The asexual freshwater planarian Dugesia japonica has emerged as a medium-throughput alternative animal model for neurotoxicology. We have previously shown that D. japonica are sensitive to organophosphorus pesticides (OPs) and characterized the in vitro inhibition profile of planarian cholinesterase (DjChE) activity using irreversible and reversible inhibitors. We found that DjChE has intermediate features of acetylcholinesterase (AChE) and butyrylcholinesterase (BChE). Here, we identify two candidate genes (Djche1 and Djche2) responsible for DjChE activity. Sequence alignment and structural homology modeling with representative vertebrate AChE and BChE sequences confirmed our structural predictions, and show that both DjChE enzymes have intermediate sized catalytic gorges and disrupted peripheral binding sites. Djche1 and Djche2 were both expressed in the planarian nervous system, as anticipated from previous activity staining, but with distinct expression profiles. To dissect how DjChE inhibition affects planarian behavior, we acutely inhibited DjChE activity by exposing animals to either an OP (diazinon) or carbamate (physostigmine) at 1 µM for 4 days. Both inhibitors delayed the reaction of planarians to heat stress. Simultaneous knockdown of both Djche genes by RNAi similarly resulted in a delayed heat stress response. Furthermore, chemical inhibition of DjChE activity increased the worms' ability to adhere to a substrate. However, increased substrate adhesion was not observed in Djche1/Djche2 (RNAi) animals or in inhibitor-treated day 11 regenerates, suggesting this phenotype may be modulated by other mechanisms besides ChE inhibition. Together, our study characterizes DjChE expression and function, providing the basis for future studies in this system to dissect alternative mechanisms of OP toxicity.

Asexual freshwater planarians reproduce by tearing themselves into two pieces by a process called binary fission. The resulting head and tail pieces regenerate within about a week forming two new worms. Understanding this process of ripping oneself into two parts poses a challenging biomechanical problem. Since planarians stop "doing it" at the slightest disturbance, this remained a centuries-old puzzle. We focus on Dugesia japonica fission and show that it proceeds in three stages: a local constriction ("waist formation"), pulsationwhich increases waist longitudinal stresses -and transverse rupture. We developed a linear mechanical model with a planarian represented by a thin shell. The model fully captures the pulsation dynamics leading to rupture and reproduces empirical time scales and stresses. It asserts that fission execution is a mechanical process. Furthermore, we show that the location of waist formation, and thus fission, is determined by physical constraints. Together, our results demonstrate that where and how a planarian rips itself apart during asexual reproduction can be fully explained through biomechanics.Keywords: planarians, biomechanics, fission, rupture, traction forces Significance Statement: How planarians reproduce by ripping themselves into a head and a tail piece, which subsequently regenerate into two new worms, is a centuries-old biomechanics problem. Michael Faraday contemplated how this feat can be achieved in the 1800s, but it remained unanswered because it is experimentally difficult to observe planarians "doing it". We recorded Dugesia japonica planarians in the act and developed a physical model which captures pivotal steps of their reproduction dynamics. The model reproduces experimental time scales and rupture stresses without fit parameters. The key to rupture is a local reduction of the animal's cross-sectional area, which greatly amplifies the stresses exerted by the planarian's musculature and enables rupture at substrate stresses in the Pa range. \body Already Michael Faraday and his contemporaries were intrigued by the observation that asexual freshwater planarians, squishy worms a few mm in length, reproduced by tearing themselves into a head and tail offspring, in a process called binary fission (1). How was it possible for these animals to generate the forces necessary to rip themselves using only their

The concept of “bound states in the continuum” (BIC) describes an idealized physical system exhibiting zero radiative loss composed, for example, of an infinitely extended array of resonators. In principle, vanishing of radiative losses enables an infinitely high-quality factor and corresponding infinite lifetime of the resonance. As such, BIC inspired metasurfaces and photonic designs aim to achieve superior performance in various applications including sensing and lasing. We describe an analytical model based on temporal coupled mode theory to realize an “accidental” (i.e., parameter-tuned) Friedrich–Wintgen BIC. Further, we experimentally verify this model with measurements of quasi-BICs in a metallic terahertz metasurface (MS) and the corresponding complementary metasurface (CMS) using terahertz time domain spectroscopy. For the MS and CMS structures, quality factors of ∼ 20 are achieved, limited by non-radiative intrinsic loss in the materials. Our results reveal that Babinet’s principle qualitatively holds for the MS and CMS quasi-BIC structures. In addition, ultra-high electric and magnetic field enhancement MS and CMS structures, respectively, are presented.

Metasurface absorbers are of particular interest in numerous photonic applications including detectors, photovoltaic cells, and emissivity coatings. We introduce a thin membrane silicon metasurface absorber with periodic elliptical holes that, as demonstrated theoretically and experimentally, achieves very high absorption (≥90%) over a ∼500 GHz bandwidth at normal incidence. Based on the analysis of the effective medium theory, the broadband absorption is attributed to proximal electric and magnetic dipole resonances. The absorption amplitude can also be tuned by ∼20% with above-gap photoexcitation. Due to the unit cell geometry, the carrier density on the top surface and sidewalls of the membrane must be taken into account. Our dynamic membrane silicon metasurface absorber is notably thin and CMOS-compatible, providing a promising platform to realize compact terahertz devices including detectors, modulators, and switches.

During the past decade, metasurfaces have shown great potential to complement standard optics, providing novel pathways to control the phase, amplitude, and polarization of electromagnetic waves utilizing arrays of subwavelength resonators. We present dynamic surface wave (SW) switching at terahertz frequencies utilizing a mechanically reconfigurable metasurface. Our metasurface is based on a microelectromechanical system (MEMS) consisting of an array of micro-cantilever structures, enabling dynamic tuning between a plane wave (PW) and a SW for normal incidence terahertz radiation. This is realized using line-by-line voltage control of the cantilever displacements to achieve full-span ( 2 π ) phase control. Full-wave electromagnetic simulations and terahertz time-domain spectroscopy agree with coupled mode theory, which was employed to design the metasurface device. A conversion efficiency of nearly 60% has been achieved upon switching between the PW and SW configurations. Moreover, a nearly 100 GHz working bandwidth is demonstrated. The MEMS-based control modality we demonstrate can be used for numerous applications, including but not limited to terahertz multifunctional metasurface devices for spatial light modulation, dynamic beam steering, focusing, and beam combining, which are crucial for future “beyond 5G” communication systems.