Fractional Brownian motion and the critical dynamics of zipping polymers

We consider two complementary polymer strands of length $L$ attached by a common end monomer. The two strands bind through complementary monomers and at low temperatures form a double stranded conformation (zipping), while at high temperature they dissociate (unzipping). This is a simple model of DNA (or RNA) hairpin formation. Here we investigate the dynamics of the strands at the equilibrium critical temperature $T=T_c$ using Monte Carlo Rouse dynamics. We find that the dynamics is anomalous, with a characteristic time scaling as $\tau \sim L^{2.26(2)}$, exceeding the Rouse time $\sim L^{2.18}$. We investigate the probability distribution function, the velocity autocorrelation function, the survival probability and boundary behaviour of the underlying stochastic process. These quantities scale as expected from a fractional Brownian motion with a Hurst exponent $H=0.44(1)$. We discuss similarities and differences with unbiased polymer translocation.Comment: 7 pages, 8 figure

Real-time optical manipulation of cardiac conduction in intact hearts

Optogenetics has provided new insights in cardiovascular research, leading to new methods for cardiac pacing, resynchronization therapy and cardioversion. Although these interventions have clearly demonstrated the feasibility of cardiac manipulation, current optical stimulation strategies do not take into account cardiac wave dynamics in real time. Here, we developed an all‐optical platform complemented by integrated, newly developed software to monitor and control electrical activity in intact mouse hearts. The system combined a wide‐field mesoscope with a digital projector for optogenetic activation. Cardiac functionality could be manipulated either in free‐run mode with submillisecond temporal resolution or in a closed‐loop fashion: a tailored hardware and software platform allowed real‐time intervention capable of reacting within 2 ms. The methodology was applied to restore normal electrical activity after atrioventricular block, by triggering the ventricle in response to optically mapped atrial activity with appropriate timing. Real‐time intraventricular manipulation of the propagating electrical wavefront was also demonstrated, opening the prospect for real‐time resynchronization therapy and cardiac defibrillation. Furthermore, the closed‐loop approach was applied to simulate a re‐entrant circuit across the ventricle demonstrating the capability of our system to manipulate heart conduction with high versatility even in arrhythmogenic conditions. The development of this innovative optical methodology provides the first proof‐of‐concept that a real‐time optically based stimulation can control cardiac rhythm in normal and abnormal conditions, promising a new approach for the investigation of the (patho)physiology of the heart

The transverse-axial tubular system of cardiomyocytes

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Photoresponsive Polymer-Based Biomimetic Contractile Units as Building Block for Artificial Muscles

Loss of muscular mechanical function occurs in several diseases affecting millions of people worldwide, including heart failure, stroke, and neuromuscular disorders. To date, no medical or surgical treatments can restore muscular contractility, and the development of artificial muscles is of extreme interest. Mimicking biological muscles, which are optimized systems displaying quick reaction times, is not trivial; only few examples are reported, mainly focused on the use of biomimetic smart materials. Among them, liquid crystalline elastomers (LCEs) can be biocompatible, show contraction parameters comparable to those of native striated muscles, and are able to effectively potentiate cardiac contraction in vitro. To go further and develop in vivo implantable devices, the integration of the stimulation system with the LCE material represents an essential step. Here, a light-stimulated biomimetic contractile unit (BCU), combining ultra-thin photoresponsive LCE films and mini-LED (mLED) matrixes is described. BCU performance (in terms of extent and kinetics of contractile force and shortening) can be fine-tuned by modulating both mLED light power and spatial stimulation patterns, allowing to reproduce mechanical dynamics of native muscles. These results pave the way for the development of novel LCE-based contraction assist devices for cardiac, skeletal, or smooth muscle support by assembling multiple BCUs