similartophysicalexercise,acupunctureimprovesmuscle function restoration and stimulates muscle regeneration especially in patients with muscle atrophy after chronic diseases.However,thereislimitedsuccessfortheregenerationof largevolumemuscledefectsaftertraumaortumorresection. Furthermore, more work needs to be done to determine the optimal timing and intensity of Acu-LFES as a standard treatmentformuscleatrophy. 2.4. Biological Scaffolds. Biological scaffolds composed of extracellular matrix (ECM) proteins are commonly used in regenerative medicine and in surgical procedures for tissue reconstruction and regeneration. The scaffolds can promote therepairofVMLbyprovidingastructuralandbiochemical framework[60].Forsmalleramountsofmuscleloss,several tissue-derived scaffolds have been tested in animal models and translated into the clinic for surgical application [6]. Xenogeneic extracellular matrix and autologous tissue have beenutilizedtorestorefunctionalmuscleandsimultaneously generate a biological niche for recovery [61]. A multilayered scaffold made of ECM derived from porcine intestinal submucosa has been applied for reconstruction of vastus medialismuscleinpatients[16].Thepatientshowedmarked gains in isokinetic performance 4 months after surgery and new muscle tissue at the implant site was demonstrated by computer tomography. Porcine small intestinal submucosaextracellularmatrixhasalsobeenutilizedforthetreatmentof abdominalmusculoskeletalwalldefects,whereitwassutured atthedefectcornersandsubcuticularlyclosedwithavicrylsuture[61].Also,porcineECMfromurinarybladderhasbeen implantedinanattempttotreatVMLinhumanbeings[60]. Functionalimprovementwithformationofmuscletissuewas observedinthreeofthefivehumanpatientsinthisstudy. However,allograftorxenogeneicscaffoldscanstillinduce adverse immune response after decellularization and there might be potential risk of infectious disease transmission. Therefore, there is a clinical need to develop new strategies thatcanfacilitatesafebiggermuscletissuerepairandregeneration.
3. Developing Technologies for Muscle Tissue Engineering and Regeneration To address remaining clinical problems and explore novel strategies for muscle tissue engineering and regeneration, new technologies have been investigated intensively. While tissue bioengineering approaches aim to construct complex muscle structures in vitro for subsequent implantation and replacement of the missing muscles, tissue regeneration approaches develop tissue-like scaffolds that can be implantedtoenhancenewmuscleformationfromremaining tissueinvivo[62].Bothapproachesmainlyrelyoncombinationsofscaffolds,cells,andmolecularsignalingwithdiffering focus. 3.1.Scaffold-BasedStrategies. Biomaterialscanprovidechemical and physical cues to transplanted cells or host muscle cells to enhance their survival, promote their functional maturation, protect them from the foreign body responses,
and recruit host cells and regenerate muscle tissues [63]. Biological scaffolds are used in a variety of clinical tissue engineeringapplicationsandhavebeenstudiedinpreclinical skeletal muscle VML injury models frequently over the last decade.Theyaremainlymadeofnaturalpolymers,synthetic polymers,orECMandattempttocreateamicroenvironment nichetofavorablycontrolthebehaviorofresidentcells. Natural polymers such as alginate, collagen, and fibrin havebeenutilizedextensivelyinskeletalmuscleengineering [64–66]. They possess intrinsic bioactive signaling cues to enhance cell behavior [67–69]. Alginate gels with a stiffness of13–45kPawerefoundtomaximizemyoblastproliferation anddifferentiation[70].Freeze-driedcollagenscaffoldsfacilitatedtheintegrationofalignedmyotubesintoalargemuscle defect,whichwerecapableofproducingforceuponelectrical stimulation[71].Collagencouldalsosupplynecessarygrowth factorstothewoundsitetoincreasemusclecellmigration[72, 73]. Fibrin gels were reported to promote myoblast survival anddifferentiationintomyofiberswhenintegratedintissues [74].Fibrinscaffoldswithmicrothreadarchitecturewerealso showntosupportthehealingofVMLinmousemodels[75]. As the natural polymer only offers limited mechanical stiffness and can be easily degraded, a variety of synthetic materials have been used for skeletal muscle regeneration suchasPGA,PLA,andPLGA[66,76–78].Myoblastsseeded onto electrospun meshes with aligned nanofiber orientation can fuse into highly aligned myotubes [78]. Furthermore, synthetic scaffolds can be easily engineered to facilitate the controlled release of growth factors for inducing muscle regeneration [75, 79]. The main disadvantages include typically poorer cell affinity compared to natural polymers and the risk of stimulation of a foreign body response by the polymeroritsdegradationproducts[79]. To improve regeneration of muscle tissues, the in vivo microenvironment of the scaffolds ideally would mimic native tissues and thereby facilitate remodeling of the neotissue [80]. An attractive approach for the repair of VML is therefore the transplantation of a myoinductive decellularized scaffold that attracts the cells required for myogenesis from the host. That is why muscle-derived ECM scaffolds are popularly investigated. These ECM scaffolds can fill the defect and restore morphology temporarily [17]. They can furtherbefilledbybone-marrowderivedmesenchymalstem cells (MSCs) after implantation. This enriched matrix gains more blood vessels and regenerates more myofibers than “conventional”extracellularmatrix[17,81].Indeed,hydrogels derivedfromdecellularizedskeletalmusclematrixhavebeen shown to enhance the proliferation of skeletal myoblasts wheninjectedintoanischemicratlimb[82].Analternative method could be to utilize minced skeletal muscle tissue that has not been decellularized, which has been reported toshowbettermuscleregenerationthandevitalizedscaffolds [83]. Comparable to muscle-derived matrix, small intestinal submucosa-extracellularmatrixcanleadtocontractilesheets ofskeletalmusclewithcomparablecontractileforce[61].For in vitro muscle tissue engineering, rat myoblasts have also been preconditioned on a porcine bladder acellular matrix inabioreactorandthenimplantedinnudemiceatamuscle defecttorestoremusculartissue[80].