A tissue-engineered jellyfish with biomimetic propulsion

Abstract

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 4,5 and traditional soft robotics, and increasing aptitude in generating structural and chemical micro environments 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.

Figure 1

Key concepts of fluid transport in jellyfish and in vitro implementation. (a) Schematics of jellyfish stroke cycle generating thrust during the power stroke, and feeding currents during the recovery stroke. (b) Controlled configurational change. Symmetric, complete bell contraction is mediated by anisotropic striated muscle tissue, a functional syncytium synchronized by a system of distributed neuronal pacemaker centers (jellyfish, top). This mechanism can be approximated by electrical field stimulation of electromechanically coupled, anisotropic cardiac muscle (medusoid, bottom). In either case, lobed geometry facilitates circumferential constriction of the bell. (c) Stroke kinematics. In the bilayered design of jellyfish (top) and medusoid (bottom), a flexible elastomer opposes an actuator, which promotes asymmetric stroke patterns: active, fast contraction and passive, slow recoil. (d) Fluid dynamics. Fluid velocity gradients, so-called boundary layers, extend effective reach of lobes (top). Overlapping boundary layers close interlobate gaps to oncoming flow. This prevents leakage and inefficient fluid transport despite the presence of gaps (center). Optimized medusoid body geometry favors the formation of boundary layer overlap and thus efficient fluid transport (bottom). (e) Body design of jellyfish (top) and free-swimming medusoid construct (bottom). Comparison demonstrates similar geometry and dimensions but also illustrates that the medusoid constitutes a simplified version of a jellyfish, reduced to elements necessary for propulsive function. (f) Jellyfish 2D muscle architecture (top) was reverse-engineered in medusoids (bottom). Left: Composite brightfield image overlaid with F-actin stain (green) of muscle cell monolayer. Square inset: Close-up on muscle organization at lobe-body junction; F-actin stain (green). Note that jellyfish muscle tissue consists of a single layer of myofibrils, here in focus, whereas engineered medusoid muscle tissue contains a stack of myofibrils, most of them being out of focus and blurring the image at this resolution. Circular inset: microstructure of single myofibril layer; F-actin stain (green), sarcomeric α-actinin (gray). (g) Distribution of actin fiber orientation angles within single myofibril layer (centered on zero). Quantitative analysis of multiple fields of view revealed no significant difference in the orientation organization parameter (OOP) (P = 0.61, n = 10; two-sample t-test).

Figure 2

Medusoids were engineered to exhibit jellyfish-like stroke kinetics. (a) Time lapse of stroke cycle in jellyfish (top) and medusoid paced at 1 Hz (bottom); t, time (sec) elapsed since start of stroke cycle; T, duration of stroke cycle; here: jellyfish, T = 0.3 s; medusoid, T = 1.0 s. (b) Average trace of angular velocity of individual bell lobes throughout stroke cycle in juvenile jellyfish and medusoids (n = 9 lobes each). Inset illustrates characteristic parameters of stroke cycle (top) and velocity-time graph (bottom). T_{power/recovery}, duration of power/recovery stroke; U_{power/recovery}, velocity of power/recovery stroke; Û_{power/recovery}, peak velocity of power/recovery stroke; t_{power/recovery}, time point of peak power/recovery stroke velocity. (c,d) Box-plot representation. Bull's eyes, median; lower edge of box, 25th percentile; upper edge of box, 75th percentile; whiskers, extreme data points. (c) Relative asynchrony of lobe contraction. Asynchrony did not differ significantly in jellyfish and medusoids (P = 0.7, Wilcoxon rank sum test, n = 4 lobe pairs each). Δt_power, difference between time points of peak power stroke velocities in pair of lobes. (d) Ratio of maximal lobe velocities during power and recovery stroke did not differ significantly in jellyfish and medusoids (P = 0.7, Wilcoxon rank sum test, n = 9 lobes each).

Figure 3

Medusoids generated jellyfish-like flow fields. (a–c) Velocity field at end of power stroke reveals similar thrust generation in a, jellyfish and b, medusoids, and reduced thrust in c, sieve-designed medusoids. White arrows, lobe motion. (d–f) Vorticity field during recovery stroke reveals similar formation of stopping vortices in jellyfish (d), medusoids (e) and sieve-designed medusoid (f). Blue and red contours, counter-rotating cross-sections of stopping vortex ring; gray arrows, lobe motion.

Figure 4

Medusoids achieved functional performance of jellyfish. (a) Box-plot representation of relative propulsion performances in BL/S. Performance in jellyfish (n = 7, red) and optimally designed medusoids (n = 11, black) spans similar range. Compared to optimal conditions, jellyfish in sieve conditions (n = 7, blue) and sieve-designed medusoids (n = 6, gray) performed significantly worse (P = 0.02 and P = 0.00015, respectively; Wilcoxon rank sum test). Bull's eyes, median; lower edge of box, 25th percentile; upper edge of box, 75th percentile; whiskers, extreme data points not considered outliers; circle, outlier. Asterisks denote statistically significant difference, P < 0.05. (b) For each stage of the recovery stroke, flow profile (i) and volume flow rate (ii) across subumbrellar reference section reveal similar fluid transport, that is, ‘feeding current’ in jellyfish and medusoids, and reduced performance—including flow reversal—in sieve-designed medusoids. Data shown for one representative sample each. Left: reference section at each stage of recovery stroke. Measured quantities are nondimensionalized to facilitate comparison. R, bell radius (mm); jellyfish: R = 9 mm; medusoid: R = 9 mm; sieve-designed medusoid: R = 9 mm; r, radial distance (mm) from center. T_recovery, duration (s) of recovery stroke; jellyfish: T = 0.2 s, medusoid, T = 0.48 s; sieve-designed medusoid, T = 0.55 s; t, time (s) elapsed since start of recovery stroke. U, flow velocity [mm/s] normal to reference section; jellyfish: U_max = 9 mm/s; medusoid: U_max = 2.4 mm/s; sieve-designed medusoid: U_max = 1.6 mm/s. Q, volume flow rate (mm³/s) passing across reference area (circle with diameter reference section); jellyfish: Q_max = 100 mm³/s; medusoid: Q_max = 40 mm³/s; sieve-designed medusoid: Q_max = 15 mm³/s.