Publications

The brown pelican (Pelecanus occidentalis) wields one of the largest bills of any bird and is distinguished by the deployable throat pouch of extensible tissue used to capture prey. Here we report on mechanical properties and microstructure of the pouch skin. It exhibits significant anisotropy, with the transverse direction having maximum nominal tensile strains of 200% to 300%, triple the value in the longitudinal direction. This is a higher extensibility than most conventional skin and is the result of the requirement of the sac to net fish; it should expand laterally, with controlled longitudinal stretch. Transmission electron microscopy provides microstructural evidence of the directionality of the collagen fibers and reveals the individual collagen fibrils with a bimodal diameter distribution having peaks at 100 and 170 nm. These dimensions are similar to collagen in mammal skin. In the lateral direction, the fibers form a curvy pattern with a radius of approximately 2 µm wherein the fibrils reorient, straighten, slide, and stretch elastically under tensile load. A second mechanism operates in the transverse direction; the membrane forms a corrugated pattern that, upon straightening of collagen fibrils, confers additional extensibility. This elicits the anisotropic response observed in tensile testing. This work focuses on the mechanical characterization based on the effect of relative bird age, sample location on the pouch, and strain rate. Anterior-posterior location and strain rate are not major influencers on exhibited strengths and extensibilities. However, bird age and dorsal-ventral location are found to affect the mechanical response of the pouch significantly. A physically-based constitutive model is developed for the middle layer of the gular sac, based on observations, which predicts maximum stresses, strains, and the shape of the stress-strain curve consistent with the experimental results.

Reinforcement learning and sim-to-real transfer have made significant progress in dexterous manipulation. However, progress remains limited by the difficulty of simulating complex contact dynamics and multisensory signals, especially tactile feedback. In this work, we propose \ours, a sim-to-real framework that addresses these limitations and demonstrates its effectiveness on nut-bolt fastening and screwdriving with multi-fingered hands. The framework has three stages. First, we train reinforcement learning policies in simulation using simplified object models that lead to the emergence of correct finger gaits. We then use the learned policy as a skill primitive within a teleoperation system to collect real-world demonstrations that contain tactile and proprioceptive information. Finally, we train a behavior cloning policy that incorporates tactile sensing and show that it generalizes to nuts and screwdrivers with diverse geometries. Experiments across both tasks show high task progress ratios compared to direct sim-to-real transfer and robust performance even on unseen object shapes and under external perturbations. Videos and code are available on https://dexscrew.github.io.

The jet compositions, central engines, and progenitors of gamma-ray bursts (GRBs) remain open questions in GRB physics. Applying broadband observations, including GRB prompt emission and afterglow properties derived from {\em Fermi} and {\em Swift} data, as well as from Keck host-galaxy observations, we address these questions for the peculiar, bright GRB 110731A. By using the pair-opacity method, we derive $Γ_{0}>190$ during the prompt emission phase. Alternatively, we derive $Γ_{0} \approx 580$ and $Γ_{0} \approx 154$ by invoking the early-afterglow phase within the homogeneous density and wind cases, respectively. On the other hand, nondetection of a thermal component in the spectra suggests that the prompt emission is likely powered by dissipation of a Poynting-flux-dominated jet leading to synchrotron radiation in an optically thin region. The nondetection of a jet break in the X-ray and optical bands allows us to place a lower limit on the jet opening angle $θ_{\rm j}>5.5^{\circ}$. Within a millisecond magnetar central engine scenario, we derive the period $P_{0}$ and polar magnetic field strength $B_{\rm p}$, which have extreme (but still allowed) values. The moderately short observed duration (7.3\,s) and relatively large redshift ($z=2.83$) places the burst as a "rest-frame short" GRB, so the progenitor of the burst is subject to debate. Its relatively large $f_{{\rm eff}, z}$ parameter (ratio of the 1\,s peak flux of a pseudo-GRB and the background flux) and a large physical offset from a potential host galaxy suggest that the progenitor of GRB 110731A may be a compact-star merger.

Abstract When discovered, SN 2017egm was the closest (redshift z = 0.03) hydrogen-poor superluminous supernova (SLSN-I) and a rare case that exploded in a massive and metal-rich galaxy. Thus, it has since been extensively observed and studied. We report spectroscopic data showing strong emission at around He i λ 10830 and four He i absorption lines in the optical. Consequently, we classify SN 2017egm as a member of an emerging population of helium-rich SLSNe-I (i.e., SLSNe-Ib). We also present our late-time photometric observations. By combining them with archival data, we analyze high-cadence ultraviolet, optical, and near-infrared light curves spanning from early pre-peak (∼−20 days) to late phases (∼+300 days). We obtain its most complete bolometric light curve, in which multiple bumps are identified. None of the previously proposed models can satisfactorily explain all main light-curve features, while multiple interactions between the ejecta and circumstellar material (CSM) may explain the undulating features. The prominent infrared excess with a blackbody luminosity of 10 7 –10 8 L ⊙ detected in SN 2017egm could originate from the emission of either an echo of a pre-existing dust shell or newly formed dust, offering an additional piece of evidence supporting the ejecta–CSM interaction model. Moreover, our analysis of deep Chandra observations yields the tightest-ever constraint on the X-ray emission of an SLSN-I, amounting to an X-ray-to-optical luminosity ratio ≲10 −3 at late phases (∼100–200 days), which could help explore its close environment and central engine.

Abstract We present observations of SN 2021csp, the second example of a newly identified type of supernova (SN) hallmarked by strong, narrow, P Cygni carbon features at early times (Type Icn). The SN appears as a fast and luminous blue transient at early times, reaching a peak absolute magnitude of −20 within 3 days due to strong interaction between fast SN ejecta ( v ≈ 30,000 km s −1 ) and a massive, dense, fast-moving C/O wind shed by the WC-like progenitor months before explosion. The narrow-line features disappear from the spectrum 10–20 days after explosion and are replaced by a blue continuum dominated by broad Fe features, reminiscent of Type Ibn and IIn supernovae and indicative of weaker interaction with more extended H/He-poor material. The transient then abruptly fades ∼60 days post-explosion when interaction ceases. Deep limits at later phases suggest minimal heavy-element nucleosynthesis, a low ejecta mass, or both, and imply an origin distinct from that of classical Type Ic SNe. We place SN 2021csp in context with other fast-evolving interacting transients, and discuss various progenitor scenarios: an ultrastripped progenitor star, a pulsational pair-instability eruption, or a jet-driven fallback SN from a Wolf–Rayet (W-R) star. The fallback scenario would naturally explain the similarity between these events and radio-loud fast transients, and suggests a picture in which most stars massive enough to undergo a W-R phase collapse directly to black holes at the end of their lives.

The study of intracellular metabolic fluxes and inter-species metabolite exchange for microbial communities is of crucial importance to understand and predict their behaviour. The most authoritative method of measuring intracellular fluxes, 13C Metabolic Flux Analysis (13C MFA), uses the labeling pattern obtained from metabolites (typically amino acids) during 13C labeling experiments to derive intracellular fluxes. However, these metabolite labeling patterns cannot easily be obtained for each of the members of the community. Here we propose a new type of 13C MFA that infers fluxes based on peptide labeling, instead of amino acid labeling. The advantage of this method resides in the fact that the peptide sequence can be used to identify the microbial species it originates from and, simultaneously, the peptide labeling can be used to infer intracellular metabolic fluxes. Peptide identity and labeling patterns can be obtained in a high-throughput manner from modern proteomics techniques. We show that, using this method, it is theoretically possible to recover intracellular metabolic fluxes in the same way as through the standard amino acid based 13C MFA, and quantify the amount of information lost as a consequence of using peptides instead of amino acids. We show that by using a relatively small number of peptides we can counter this information loss. We computationally tested this method with a well-characterized simple microbial community consisting of two species.

Surface properties can markedly affect the mechanical behavior of structural thin films used in microelectromechanical systems (MEMS) applications. This study highlights the striking difference in the sidewall surface morphology of n+-type polysilicon films from two popular MEMS processes and its effect on fracture and fatigue properties. The sidewall surface roughness was measured using atomic force microscopy, whereas silicon oxide thickness and grain size were measured using (energy-filtered) transmission electron microscopy. These measurements show that the oxide layers are not always thin native oxides, as often assumed; moreover, the roughness of the silicon/silicon oxide interface is significantly influenced by the oxidation mechanism. Thick silicon oxides (20±5nm) found in PolyMUMPs™ films are caused by galvanic corrosion from the presence of gold on the chip, whereas in SUMMiT V™ films a much thinner (3.5±1.0nm) native oxide was observed. The thicker oxide layers, in combination with differences in sidewall roughness (14±5nm for PolyMUMPs™ and 10±2nm for SUMMiT V™), can have a significant effect on the reliability of polysilicon structures subjecting to bending loads; this is shown by measurements of the fracture strength (3.8±0.3GPa for PolyMUMPs™ and 4.8±0.2GPa for SUMMiT V™) and differences in the stress-lifetime cyclic fatigue behavior.

Supplementary Figure Legends 1-3 from Chemotherapy-Induced Genotoxic Stress Promotes Sensitivity to Natural Killer Cell Cytotoxicity by Enabling Missing-Self Recognition