Synthetic Muscle Tissue:An Observation of Mechanics, Materials and BiocompatibilityBy: Xavier Costelazo, Daniel Genthe, Brandon Johnson, Tyler Torgerson, and Justin Vincik IntroductionAdvancements in synthetic muscle tissues have become a fast growing biomaterial field. These advancements have lasting impact on the world at large. Whether it is a veteran injured in a war zone or a person with a degenerative disease; the prospect of being able to replace damaged muscle with biologically compatible synthetic options is a great achievement for all. The various materials chosen also have lasting mechanical and biological impact for the host. The weight, strength, ductility, elasticity are all mechanical properties that are important to the effectiveness of the tissue. The materials and construction process will affect these properties. This study is an analysis of the mechanics, materials and biocompatibility of synthetic muscle tissue for the purpose of education in biomaterial engineering.

Biomechanical OperationsTo understand how artificial muscle is to be used we must first understand the functions of organic human muscular tissue. The basis of a functioning replacement is in direct comparison of what nature has already perfected, to an extent. Organic muscle tissue is relatively fragile with it needing constant nourishment and protection. Overworking or lack of exercise can be detrimental to its function. In addition, the structure and composition are vulnerable to injury and damage that may not be repairable rendering it useless. Current technological innovations are providing hopeful substitutions for the fibers of life.

Muscle tissue is a system composed of a collection of smaller strands acting in unison, over many cycles to provide a force to act on our skeletal structure to produce movement and functionality. All muscles, with the exception of smooth muscle found in heart, is composed of muscle fibers. These fibers are strands of smaller units called myofibril. Myofibril are interconnected cylinders within muscle. These cylinders contain hexagonal arrangement of filaments. Thick and thin filaments are the foundation of the mechanics at work in the complete tissue system. Thick filaments are myosin while the thin filaments are actin. The thin actin strands contain binding sites for the myosin heads. The chemicals allowing the function of the muscle are Adenosine Triphospaste (ATP), Adenosine Diphosphate (ADP), and Inorganic Phosphate(P).

To begin the cycle of movement, the brain sends an electrical impulse to trigger a receptor in the myofibril. When the signal is received the receptor releases calcium ions that bind to the tropinin complex allowing it to uncover the binding site for the myosin head. When the head binds to the actin, ADP and P are released causing the head to pull the actin strand past, causing movement. When the pulling motion is completed, ATP is introduced to release the myosin head from the binding site on the actin strand. As the ATP is broken down into ADP and P the myosin head returns to the initial position, ready for another cycle. The cycle of motion within the myofibril produces a 1% contraction. The process is repeated several times in every fiber to produce an overall contraction of 35% to 50% in the overall muscle. https://cyhsanatomy1.wikispaces.com/file/view/image005.jpg/49897235/image005.jpg

The forces present while a muscle contracts are dependent on the desired task or motion. From a stress vs. elongation chart of organic muscle there is a baseline in which to evaluate the performance of a synthetic alternative. This alternative must cope with the demands of the system. Forces required for motion equate to muscle work need. Therefore, with a smaller work load, less muscle power is needed, but the same range of motion must still be attainable. http://www.ibnm.uni-hannover.de/de/forschung/biomechanics/studies-on-the-biomechanical-compatibility-of-hip-joint-endoprostheses/details/

While organic muscle contracts by pulling actin filaments past myosin filaments in a direction parallel to the two, polymers actuate in many different configurations and motions to perform desired tasks. Methods of actuation in addition to contraction, utilize rotational motion and bending. Unlike current methods to produce movement, polymeric actuation can produce a more compact and more efficient alternative with its wide variety of polymer options and unique abilities of each.https://figures.boundless.com/19641/full/figure-38-04-03.jpeMaterialsWhile the previous section examined the mechanical needs that are provided by basic muscle operation and certain mechanical properties that are necessary this shall enumerate several of the most viable and widely studied options for synthetic muscle. This will be done through a comparison of the properties of skeletal muscle to the properties of various engineered materials.Table 1: Properties of mammalian skeletal musclesPropertyTypical valueMaximum value

Strain (%)20>40

Stress (MPa)0.1 (sustainable)0.35

Work density (KJ/m^3)8

Density (kg/m^3)1037

Strain rate (%/s)500

Power to mass (W/kg)50200

Efficiency (%)40

Cycle Life10^9

Modulus (MPa)10-60

(Baughman 2007)A plethora of inorganic and organic materials have been developed during the last 30 years. The artificial muscles based on polymeric materials are the closest ones to natural muscles (Martinez 2014). There are a variety of different polymeric materials that can be incorporated into synthetic muscle tissue. Electronic electroactive polymers and ionic electroactive polymers are the two basic groups of polymeric actuators. These actuators take advantage of the material properties of polymers to contract, expand and bend based on either ionic diffusion or an electrical signal caused by metal electrodes. Combinations of polymers and metals are determined by the types of materials necessary to derive specific chemical and mechanical properties which can provide muscle- like movements in an efficient manner.

Dielectric elastomer actuator are a subset of electroactive polymers. In order to produce displacement of the actuator, electrical energy is converted into mechanical energy because of the electrodes conducting through the elastomer. Strains are 10-100%, which compares favorably with the 20% observed in our skeletal muscle. (Baughman 2007). These strains are caused from the millions of polymeric chains, intertwined throughout the elastomer, ability to straighten when electrically excited. When the current is removed from the electrodes, the polymeric chains relax back to their normal position. The desired affect is for the elastomer to produce high strains, so the materials used must require a low stiffness along with a high dielectric constant to maximize the strain. This is a reason elastomeric materials are best suited for actuators as of now. The basic format for making a dielectric actuator involves an elastomeric film placed between two electrodes with some voltage for actuation. The elastomer employed is often a silicone or acrylic elastomer, sometimes loaded with heavy particles such as TiO2 to increase the dielectric constant. The compliant electrodes can be made of conductive C or Ag pastes, spin-coated conductive rubbers, sprayed graphite particles, or superelastic CNT sheets. The key to large strain is to make both the electrodes and the elastomer highly compliant without sacrificing dielectric strength and conductivity. (Baughman 2007). This process works well with a silicone elastomer because of its good conductivity and can be manufactured with relatively low costs. However, the specific material for electrodes is crucial to the success of dielectric elastomers because different material interactions promote changes in the actuators response. In order to maximize the strain for dielectric elastomer, it must contain a high dielectric constant for the elastomer with electrodes that are very conductive. This must be taken into consideration when picking the materials.(Baughman 2007)

There are three main electrostrictive materials: electrostrictive relaxor ferroelectric polymer, electrostrictive graft elastomer, and liquid crystal elastomers. Of these materials, the ferroelectric polymer is the most viable option for an artificial muscle tissue. However, each of them offer insight into how the different material applications exhibit interesting effects macroscopically. Research in polymer artificial muscles by Tissaphern Mirfakhrai, John Madden, and Ray Baughman describe the best ferroelectric polymer involved for high performance. Ferroelectric polymers, which can strain by an incredible 10%...The best actuator performance to date has been obtained from poly (vinylidene fluoride)-based (PVDF) polymers copolymerized with trifluoroethylene, forming P(VDF-TrFE). The electronegativity of the fluorine makes these polymer backbones highly polar. Field-driven alignment of polar groups produces reversible conformational changes that are used for actuation, as depicted in Fig. 4. The application of a field perpendicular to the chains leads to a transition between the nonpolar (alpha phase) and polar (ferroelectric beta phase) forms. The result is a contraction in the direction of polarization and an expansion perpendicular to it. (Baughman 2007)

An interesting quality of a ferroelectric is the material becomes piezoelectric which causes the ferroelectric polymer to stretch or compress when an electric field or stress is introduced. Because of this, ferroelectric polymers generate a strong contractions that reacts to stress similar to a natural muscle. Also, they provide a high strain rate because of permeant electrical polarization which allows the polymer a high displacement that will sustain from relaxing, which is a challenge with some ionic polymers.

Fig. 4 (Baughman 2007)

Electrostrictive graft elastomers and liquid crystal elastomers take advantage of polar crystalline areas that expand at low applied voltage, but the overall stain is low. Because of this fact, ferroelectric actuators are the best electrostricive actuation because of the incredible strain and performance by poly (vinylidene fluoride)-based (PVDF) polymers when activated.

Ionic actuation occurs from the transfer of ions into a material causing swelling usually involving carbon nanotube bundles or conducting polymers to facilitate the ionic diffusion. Carbon nanotube are being researched in a wide variety of applications because of their high electrical conductivity, good thermal conductivity and massive mechanical strength. Single-walled CNTs (SWNTs) can be thought of as a single layer of graphite (graphene) rolled into a cylinder of nanometer diameter Multiwalled CNTs (MWNTs) are nested SWNTs... Individual SWNTs or very long MWNTs have exceptional mechanical properties. The tensile modulus of SWNTs (640 GPa) approaches that of diamond, while their tensile strength is thought to be 20-40 GPa, about ten times higher than any other type of continuous fiber. (Baughman 2007). However, Carbon nanotube actuators are not fully viable yet for synthetic muscles tissue, but because of it high strain rates a lot of research is going into developing them. Another effective benefit of carbon nanotube is they can be mass produced relatively quickly. Actuation is generally achieved in films or yarns composed of many nanotubes. The porous nature of the films and fibers enables fast ion transport with response times of