Cohesive nonsticky electrically conductive gel composition

A cohesive nonsticky electrically conductive gel is disclosed, for facilitating low resistance contact between a metal electrode and a biological body. The gel comprises an aqueous solution of up to saturated concentrations of ionized salts as the conducting agent, a natural gum capable of crosslinking, and a crosslinking material which provides the electrically conductive gel with sufficient internal strength to remain cohesive without reinforcement. The gel has good electrical characteristics and improved physical properties which prevent the gel from leaving a messy residue on the skin of the patient or on the electrode.

BACKGROUND OF THE INVENTION 
The present invention relates generally to electrically conductive gels 
which are used to transmit an electrical signal between the human skin and 
an electrode attached to an electrical recording or stimulating device. 
Frequently in the practice of medicine it is desirable to make electrical 
contact with the body. Such contact may be for the purpose of measuring 
electrical signals, as in the making of electrocardiograms or 
electroencephalograms, or applying electrical impulses to the body during 
electrotherapy. 
The skin is a difficult structure with which to make reliable, low 
resistance, electrical contact. Accordingly, it has become customary in 
the art to utilize a conductive medium between the electrode and the skin 
to enhance conductivity. This medium normally takes the form of a 
conductive paste or gel which makes intimate contact with the skin, by 
conforming to the contours of the skin, and fills the gaps between the 
skin and the electrode, thus providing a more reliable path for the 
electrical current than is afforded by dry surface contact between 
electrode and skin. These gels or pastes are normally made of a thickened 
aqueous mixture containing a conductive salt, such as sodium chloride. 
Conventional thickening agents typically include polymers, such as 
polyvinylalcohol (commonly referred to as PVA), polyethylene glycol or 
polypropylene glycol; glycerol and glycerol derivatives, such as glycerol 
monostearate; and a number of naturally occurring gummy materials, such as 
gum tragacanth, sodium alginate, locust bean gum and guar gum. A number of 
synthetic gummy materials and thickeners have also been used, including 
carboxymethyl cellulose, and proprietary materials such as Ganatrez 
materials sold by General Aniline and Film Corporation and Carbopols sold 
by the B. F. Goodrich Co. 
Examples of the gels or pastes of the prior art can be found in U.S. Pat. 
Nos. 4,016,869; 3,998,215; 3,989,050; 3,658,726; and 3,265,638. These gels 
and creams are comprised of a thickened aqueous mixture and a salt or 
polarizing substance and do a reasonably effective job of making 
electrical contact with the skin. In particular, they make possible a 
contact which is largely free of voids and areas of poor or intermittent 
contact, which, when present, result in the generation of spurious 
electrical signals. Such spurious signals interfere with the collection of 
desired electrical data. However, all of these gels have one major 
disadvantage. They are sticky, messy materials which are unpleasant to use 
and are hard to remove from surfaces they have contacted. This problem has 
been addressed in the art by reinforcing the gelatinous or creamy 
conductive materials with porous or fibrous substances, which help to 
contain the gel or cream in a cohesive matrix, see U.S. Pat. No. 
3,998,215. These structures, often referred to as gel pads, function well 
in regard to making good electrical contact with skin. However, the 
addition of nonconductive structural members within the conductive gel 
inevitably alters the resistance of a gel pad relative to that of the pure 
gel. 
Germam Offenlegangschrift No. 27 27 396 discloses a viscoelastic conductive 
gel comprising a high molecular weight polysaccharide and a polyol, which 
is said to leave behind no residue on the skin. The gels disclosed therein 
are not crosslinked, have a low water content, are capable of carrying 
little salt and require the use of high molecular weight (at least about 
10.sup.6) polysaccharides in order to provide the necessary cohesivity to 
be removed without leaving a residue. The low water content of these gels 
and their consequent inability to tolerate high salt levels limits their 
conductivity and sensitivity to electrical stimuli. 
The gels of the present invention are an improvement over prior art gels. 
They maintain themselves as a cohesive mass without the need for 
mechanical reinforcement. They do not leave a residue on the skin or the 
electrode. Furthermore, they are capable of tolerating high concentrations 
of salt without breakdown of the gel. The gels of the present invention 
are less expensive to produce than the gels of the prior art since they 
can contain relatively less thickener and more water while still 
maintaining sufficient cohesive strength. 
SUMMARY OF THE INVENTION 
The present invention provides an electrically conductive gel for use in 
establishing a low resistance contact between an electrode and a 
biological body, comprising an aqueous solution of a natural gum capable 
of crosslinking and a crosslinking agent. The gum and crosslinking agent 
are present in quantities sufficient to impart a gel-like body to the 
material and to provide the electrically conductive gel with sufficient 
internal strength to remain cohesive without reinforcement. The gel 
material is capable of containing up to saturated concentrations of 
ionized salt without breakdown of the crosslinked gel. The gel material is 
nonsticky in character. 
The gel of the instant invention provides a conductive, conformable 
interface between the skin and the electrodes placed thereon thus 
preventing electrical noise interference, and additionally is easy to 
apply, removable without leaving a residue, and has sufficient strength of 
itself to perform well without reinforcement. 
Although approximately 70% water, the gel stays together in a cohesive mass 
rather than spreading and sticking to surfaces with which it comes in 
contact. In this connection "cohesive" should be interpreted to mean that 
the gel has more adhesion to itself than to the surface of the skin and, 
thus, is capable of maintaining internal integrity and lifting from the 
skin without leaving a residue. 
The instant invention provides a gel which conducts small electrical 
signals faithfully and which produces no artifacts of its own to degrade 
the signal. 
The gel is physically stable over a wide temperature range, i.e., its flow 
and cohesive properties are essentially the same over the range of 
0.degree. to 60.degree. C. 
The gel of the present invention is resistant to drying out. 
The gel can be used on the skin routinely with a minimum of irritation to 
the skin. 
In addition, the gels of the present invention are stable in the presence 
of any practical salt concentration. Thus, even in the presence of 
saturated sodium chloride the crosslinked gels of the present invention 
will not break down. This feature is in contrast to crosslinked gels based 
on polyvinyl chloride which will break down in the presence of salt 
concentrations much lower than saturation, i.e., 10 percent NaCl higher 
than about 5 percent. Furthermore, the gel is not adversely affected by 
exudates from the skin, such as perspiration. 
The gels of the present invention can be used as a conductive medium on a 
patient's skin before emplacing an electrode or in a pre-assembled 
electrode. An example of the former use is in emergency situations where a 
patient is suffering from cardiac distress. Dabs of gel are dispensed onto 
the patient's skin in a standard pattern over the heart area. Electrodes 
are attached to these portions and are connected to an electrocardiograph, 
the read-out of which, commonly called an E.C.G., provides an indication 
of the patient's heart condition. For long-term monitoring of 
heart-function it is preferred to use the gel in a pre-assembled 
electrode, referred to as a "monitoring electrode". Such an electrode 
comprises an electrode plate having on one surface thereof means for 
electrical connection to an electro-medical apparatus and on the opposite, 
body contacting surface thereof, the electrically conductive gel material 
of the present invention. Descriptions of pre-assembled electrodes are 
contained in assignee's copending patent applications, U.S. Ser. Nos. 
940,735 and 940,734, both filed on Sept. 8, 1978 and incorporated herein 
by reference. In both uses the gel is applied and electrical contact 
achieved with light finger pressure. After use the gel may simply be 
lifted off the skin in a cohesive mass without leaving a sticky residue. 
Although the gel of this invention is particularly useful as a conductive 
medium between the skin and a biopotential monitoring electrode suitable 
for detecting the very small electrical signals, such as are 
characteristic of E.C.G. measurements, it is not limited to this use. For 
example, the gel can be used as the conductive medium between 
defibrillation electrodes and the skin of a patient whose heart is in 
fibrillation. In such a case high voltages are required in order to 
electrically shock the heart into beating. A major advantage of the new 
gel in this use is that it does not smear or flow rapidly over a surface, 
thus avoiding the creation of a potentially dangerous conductive path; 
possibly over a patient's chest. An added advantage of the gel of the 
present invention is the greatly reduced chore of cleanup. Since the 
electrodes used in defibrillation are large, a substantial proportion of 
the patient's chest can become covered with conductive medium. The 
cohesive, non-sticky gel of the present invention greatly eases clean-up 
of the patient. 
Another use of the present invention is as an electroconductive medium for 
an electrosurgical ground plate. Still another use of the present gel is 
as the conductive medium between the skin and electrodes of the type used 
for transcutaneous nerve stimulation or for pain relief. These electrodes 
are often in the form of metal plates or foils. 
It should be pointed out that while the gel can be used advantageously with 
electrosurgical grounding plates or with transcutaneous nerve stimultion 
electrodes, as described above, the preferred embodiment of the gel has 
limitations in conditions where it is under pressure. The compositions 
have the ability to cold flow; that is, when placed in a vessel the gum 
will eventually acquire the shape of the inside of the vessel. By this 
means, the ability of the gel to conform accurately to the contours of, 
for example, the skin and the undersurface of an electrode, is assured. In 
practice, a momentary light finger pressure is all that is required to 
emplace an electrode properly on the skin. However, due to its ability to 
cold flow, the gel will spread slowly under pressure, and if squeezed for 
a long time, such as when placed under a supine patient undergoing lengthy 
surgery, it could be squeezed out beyond the immediate area of the 
electrode plate. Under these conditions, a restraining means can be used 
to keep the gel in place. A porous fibrous material, such as a pouch of 
inert porous woven or nonwoven fabric placed around the gel can be used as 
a restraining means. An open-cell foam, such as one of the polyurethane 
foams, impregnated with the gel may also be used. 
DETAILED DESCRIPTION 
The present invention provides an electrolyte gel based on a crosslinked 
natural gum as the thickening agent. The preferred gums are guar gum and 
locust bean gum. Structurally, the useful natural gums are high polymeric 
saccharides comprised of hexose, pentose or uronic acid groups linked 
together. 
One feature of the natural gums is their ready availability and low cost. A 
feature of guar gum is that it can be obtained in a rather pure state 
without extensive processing. Guar gum in its natural state is relatively 
pure, having very few impurities such as sulphur (sometimes found in agar) 
or extraneous ions (as found in many of the less pure gums). 
A useful practical feature of guar gum gels is that they can be produced at 
room temperature or at only slightly raised temperature due to the fact 
that guar gum powder mixes well with room temperature water unlike 
synthetic gels such as polyvinyl alcohol which requires heating and more 
complicated production techniques. In addition, the natural pH of guar gum 
gels of this invention is approximately 7-8.5, which is an excellent pH 
range for a composition to be used against the skin since it is close to 
the physiological pH. Gels of the prior art have been neutralized or 
buffered in order to achieve an acceptable pH. 
Natural gums are polysaccharides obtained from natural substances. For 
example, guar gum is a polysaccharide obtained from the seeds of the guar 
plant. The structure of guar gum, as illustrated below, is that of a chain 
mannose sacharide polymer with repeating single-unit galactose branches, 
referred to as galactomannan. 
##STR1## 
Guar gum is available in anionic, cationic and nonionic forms. The nonionic 
type has been found most suitable for use with Ag/AgCl electrodes and is 
preferred for use with sensitive biomonitoring electrodes. Applicant has 
surprisingly found that gels made from a hydroxy-propylated nonionic guar 
gum, sold by the Stein Hall Co. under the trademark JAGUAR.RTM. HP-11, are 
stable to concentrations of chloride ion greater than 10 percent by 
weight. Thus, this guar gum gel can be successfully utilized where the 
transmission of high currents is desired (i.e., high salt concentrations 
are required) without breakdown of the gel's cohesive structure. However, 
in applications where the electrodes are to contact the skin for periods 
longer than an hour, lower concentrations, 0.1-5 percent by weight, of 
chloride ion are preferred. The lower concentrations of chloride are also 
preferred for electrodes which have been gelled and stored a long time 
prior to use in order to avoid corrosion effects on other parts of the 
electrode. 
Both anionic and cationic guars are also useful as conductive gels. Anionic 
guar, sold by the Stein Hall Co. under the trademark JAGUAR.RTM. CMHP, and 
cationic guar, sold by the Stein Hall Co. under the trademark JAGUAR.RTM. 
C-13, have been successfully tested. Additionally, even a food-grade guar 
has been used successfully. Gels made from these gums are of different 
viscosities and achieve peak viscosity at different times than do gels 
made from nonionic guar gum. 
Mixtures of crosslinkable natural gums with other thickeners are also 
within the scope of the present invention. For example, the addition of 
polyvinyl alcohol (PVA) to guar gum increases the cohesive strength of the 
final gel, and decreases its cold flow. This formulation is not 
particularly advantageous for biomonitoring electrodes, but can be 
valuable in electrodes where the gel is under high compressive loads, such 
as in electrosurgery or in transcutaneous nerve stimulation. Other 
thickeners which can be mixed with the crosslinkable natural gums include 
hydroxyethyl cellulose, and hydroxypropyl methyl cellulose. Examples of 
other natural gums which can be mixed with the crosslinkable natural gums 
of the present invention include gum Arabic, sodium alginate and gum 
tragacanth. 
The gels of the present invention have increased internal cohesiveness and 
are able to be easily removed from surfaces with which they come in 
contact due to their crosslinked nature. The preferred crosslinking agent 
is borate ion, supplied by potassium tetraborate or sodium tetraborate. 
Borate ion reacts effectively with the preferred gums, guar gum and locust 
bean gum, to form stable gels. In addition compositions crosslinked with 
borate are acceptable for contact with human skin. 
The exact nature of the crosslinking of guar gum with borate ion is not 
well understood. A degree of ester formation between the borate anions and 
the hydroxyl groups of the gums is possible. The formation of coordinate 
bonds would also account for the observed crosslinking effect. It is noted 
that polysaccharides with cis-hydroxyl groups on adjacent chains, such as 
guar gum and locust bean gum, are those most usefully crosslinked by 
borate ions for purposes of this invention. That is, gels made with 
polysaccharides having cis-hydroxyl groups exhibit the greatest degree of 
crosslinking (e.g., the stiffest gel is produced) for given concentrations 
of gum and borate. It is possible that borate ion reacts with 
polysaccharides containing cis-hydroxyl groups to form bridges between 
adjacent cis-polyhydroxy moieties on different polymeric molecules. 
Other crosslinking agents useful in the gels of the present invention 
include salts, such as ferric chloride, calcium chloride and the acetates 
of the multivalent cations of lead, chromium or nickel. Those skilled in 
the art will recognize that by careful manipulation of reaction 
conditions, e.g., temperature, pH, agitation, time of reaction, etc., a 
degree of crosslinking can be achieved in the gel without the use of these 
crosslinking agents. Such crosslinking can be detected by viscosity 
changes or by gel formations. However, the difficulty in preparing a 
stable medically-acceptable gel makes the above means of crosslinking less 
desirable than the borate-guar system. 
The preferred embodiment of the present invention includes within the 
crosslinked gum, any salt suitable to act as a conductor for the passage 
of electric current from an electrode to the body of a patient. However, 
crosslinked gums containing no salt are also contemplated since the gels 
of the present invention are aqueous in major portions and can conduct a 
current when subjected to high voltages. The preferred salts are 
chlorides, particularly those of sodium or potassium, since these are the 
most compatible with the normal electrolytes within the body. The 
chlorides are particularly preferred for use with the very sensitive 
Ag/AgCl (Silver/Silver Chloride) electrodes, as they take part in the cell 
reaction and contribute to the proper functioning of the electrode. As 
previously mentioned the Ag/AgCl electrodes are particularly well suited 
for measuring minute electrical bio-events. 
The electrolyte concentration is important as it affects both current 
carrying capacity and skin irritation. For monitoring purposes, where 
electrodes may be worn for days at a time, it is desirable to keep the 
salt concentration below about 3%. Higher salt concentrations become 
irritating to the skin when in contact for prolonged periods and may cause 
serious lesions in the most severe cases. 
For short-term use as in cardiac stress testing, electrotherapy or 
electrosurgery, where the total contact time may be less than one hour, 
much higher salt concentrations can be used. The low electrical resistance 
necessary for the above-mentioned uses can only be exhibited by gels with 
high concentrations of electrolyte. A surprising feature of the 
crosslinked gels of the present invention are their stability even in the 
presence of saturated sodium chloride, approximately 25 percent by weight. 
Thus, the present invention provides a gel which is stable in the presence 
of essentially any salt concentration desired. 
Electrode storage time is another factor in the determination of 
electrolyte type and concentration. Lower salt concentrations are 
preferred when electrodes are to be stored a long time between manufacture 
and use. Salt solutions of sodium chloride and potassium chloride are 
corrosive to ferrous metals, with the result that gels high in 
concentrations of these salts may corrode the electrodes when in contact 
with the electrodes over a sufficiently long period of time. Where storage 
periods are long and higher salt concentrations are desired, salts less 
corrosive than sodium chloride or potassium chloride, such as sodium 
citrate, should be used. 
The choice of electrolyte is also affected by electrode composition. Where 
electrodes made of aluminum, stainless steel or German silver (a 
silver-white alloy of copper, zinc and nickel) are employed for 
biomonitoring purposes spurious signals or electrical noise are commonly 
experienced. Such signals are thought to be generated by chemical 
reactions taking place between the electrode and corrosive conductive 
salts, such as sodium chloride. Potassium citrate can be substituted for 
more corrosive salts, in order to reduce electrical noise. 
Another aspect of the present invention may include the presence of 
humectants, plasticizers, and wetting agents in the crosslinked gel. 
Humectants increase the ability of the gel to resist drying out when 
exposed to the atmosphere or to conditions of low humidity. Plasticizers 
add smoothness and increased pliability to the gel. Wetting agents permit 
the gel powder to disperse in water in a homogeneous and lump-free manner. 
1,3-Butylene glycol, tetrahydrofurfuryl alcohol and dipropylene glycol are 
known plasticizers and humectants. Diethylene glycol and glycerol have 
been commonly utilized as humectants. However, glycerol competes with guar 
gum for borate, and can interfere with proper gel formation by inhibiting 
crosslinking if present in sufficient quantity. Propylene glycol can 
function in the gels of the present invention as a humectant, a 
plasticizer and a wetting agent for guar gum powder during manufacture. 
The gels of the present invention may also contain preservatives to prevent 
bacterial growth during storage and use. The parabens, e.g., methyl and 
propyl-p-hydroxy-benzoates, are well-accepted preservatives for use in 
medicinal preparations. 
Preferred components and concentrations for the gels of the present 
invention follow. All percentages are given in percents by weight. 
______________________________________ 
Component Percent by Weight 
______________________________________ 
Guar gum (sold by the Stein Hall 
1 to 5% 
Co. under the trademark 
JAGUAR .RTM. HP-11) 
NaCL 0.8 to 25% 
Potassium Tetraborate 
0.05 to 3.0% 
(K.sub.2 B.sub.4 O.sub.7.5H.sub.2 O) 
Propylene glycol 5 to 50% 
Propyl-p-hydroxy benzoate 
0.01 to 0.05% 
(propylparaben) 
Methyl-p-hydroxybenzoate 
0.01 to 0.9% 
(methylparaben) 
Water to 100% 
______________________________________ 
In general altering the proportions of the components has the following 
effects: 
Raising the amount of guar gum increases the viscosity of the gel, and 
conversely lowering the amount of guar gum decreases the viscosity of the 
gel. 
Raising the chloride ion concentration increases the electrical 
conductivity of the gel and decreases the gel-skin impedance, and 
conversely lowering the chloride ion concentration decreases the 
electrical conductivity of the gel and increases gel-skin impedance. 
Raising the borate ion concentration increases the degree of crosslinking 
and the stiffness of the gel, and conversely lowering the borate ion 
concentration decreases the degree of crosslinking and thus the stiffness 
of the gel. 
Raising the amount of propylene glycol, a humectant, increases the ability 
of the gel to resist drying out. 
Raising the concentration of the parabens increases the bacteriostatic 
ability of the gel. 
An especially preferred composition for use in the practice of the present 
invention, particularly with a biomonitoring electrode, is the following: 
______________________________________ 
Component Percent by weight 
______________________________________ 
Guar gum (HP-11, Stein Hall & Co.) 
2.0 
NaCl 2.4 
Propylene glycol 15.0 
Methyl-p-hydroxy benzoate 
0.1 
Propyl-p-hydroxy benzoate 
0.02 
Potassium Tetraborate 0.57 
Water to 100 
______________________________________ 
This composition has excellent electrical properties in addition to a 
useful combination of physical properties. The gel makes good contact with 
both skin and electrode, is stable with regard to moisture loss (a major 
factor affecting shelf-life and useful life on patient), and possesses 
excellent cohesive strength.

The following examples further illustrate the present invention. In these 
Examples, all parts and percents are by weight, unless otherwise 
indicated. 
EXAMPLE 1 
Approximately 300 ml of distilled water is heated in a 600 ml beaker to a 
temperature of 60.degree.-75.degree. C. and 9.9 gm of sodium chloride is 
added to the heated water with stirring until dissolved. In a separate 
vessel, 0.16 gm of propyl-p-hydroxy benzoate and 0.8 gm of 
methyl-p-hydroxy benzoate are mixed well with 80.0 gm of propylene glycol 
until dissolved. To this mixture 6.4 gm of guar gum powder (commercially 
available as JAGUAR.RTM. HP-11 from the Stein Hall Co.) is added slowly 
with constant stirring until homogeneously dispersed. 
The dispersion of guar gum in paraben/propylene glycol solution is added 
slowly to the aqueous sodium chloride solution with vigorous stirring, 
e.g., with a high shear mixer (Homo-mixer commercially available from 
Gifford Wood, Inc., Hudson, N.Y.). Vigorous mixing is continued and the 
temperature is maintained at about 60.degree.-75.degree. C. until the 
mixture is smooth and the guar gum is completely dissolved (about 10-20 
minutes). The resultant mixture is a homogenous, viscous solution. The 
heat source is removed and vigorous mixing is continued while 20 ml of a 
10% w/v solution of potassium tetraborate is slowly added. The stirring is 
discontinued and the mixture is allowed to cool to room temperature. 
EXAMPLES 2-7 
Following the procedure of Example 1 gels were prepared having the 
following compositions: 
______________________________________ 
Amount 
Amount potassium 
Amount 
Example Gum tetraborate 
NaCl/KCL* 
Number Gum (% by wt.) 
(% by wt.) 
(% by wt.) 
______________________________________ 
2 Guar 1.6 0.5 2.4 
(JAGUAR .RTM. 
CMHP) 
3 Guar 1.6 0.5 2.4 
(JAGUAR .RTM. 
C-13) 
4 Locust Bean 
1.6 0.375 2.4 
5 GUAR 1.6 0.583 2.4 
(JAGUAR .RTM. 
HP-11) 
6 GUAR 1.6 0.5 30.0* 
(JAGUAR .RTM. 
C-13) 
7 GUAR 1.6 0.5 30.0* 
(JAGUAR 
HP-11) 
______________________________________ 
Examples 6 and 7 illustrate that a gel can be made according to the present 
invention which can accomodate high salt concentrations. 
EXAMPLE 8 
Approximately 300 ml of distilled water is heated in a 600 ml beaker to a 
temperature of 60.degree.-75.degree. C. 9.9 gm of sodium chloride, 0.16 gm 
of propyl-p-hydroxy benzoate and 6.4 gm of guar gum powder (commercially 
available as JAGUAR.RTM. A2S from the Stein Hall Co.) are added to the 
water and the mixture is stirred vigorously, e.g., with a Homo-mixer, 
until a homogeneous mixture is obtained (15-20 minutes). The heat source 
is removed and, using moderate stirring (e.g., with a propeller-type 
stirrer), a 10% w/v solution of potassium tetraborate, and propylene 
glycol are slowly added in alternate aliquots over a period of about 5-10 
minutes as follows: 
1. 2-5 ml 10% w/v solution of potassium tetraborate (until gelation 
starts). 
2. 10 gms propylene glycol. 
Thereafter 2 ml aliquots of the 10% potassium tetraborate solution are 
alternated with 10 gm aliquots of propylene glycol until a total of 20 ml 
of the potassium tetraborate solution and 80 gms of propylene glycol have 
been added. Upon cooling, a gel of this invention is obtained. 
EXAMPLE 9 
Following the procedure of Example 8 a gel was prepared having the 
following composition: 
______________________________________ 
Amount 
potassium 
Amount 
Example Amount Gum tetraborate 
NaCl 
Number Gum (% by wt.) (% by wt.) 
(% by wt.) 
______________________________________ 
9 GUAR 1.6 0.25 2.4 
(JAGUAR .RTM. 
A-40-(F)) 
______________________________________ 
The following table (Table I) is a list of the physical properties of the 
gels of Examples (1-9). 
TABLE I 
__________________________________________________________________________ 
Electrical 
Amount 
Amount 
pH of Resistivity Viscosity (Poise) 
Example Gum potassium 
crosslinked 
ohm-cm 
NaCl/KCL* 
at Shear Rate 
Number 
Gum (% by wt.) 
tetraborate 
gel at 10 KHz 
(% by wt.) 
0.025/sec. 
0.1/sec. 
__________________________________________________________________________ 
1 Guar (HP-11) 
1.6 0.50 7.66 42.8 2.4 11.8 .times. 10.sup.3 
5.2 .times. 10.sup.3 
2 Guar (CMHP) 
1.6 0.50 7.70 44.0 2.4 12.0 .times. 10.sup.3 
5.5 .times. 10.sup.3 
3 Guar (C-13) 
1.6 0.50 7.66 43.7 2.4 20.0 .times. 10.sup.3 
9.6 .times. 10.sup.3 
4 Locust Bean 
1.6 0.375 7.65 42.8 2.4 20.0 .times. 10.sup.3 
-- 
5 Guar (HP-11) 
1.6 0.583 7.6 18.4 6.4 9.2 .times. 10.sup.3 
-- 
6 Guar (C-13) 
1.6 0.5 7.6 6.8 30.0* -- -- 
7 Guar (HP-11) 
1.6 0.5 7.8 5.4 30.0* -- -- 
8 Guar (A2S) 
1.6 0.50 7.35 41.5 2.4 2.0 .times. 10.sup.3 
1.7 .times. 10.sup.3 
9 Guar (A40F) 
1.6 0.25 7.60 43.8 2.4 11.7 .times. 10.sup.3 
8.3 .times. 10.sup.3 
__________________________________________________________________________ 
The viscosities of the gels of Examples 6 and 7 were not measured since 
these gels were prepared to show high salt concentration capability. 
Electrical resistivity was measured using a plastic cell of approximately 3 
c.c. volume. The cell consisted of two circular platinized platinum 
electrodes approximately 0.7 cm in diameter, which faced each other and 
were approximately 0.8 cm. apart. The cell constant (K cell) was 
calculated according to known experimental technique (see American Society 
of Testing Materials Standards, report Number D202-77, part 39, section 
48, pp. 73, 1978 Annual) and found to be equal to 1.39 at 10 KHz 
(sinusoidal signal). 
Resistivity measurements were taken at 10 KHz (sinusoidal signal) using a 
Hewlett Packard Model 4800 A vector impedance meter. A 10 KHz frequency 
was chosen to minimize electrode polarization effects. The cell was filled 
with the appropriate gel and its measured resistance (Rm) was obtained. 
Resistivity (.rho.) is given in ohm-cm by the equation 
EQU (.rho.)=Rm.times.Kcell=Rm.times.1.39 at 10 KHz 
All viscosity measurements were made using a mechanical spectrometer (Model 
RMS-7200 made by Rheometrics, Inc.) and according to the instrument 
instruction manual, using a 72 mm diameter cone and plate, a 0.04 radian 
angle and a 0.05 mm gap. All measurements were made at room temperature 
(18 25.degree. C.). 
EXAMPLE 10 
A 1.6% by weight solution of JAGUAR.RTM. HP-11 in distilled water was 
prepared. To a 40 c.c. sample of the guar gum solution approximately 1 
c.c. of a 10% by weight solution of FeCl.sub.3 in water was added with 
stirring. To this a concentrated solution of potassium hydroxide was added 
dropwise and the pH of the mixture was monitored. When the pH rose to an 
alkaline pH of about 11.2, from a starting pH of about 2.25, a 
crosslinked, cohesive, non-sticky gel was obtained. 
EXAMPLE 11 
A 1.6% by weight solution of JAGUAR.RTM. CMHP in distilled water was 
prepared. To a 20 gm sample of the guar gum solution, 15 drops of a 10% by 
weight solution of chromium acetate was added with stirring. A 
concentrated solution of potassium hydroxide was then added dropwise to 
the mixture with stirring and the pH was monitored. At an alkaline pH of 
above about 9, an excellent crosslinked gel of the present invention was 
obtained. Subsequently 20 drops of a saturated solution of potassium 
chloride was mixed with the gel. The gel remained crosslinked, cohesive 
and non-sticky.