derived from aminoacids or hydroxy functional fatty acids can be substantially biocompatible, which means that the power source according to the invention may be used in vivo.

According to a second aspect of the invention, there is provided an electrical power source which comprises at least one cell each having at least two primary opposed electrodes, and at least two opposed secondary electrodes, the cell being substantially filled by a viscous hydrogel having fixed ions oriented substantially transverse to the primary electrodes, the gel containing mobile ions capable of migration through said hydrogel.

The power source preferably includes a plurality of such cells, in which each cell has a primary electrode thereof connected to an adjacent primary electrode of an adjacent one of the aforementioned cells.

It is further preferred that the power source includes a plurality of such cells, in which each cell has a secondary electrode thereof connected to an adjacent secondary electrode of an adjacent one of the aforementioned cells.

The results obtained using a power source according to the invention suggest that during polymerisation in the presence of an applied electric (or magnetic) field, the fixed ionic groups which are electrically orientated, are drawn from their"normal"alignment/molecular structure, towards the primary electrodes of opposite polarity. While this process continues, the polymerisation system increases in viscosity to form a viscous hydrogel, thus restricting the movement of the fixed ionic groups provided by or associated with the polymerisable monomer.

These results indicate that, following polymerisation in the presence of an applied electric or magnetic field, there is an electric potential between longitudinally spaced locations within the resulting viscous hydrogel.

The longitudinally spaced locations may be the primary electrodes. If the fixed ionic groups are drawn into a "virtual re-alignment", then the polar groups in the polymer may form magnetic dipole moments also drawn into a "virtual re-alignment". Thus when the primary electrodes are connected to one another, mobile ions will pass through the hydrogel as a result of the electric potential difference or magnetic field; at the same time the mobile ions pass through the re-structured magnetic dipole moments causing a transverse voltage to be produced at right angles thereto.

In a hydrogel, the fixed ionic groups will attract around themselves mobile ions of opposite electrical polarity, forming an electrical double layer. The symmetry of this double layer can be expected to be distorted in the presence of an applied electrical field. The distortion of the electrical layer and the creation of Maxwell-Wagner interfacial charges lend to the ions the properties of an electric dipole moment. The magnitudes of these induced dipoles are very large in comparison to those usually associated with molecular species because, although the magnitude of the charges involved may not be large, the distance between their oppositely charged poles is large, so that the induced dipole moment arising from the two induced charges + and-located at r-and r+ respectively, is given by- m = (a +) r,- (a-) r = aq. r where r is the particle radius. Even when the separated charge q is equivalent to just one electronic charge, then for a cell of diameter 5 microns the dipole moment will be of value around 2.5 x 105 debye units (cf.. 1.84 debye for a water molecule).

The relative proportions of the various ingredients of the polymerisable system are selected so that the resulting aqueous medium is in the form of a viscous hydrogel (or hydrated gel), which substantially fills the well up to the cover.

When the polymerisation process is carried out in the presence of a direct current electric field, the electric and magnetic dipole moments of the resulting polymer forming the hydrogel are drawn towards the appropriate primary electrode, with the polar groups oriented substantially transverse to the respective primary electrodes. The polar groups are"stretched"from their equilibrium positions and held in fixed positions by the viscosity of the viscous hydrogel.

The electrodes in this case (that is, when polymerisation is in the presence of a direct current electric field) are typically of a suitable conductive metal of, for example, platinum, aluminium, tungsten or the like.

When the field applied during polymerisation is a magnetic field, the primary electrodes are preferably in the form of electromagnets each having a core surrounded by conductive windings.

The resulting orientation of the polar groups following polymerisation in the method according to the invention is illustrated schematically in Figure 1 of the accompanying drawings, compared to the more random orientation which arises if polymerisation is carried out in the absence cf an applied field (see for example, the orientation illustrated schematically in Figure 2).

In the accompanying drawings, Figure 1 is a schematic sectional view showing the arrangement of polymer chains obtained in a power source according to the invention, whereas Figure 2 is a schematic sectional view showing the arrangement of a similar system in which polymerisation has taken place in the absence of an applied field between primary electrodes.

In more detail, Figure 1 shows a cell 1 having a pair of opposed primary electrodes 2,3 and, substantially transverse thereto, a pair of secondary electrodes 4,5.

Extending between the primary electrodes are a plurality of polymer chains 6 in which the polymer chains having backbones tend towards a linear or stretched orientation, with reticulated bridges extending between the backbones.

The polymer backbones have side chains 7 including fixed ionic groups, the side chains being aligned substantially transverse to the. spacing between primary electrodes, and substantially parallel to the spacing between the secondary electrodes. In Figure 2, in which like elements are denoted by like reference numerals, it will be seen that the polymer backbones are somewhat more random in orientation and the side chains including the fixed polar groups are also somewhat more random in orientation.

As a result, in the power source according to the invention, electric dipole moments present on the polymer chain are drawn towards primary electrodes of opposite polarity and the fixed ionic groups on the polymer chain will be oriented in a"spring loaded"fashion. There may thus be a substantially constant potential difference across the gel, and when the secondary electrodes are electrically connected to one another so as to complete a circuit, mobile ions (such as hydroxyl ions) may start to migrate towards the secondary electrode of opposite polarity and at the same time pass through aligned magnetic dipole moments formed by the fixed ionic (polar) groups.

This process is similar to the Hall effect in macro physics and the resulting transverse voltage can be associated with a voltage or current output drawn from the secondary electrodes.

From investigations carried out on such systems, it is believed that the conductive mechanism is not governed by co-operative motion of the polymer chains but by the mobile ions or charge carriers (typically hydroxyl ions) moving through water-filled micro pores of the hydrogel. If a direct current electric field of uniform intensity is applied to a system containing mobile ions suspended in a viscous hydrogel matrix, at least three effects are expected to occur, as follows: 1. the mobile ions will move towards the electrode of opposite polarity; 2. an electrical double layer surrounding the mobile ions will be distorted; and 3. electrical charges are likely to be induced between moving mobile ions.

It is particularly preferred that the wells employed in the polymerisation system according to the invention are sealed against ingress of air both during and after polymerisation. The wells are preferably provided with an air-tight cover, after having been filled with the polymerisable system.

It is particularly preferred that a series of power sources obtained according to the invention can be connected in order with the primary anode of a first well being connected to the primary cathode of a second well, the primary anode of the second well connected to the primary cathode of a third well, and so on. An example of such an arrangement is illustrated in Figure 3 of the accompanying drawing, which shows a series of nine wells 1,2,3,4,5,6,7,8,9 (in order of connection of the primary electrodes, as will be described below). Well 1 has a pair of opposed primary electrodes la and lb ; well 2 has a pair of opposed primary electrodes 2a, 2b, well 3 has a pair of opposed primary electrodes 3a, 3b, etc.

Well 1 furthermore has a pair of opposed secondary electrodes lc, ld, well 2 has a pair of opposed secondary electrodes 2c, 2d and well 3 has a pair of opposed secondary electrodes 3c, 3d, etc.

The present invention will now be illustrated in more detail in the following example, given by way of illustration only, in which a power source is obtained as follows: A unitary moulded body of plastics material as shown in Figure 3 of the accompanying drawings had nine wells each of dimensions approx. lcm2 and a depth of 0.75cm. All nine wells were filled with a polymerisable mixture containing acrylamide, methylene bisacrylamide, ammonium persulfate, TEMED and distilled water. The wells were all sealed by a hermetically sealed lid or cover and then a 30 volt direct current applied to primary electrode la of well 1 and opposed primary electrode 9b of well 9. Polymerisation was allowed to proceed in each wel--until a viscous hydrogel was produced.

Finally, a circuit was completed between secondary electrode lc of well 1 and opposed secondary electrode 9d of well 9. The resultant circuit provided a long lasting output as shown in Figures 4a, 4b, 4c and 4^ of the accompanying drawings.

In Figures 4a and 4c, the vertical axis is the voltage (the applied voltage between the primary electrodes in Figure 4a and the transverse voltage between the secondary electrodes in Figure 4c).

In Figures 4b and 4d, the vertical axis is the current in milliamps (the applied current in Figure 4b and the transverse current between the secondary electrodes in Figure 4d).

The horizontal axis in all Figures have units of time (seconds).