Hyungsuk Lee

A tissue-engineered jellyfish with biomimetic propulsion

Reverse engineering of biological form and function requires hierarchical design over several orders of space and time. Recent advances in the mechanistic understanding of biosynthetic compound materials, computer-aided design approaches in molecular synthetic biology and traditional soft robotics, and increasing aptitude in generating structural and chemical microenvironments that promote cellular self-organization have enhanced the ability to recapitulate such hierarchical architecture in engineered biological systems. Here we combined these capabilities in a systematic design strategy to reverse engineer a muscular pump. We report the construction of a freely swimming jellyfish from chemically dissociated rat tissue and silicone polymer as a proof of concept. The constructs, termed medusoids, were designed with computer simulations and experiments to match key determinants of jellyfish propulsion and feeding performance by quantitatively mimicking structural design, stroke kinematics and animal-fluid interactions. The combination of the engineering design algorithm with quantitative benchmarks of physiological performance suggests that our strategy is broadly applicable to reverse engineering of muscular organs or simple life forms that pump to survive.

Non-volatile organic memory with sub-millimetre bending radius

High-performance non-volatile memory that can operate under various mechanical deformations such as bending and folding is in great demand for the future smart wearable and foldable electronics. Here we demonstrate non-volatile solution-processed ferroelectric organic field-effect transistor memories operating in p- and n-type dual mode, with excellent mechanical flexibility. Our devices contain a ferroelectric poly(vinylidene fluoride-co-trifluoroethylene) thin insulator layer and use a quinoidal oligothiophene derivative (QQT(CN)4) as organic semiconductor. Our dual-mode field-effect devices are highly reliable with data retention and endurance of >6,000u2009s and 100 cycles, respectively, even after 1,000 bending cycles at both extreme bending radii as low as 500u2009μm and with sharp folding involving inelastic deformation of the device. Nano-indentation and nano scratch studies are performed to characterize the mechanical properties of organic layers and understand the crucial role played by QQT(CN)4 on the mechanical flexibility of our devices.

Measuring molecular rupture forces between single actin filaments and actin-binding proteins

Actin-binding proteins (ABPs) regulate the assembly of actin filaments (F-actin) into networks and bundles that provide the structural integrity of the cell. Two of these ABPs, filamin and α-actinin, have been extensively used to model the mechanical properties of actin networks grown in vitro; however, there is a lack in the understanding of how the molecular interactions between ABPs and F-actin regulate the dynamic properties of the cytoskeleton. Here, we present a native-like assay geometry to test the rupture force of a complex formed by an ABP linking two quasiparallel actin filaments. We readily demonstrate the adaptability of this assay by testing it with two different ABPs: filamin and α-actinin. For filamin/actin and α-actinin/actin, we measured similar rupture forces of 40–80 pN for loading rates between 4 and 50 pN/s. Both ABP unfolding and conformational transition events were observed, demonstrating that both are important and may be a significant mechanism for the temporal regulation of the mechanical properties of the actin cytoskeleton. With this modular, single-molecule assay, a wide range of ABP/actin interactions can be studied to better understand cytoskeletal and cell dynamics.

Computational analysis of viscoelastic properties of crosslinked actin networks

Mechanical force plays an important role in the physiology of eukaryotic cells whose dominant structural constituent is the actin cytoskeleton composed mainly of actin and actin crosslinking proteins (ACPs). Thus, knowledge of rheological properties of actin networks is crucial for understanding the mechanics and processes of cells. We used Brownian dynamics simulations to study the viscoelasticity of crosslinked actin networks. Two methods were employed, bulk rheology and segment-tracking rheology, where the former measures the stress in response to an applied shear strain, and the latter analyzes thermal fluctuations of individual actin segments of the network. It was demonstrated that the storage shear modulus (G′) increases more by the addition of ACPs that form orthogonal crosslinks than by those that form parallel bundles. In networks with orthogonal crosslinks, as crosslink density increases, the power law exponent of G′ as a function of the oscillation frequency decreases from 0.75, which reflects the transverse thermal motion of actin filaments, to near zero at low frequency. Under increasing prestrain, the network becomes more elastic, and three regimes of behavior are observed, each dominated by different mechanisms: bending of actin filaments, bending of ACPs, and at the highest prestrain tested (55%), stretching of actin filaments and ACPs. In the last case, only a small portion of actin filaments connected via highly stressed ACPs support the strain. We thus introduce the concept of a ‘supportive framework,’ as a subset of the full network, which is responsible for high elasticity. Notably, entropic effects due to thermal fluctuations appear to be important only at relatively low prestrains and when the average crosslinking distance is comparable to or greater than the persistence length of the filament. Taken together, our results suggest that viscoelasticity of the actin network is attributable to different mechanisms depending on the amount of prestrain.

Cooperative coupling of cell-matrix and cell–cell adhesions in cardiac muscle

Adhesion between cardiac myocytes is essential for the heart to function as an electromechanical syncytium. Although cell-matrix and cell–cell adhesions reorganize during development and disease, the hierarchical cooperation between these subcellular structures is poorly understood. We reasoned that, during cardiac development, focal adhesions mechanically stabilize cells and tissues during myofibrillogenesis and intercalated disc assembly. As the intercalated disc matures, we postulated that focal adhesions disassemble as systolic stresses are transmitted intercellularly. Finally, we hypothesized that pathological remodeling of cardiac microenvironments induces excessive mechanical loading of intercalated discs, leading to assembly of stabilizing focal adhesions adjacent to the junction. To test our model, we engineered μtissues composed of two ventricular myocytes on deformable substrates of tunable elasticity to measure the dynamic organization and functional remodeling of myofibrils, focal adhesions, and intercalated discs as cooperative ensembles. Maturing μtissues increased systolic force while simultaneously developing into an electromechanical syncytium by disassembling focal adhesions at the cell–cell interface and forming mature intercalated discs that transmitted the systolic load. We found that engineering the microenvironment to mimic fibrosis resulted in focal adhesion formation adjacent to the cell–cell interface, suggesting that the intercalated disc required mechanical reinforcement. In these pathological microenvironments, μtissues exhibited further evidence of maladaptive remodeling, including lower work efficiency, longer contraction cycle duration, and weakened relationships between cytoskeletal organization and force generation. These results suggest that the cooperative balance between cell-matrix and cell–cell adhesions in the heart is guided by an architectural and functional hierarchy established during development and disrupted during disease.

Passive and active microrheology with optical tweezers

Efforts at understanding the behaviour of complex materials at the micro scale have led to the development of many microrheological techniques capable of probing viscoelastic behaviour. Among these, optical tweezers have been extensively developed for biophysical applications: they offer several advantages over traditional techniques, and can be employed in both passive and active microrheology. In this report, we outline several methods that can be used with optical tweezers to measure the microrheological behaviour of materials such as glycerol, methylcellulose solutions, actin matrices, and cellular membranes. In addition, we quantify the effect that the index of refraction of the solution has on the stiffness of the optical trap. Our results indicate that optical tweezers force microscopy is a versatile tool for the exploration of viscoelastic behaviour in a range of substrates at the micro scale.

Myocyte Shape Regulates Lateral Registry of Sarcomeres and Contractility

The heart actively remodels architecture in response to various physiological and pathological conditions. Gross structural change of the heart chambers is directly reflected at the cellular level by altering the morphological characteristics of individual cardiomyocytes. However, an understanding of the relationship between cardiomyocyte shape and the contractile function remains unclear. By using in vitro assays to analyze systolic stress of cardiomyocytes with controlled shape, we demonstrated that the characteristic morphological features of cardiomyocytes observed in a variety of pathophysiological conditions are correlated with mechanical performance. We found that cardiomyocyte contractility is optimized at the cell length/width ratio observed in normal hearts, and decreases in cardiomyocytes with morphological characteristics resembling those isolated from failing hearts. Quantitative analysis of sarcomeric architecture revealed that the change of contractility may arise from alteration of myofibrillar structure. Measurements of intracellular calcium in myocytes revealed unique characteristics of calcium metabolism as a function of myocyte shape. Our data suggest that cell shape is critical in determining contractile performance of single cardiomyocytes by regulating the intracellular structure and calcium handling ability.

Optical trapping for undergraduates

The detailed design of a robust and inexpensive optical trap system is presented. The system features high-sensitivity back focal plane position detection, mechanically controlled specimen stage movement, and fluorescence imaging to provide broad experimental applications. Three educational experimental modules are described to cover basic concepts in optical trapping and biophysics at a level appropriate for undergraduate students.

A potential role for integrin signaling in mechanoelectrical feedback.

Certain forms of heart disease involve gross morphological changes to the myocardium that alter its hemodynamic loading conditions. These changes can ultimately lead to the increased deposition of extracellular matrix (ECM) proteins, such as collagen and fibronectin, which together work to pathologically alter the myocardiums bulk tissue mechanics. In addition to changing the mechanical properties of the heart, this maladaptive remodeling gives rise to changes in myocardium electrical conductivity and synchrony since the tissues mechanical properties are intimately tied to its electrical characteristics. This phenomenon, called mechanoelectrical coupling (MEC), can render individuals affected by heart disease arrhythmogenic and susceptible to Sudden Cardiac Death (SCD). The underlying mechanisms of MEC have been attributed to various processes, including the action of stretch activated channels and changes in troponin C-Ca(2+) binding affinity. However, changes in the heart post infarction or due to congenital myopathies are also accompanied by shifts in the expression of various molecular components of cardiomyocytes, including the mechanosensitive family of integrin proteins. As transmembrane proteins, integrins mechanically couple the ECM with the intracellular cytoskeleton and have been implicated in mediating ion homeostasis in various cell types, including neurons and smooth muscle. Given evidence of altered integrin expression in the setting of heart disease coupled with the associated increased risk for arrhythmia, we argue in this review that integrin signaling contributes to MEC. In light of the significant mortality associated with arrhythmia and SCD, close examination of all culpable mechanisms, including integrin-mediated MEC, is necessary.

Micropatterned Pyramidal Ionic Gels for Sensing Broad-Range Pressures with High Sensitivity

The development of pressure sensors that are effective over a broad range of pressures is crucial for the future development of electronic skin applicable to the detection of a wide pressure range from acoustic wave to dynamic human motion. Here, we present flexible capacitive pressure sensors that incorporate micropatterned pyramidal ionic gels to enable ultrasensitive pressure detection. Our devices show superior pressure-sensing performance, with a broad sensing range from a few pascals up to 50 kPa, with fast response times of <20 ms and a low operating voltage of 0.25 V. Since high-dielectric-constant ionic gels were employed as constituent sensing materials, an unprecedented sensitivity of 41 kPa-1 in the low-pressure regime of <400 Pa could be realized in the context of a metal-insulator-metal platform. This broad-range capacitive pressure sensor allows for the efficient detection of pressure from a variety of sources, including sound waves, a lightweight object, jugular venous pulses, radial artery pulses, and human finger touch. This platform offers a simple, robust approach to low-cost, scalable device design, enabling practical applications of electronic skin.