Dynamic loading of human engineered heart tissue enhances contractile function and drives a desmosome-linked disease phenotype.

The role that mechanical forces play in shaping the structure and function of the heart is critical to understanding heart formation and the etiology of disease but is challenging to study in patients. Engineered heart tissues (EHTs) incorporating human induced pluripotent stem cell (hiPSC)-derived cardiomyocytes have the potential to provide insight into these adaptive and maladaptive changes. However, most EHT systems cannot model both preload (stretch during chamber filling) and afterload (pressure the heart must work against to eject blood). Here, we have developed a new dynamic EHT (dyn-EHT) model that enables us to tune preload and have unconstrained contractile shortening of >10%. To do this, three-dimensional (3D) EHTs were integrated with an elastic polydimethylsiloxane strip providing mechanical preload and afterload in addition to enabling contractile force measurements based on strip bending. Our results demonstrated that dynamic loading improves the function of wild-type EHTs on the basis of the magnitude of the applied force, leading to improved alignment, conduction velocity, and contractility. For disease modeling, we used hiPSC-derived cardiomyocytes from a patient with arrhythmogenic cardiomyopathy due to mutations in the desmoplakin gene. We demonstrated that manifestation of this desmosome-linked disease state required dyn-EHT conditioning and that it could not be induced using 2D or standard 3D EHT approaches. Thus, a dynamic loading strategy is necessary to provoke the disease phenotype of diastolic lengthening, reduction of desmosome counts, and reduced contractility, which are related to primary end points of clinical disease, such as chamber thinning and reduced cardiac output.

Understanding the Role of ECM Protein Composition and Geometric Micropatterning for Engineering Human Skeletal Muscle.

Skeletal muscle lost through trauma or disease has proven difficult to regenerate due to the challenge of differentiating human myoblasts into aligned, contractile tissue. To address this, we investigated microenvironmental cues that drive myoblast differentiation into aligned myotubes for potential applications in skeletal muscle repair, organ-on-chip disease models and actuators for soft robotics. We used a 2D in vitro system to systematically evaluate the role of extracellular matrix (ECM) protein composition and geometric patterning for controlling the formation of highly aligned myotubes. Specifically, we analyzed myotubes differentiated from murine C2C12 cells and human skeletal muscle derived cells (SkMDCs) on micropatterned lines of laminin compared to fibronectin, collagen type I, and collagen type IV. Results showed that laminin supported significantly greater myotube formation from both cells types, resulting in greater than twofold increase in myotube area on these surfaces compared to the other ECM proteins. Species specific differences revealed that human SkMDCs uniaxially aligned over a wide range of micropatterned line dimensions, while C2C12s required specific line widths and spacings to do the same. Future work will incorporate these results to engineer aligned human skeletal muscle tissue in 2D for in vitro applications in disease modeling, drug discovery and toxicity screening.

Engineered skeletal muscle tissue for soft robotics: fabrication strategies, current applications, and future challenges.

Skeletal muscle is a scalable actuator system used throughout nature from the millimeter to meter length scales and over a wide range of frequencies and force regimes. This adaptability has spurred interest in using engineered skeletal muscle to power soft robotics devices and in biotechnology and medical applications. However, the challenges to doing this are similar to those facing the tissue engineering and regenerative medicine fields; specifically, how do we translate our understanding of myogenesis in vivo to the engineering of muscle constructs in vitro to achieve functional integration with devices. To do this researchers are developing a number of ways to engineer the cellular microenvironment to guide skeletal muscle tissue formation. This includes understanding the role of substrate stiffness and the mechanical environment, engineering the spatial organization of biochemical and physical cues to guide muscle alignment, and developing bioreactors for mechanical and electrical conditioning. Examples of engineered skeletal muscle that can potentially be used in soft robotics include 2D cantilever-based skeletal muscle actuators and 3D skeletal muscle tissues engineered using scaffolds or directed self-organization. Integration into devices has led to basic muscle-powered devices such as grippers and pumps as well as more sophisticated muscle-powered soft robots that walk and swim. Looking forward, current, and future challenges include identifying the best source of muscle precursor cells to expand and differentiate into myotubes, replacing cardiomyocytes with skeletal muscle tissue as the bio-actuator of choice for soft robots, and vascularization and innervation to enable control and nourishment of larger muscle tissue constructs.