Artificial muscle
United States Patent 3882551

A means of totally replacing one member of an antagonistic skeletal muscle pair with an elongated elastic structure comprised of a biologically compatible element in a state of tension opposing the natural member of the muscle pair with the magnitude of the tension force in the range of from 5 to 25 percent of the maximal tension force of a normal muscle identical to the muscle being replaced.

Helmer, Jerry D. (Columbus, OH)
Hughes, Kenneth E. (Columbus, OH)
Application Number:
Publication Date:
Filing Date:
Primary Class:
Other Classes:
International Classes:
A61F2/08; (IPC1-7): A61F1/24
Field of Search:
3/1 128
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US Patent References:
3545008TENDON PROSTHESIS1970-12-08Bader

Primary Examiner:
Frinks, Ronald L.
Attorney, Agent or Firm:
Peterson, Stephen Mase William L. J.
We claim

1. An artificial muscle for complete replacement of a natural muscle comprising:

2. The muscle of claim 1 wherein the means on the extremities of said elastic member are comprised of a woven fabric having a biologically inert covering.

3. The muscle of claim 1 where said elastic polymeric material is medical grade silicone rubber.

4. An artificial muscle comprising:

5. The muscle of claim 4 where said core consists of a woven fabric tube consisting of a polyester terephthalate polymer.

6. The muscle of claim 5 where said inert covering is medical grade silicone rubber.

7. A method of totally replacing a muscle comprising one member of an antagonistic pair of muscles by opposing the intact biological muscle with a tension force in the range of from 5 to 25 percent of the maximal tension force of an identical healthy muscle with said tension generated by the elastic deformation of an elongated member.

8. The method of claim 7 where said antagonistic pair are major skeletal muscles and the tension generated by the elastic deformation of said elongated member is in the range of from 2 to 200 kilograms.

9. The method of claim 7 where said tension generated by the elastic deformation of said elongated member is approximately 20 percent of the maximal tension force of an identical healthy muscle.


This invention is related to the field of the art where members of the body are replaced by artificial means disposed to duplicate the effect of a missing natural portion of the body. The present invention comprises both a method of muscle replacement and a specific embodiment that comprises an artificial muscle.

The loss of muscular control or the physical loss of one muscle in an antagonistic muscle pair normally necessitates the use of external bracing. The use of external bracing is both bothersome to the wearer and is cosmetically unattractive. The replacement of a muscle with an internal elastic member that provides a predetermined tensile force allows muscular control to be exerted by the remaining natural muscle in the antagonistic pair without the necessity for external bracing.

There are no prior art references that teach or suggest that skeletal muscles may be totally replaced by an elastic member so as to restore natural skeletal control with the remaining natural muscle in an antagonistic pair.

U.S. Pat. No. 3,646,615, Ness, discloses a structure disposed to increase the tension exerted by one member of an imbalanced pair of muscles by adding the tension of the elastic member to the tension exerted by the weak but operable natural muscle opposed by the normal muscle of the pair. The muscle pairs contemplated in this reference are the extraocular or oculorotary muscle which impart movements to the eyes and eyelids. This reference does not contemplate total muscle replacement or the use of such an element on major skeletal muscles.


This invention relates to a means of reinstating muscular control to a body member suffering the loss of control or physical loss of one of a pair of antagonistic skeletal muscles. Since normal skeletal muscles provide only tensile forces, one muscle in the pair is replaced by an internal elastic member generating a predetermined static tensile force against the oppositely paired muscle. The magnitude of the tension force in the artificial muscle is determined from the maximal tension force exerted by a healthy muscle identical to the one being replaced. The tension force should be between 5 and 25 percent of the maximal tension that can be induced in a healthy muscle equivalent to the one being replaced. The present invention also contemplates that the elastic artificial muscle may have a predetermined maximum extension so as to prevent overextension by the biological muscle in the pair. One embodiment of such an artificial muscle would consist of a woven fabric core with a covering of a biologically compatible polymer in a state of tension. The magnitude of the state of tension determines the static tension force exerted by the artificial muscle.


FIG. 1 is a schematic representation of an antagonistic muscle pair.

FIG. 2 shows a cross section of one embodiment of an artificial muscle.

FIG. 3 is a graph of Force in kg versus Length in cm of one artificial muscle.

FIG. 4 is a cross-sectional view of the tendon portion of the artificial muscle showing a method of attachment to the natural tendon.


The elongated muscles that provide voluntary movement to various body members do so solely by the generation of a tension force. As a result, for a body member to move reversibly there must be an opposing muscle to return the body member to the position it had before the operation of the first muscle in the pair. A pair of muscles operating on the same body member that would move the member in opposite directions if they exert a tension are called "antagonistic muscles" or "muscles of an antagonistic pair."

FIG. 1 shows an antagonistic muscle pair schematically. The figure represents the lower portion of the rear leg of a sheep with 1 being the posterior tibialis muscle and 2 being the anterior tibialis muscle. The present invention was utilized to replace the anterior muscle in several sheep. Elements 3, 4, 5, and 6 are the tendons that attach the muscles 1 and 2 to the bones comprising the lower leg. Tendons 3 and 5 connect to the tibia bone 20 thereby immobilizing one end of the respective muscle while tendons 4 and 6 connect to 10 comprising the foot bones and impart motion to those bones through the pivoting action of the ankle joint 15. Therefore, when the posterior muscle 1 contracts and exerts a tension on the bone 10 that is not opposed by a tension in muscle 2 sufficient to negate the moment about the joint 15, then the heel 14 will move up and the toe 12 will move down. Conversely, when the muscle 2 contracts and generates a tension force, the toe 12 will move up when the moment about the joint 15 created by the anterior muscle 2 exceeds that generated by the posterior muscle 1. For purposes of definition "moment" is defined as "the mathematical product of the tension force exerted through the tendons of the muscle and the distance from the pivot point of the joint to the point of attachment of the tendon." While this definition is at slight variance with the accepted technical definition of a moment, the difference between a moment calculated by each of the different methods will be slight since the distance between the pivot and the point of application of the tension force is small. Furthermore, the use of the accepted definition of a moment would introduce unnecessary complications since the normal joint structure is not rectilinear and the exact pivot point may not easily be determined. For purposes of this invention it should be noted that it is the relative moments generated by the opposing muscles (whether both are natural or one is artificial) that control skeletal movement, however, since natural tendons are normally used when an artificial muscle is implanted the tension force is the only variable since the distance between the joint pivot point and the point of tendon attachment has not been altered. It should be obvious that if one were not constrained by the use of the natural tendons both the tension force in the artificial muscle and the distance from the pivot point in the joint to the point of tendon attachment could be manipulated to generate predetermined moment that would be compatible with the remaining natural antagonistic muscle.

The present invention operates to restore skeletal control to an antagonistic pair of muscles having one member of the pair totally inoperable by replacing the inoperable muscle with an elongated member that will generate a static tension force to the skeletal system that will generate a moment about some pivot that can be overcome by exertion of a tension by the remaining natural muscle of the antagonistic pair. The tension force may be nearly constant or may be variable depending on the deformation characteristics of the tension producing means. The actual magnitude of the tension force required depends upon the antagonistic muscle pair in question but for major skeletal muscles, the static tension force normally lies in the range of from 2 to 200 kg. To determine the actual tension value required for a specific muscle requires some knowledge of the maximal tension force such a muscle should normally produce. The maximal tension force is defined as that tension (as measured in units of force) that is the maximum value that can be induced in the particular muscle. If the value of the maximal tension force is unknown it can be determined by exciting an equivalent normal muscle electrically by placing a voltage on the motor nerve and increasing the voltage until no appreciable increase in tension is noted. The tension can be measured in any number of ways but particular success has been experienced using a simple spring balance attached to a tendon on the muscle being excited. One having ordinary skill in the art needs no further teaching in inducing or measuring such a tension as it is a standard medical technique.

Once the maximal tension force is determined the artificial muscle should, when implanted as a replacement, impart a tension force in the range of from 5 to 25 percent of the maximal tension force. Particular success has been experienced when the artificial muscle imparts a tension of approximately 20% of the maximal tension force.

One embodiment of the present invention is shown in FIG. 2. The artificial muscle 40 is comprised of a fabric core 32 that has a dual function. At the extremities of the muscle the core 32 is fully extended and comprises an artificial tendon to be attached either to the remnants of the natural tendons or directly to the skeletal system. The artificial tendons 32 are covered with a biologically compatible material 31 to prevent the surrounding tissue from growing into the fabric. Particular success has been experienced using a woven fabric core 32 consisting of Dacron, a trademark of the E. I. du Pont de Nemours Company consisting of a polyester terephthalate polymer, and using medical grade silicone rubber as the sheath 31. Irrespective of the materials used, at the extremities 35 of the muscle the tension force is transmitted by the fabric core 32 to the skeletal system. By contrast in the actual muscle portion 36 of the artificial muscle, the fabric core 32 is unextended and the elastic portion 30 transmits and generates the tension force. The two extremities of the element 40 are attached to the fabric core 32 at the juncture of the tendon portion 35 and the muscle portion 36. An advantage of having the fabric core 32 continuous through the entire member 40 is that the unextended portion of the core 32 can be used to impart a limit to the elastic deformation of the elastic portion 30. Upon the full extension of the fabric core in the muscle portion 36 further skeletal movement will be resisted by the core 32 rather than the elastic member 30. In this manner limits of skeletal movement can be easily imposed without relying on the elastic member 30. It should be noted that the presence of the core 32 in the elastic portion 36 is only a preferred embodiment and the invention is fully operable without the fabric core in that portion of the artificial muscle.

The elastic member 30 can consist of any elastic material having the appropriate deformation characteristics but particular success has been experienced with medical grade silicone rubber. Medical grade silicone rubber has the deformation characteristics required and has the added advantages of being relatively stable and biologically compatible. Furthermore medical grade silicone rubber has demonstrated the ability to promote the growth of a natural tissue sheath that is nonadherent and well suited for tolerating the sliding motion generated by relative movement between the artificial muscle and surrounding tissue.

The deformation characteristics of the elastic material while not known to be critical do have an effect on the performance of the artificial muscle. FIG. 3 illustrates a force V extension relationship for an operable muscle. The region so defines the upper and lower limits of tension exerted in an operable artificial muscle. Preferably the elastic material should be utilized in a reasonably linear portion of its force V extension curve although those skilled in the art may use non-linear deformation characteristics for some special reason. The elastic material must not plastically deform during use so as to appreciably change the tension exerted therefore the elastic material should not exceed its elastic limit during use. The elastic modulus (the slope of the force V extension curve) is not known to be critical except that the modulus must be such that the extension of the muscle would not be sufficient to exert an excessive tension force.

The artificial muscle is normally attached to the natural tendon in the manner shown in FIG. 4. The natural tendon 33 is sutured to the fabric core 32 and the junction is covered with the material 31 disposed to prevent tissue growth into the fabric core.


An artificial muscle of the configuration shown in FIG. 2 was implanted in three sheep so as to replace the tibialis anterior muscle which dorsally flexes the foot. In the first sheep, the distal artificial tendon was attached to the periosteal flap and the proximal artificial tendon was attached to the proximal anterior tibialis tendon. During implantation the rubber sheath tube covering the fabric core was inverted as illustrated in FIG. 4 prior to suturing the Dacron fabric core to the natural tendon. Two FIG. 8 sutures were placed through each tendon junction and subsequently the inverted sheath was pulled down over the fabric core tendon. One week post operatively the artificial distal tendon in the first sheep tore loose from the periosteal flap. The distal tendon site was surgically opened and the artificial tendon reattached to the distal tibialis anterior tendon. The tension force was estimated to be 7 percent of the maximal tension force. A second sheep initially had the artificial tendon sutured to the distal tibialis anterior tendon instead of the periosteal flap as in the first sheep. In the second sheep the artificial muscle was implanted so the tensile muscle force was approximately 5 percent of the maximal tensile force when the foot was dorsally flexed. By the second post-operative week the first and second sheep were able to walk near normally but could not run, after 4 weeks the first and second sheep could stand, walk, and run at a near normal gait. They had relearned to use their posterior tibialis muscle and were able to compensate for the static tension of the artificial muscle. When the sheep were lying at rest, the foot with the artificial muscle would slightly favor the dorsally flexed position. To maintain that position the antagonist muscle tonal force was minimal. The prosthesis of the first sheep was surgically explored 6 weeks after the operation. Both the distal and proximal tendons were secure and the artificial muscle was operative inside a tissue capsule that had formed around the silicone rubber member. The tendon attachment was totally dependent on the original surgical FIG. 8 sutures and the examination revealed that the tendon had not ingrown the Dacron fabric core.