From the Cover: Neural interfacing architecture enables enhanced motor control and residual limb functionality postamputation

Despite advancements in prosthetic technologies, patients with amputation today suffer great diminution in mobility and quality of life. We have developed a modified below-knee amputation (BKA) procedure that incorporates agonist–antagonist myoneural interfaces (AMIs), which surgically preserve and couple agonist–antagonist muscle pairs for the subtalar and ankle joints. AMIs are designed to restore physiological neuromuscular dynamics, enable bidirectional neural signaling, and offer greater neuroprosthetic controllability compared to traditional amputation techniques. In this prospective, nonrandomized, unmasked study design, 15 subjects with AMI below-knee amputation (AB) were matched with 7 subjects who underwent a traditional below-knee amputation (TB). AB subjects demonstrated significantly greater control of their residual limb musculature, production of more differentiable efferent control signals, and greater precision of movement compared to TB subjects (P < 0.008). This may be due to the presence of greater proprioceptive inputs facilitated by the significantly higher fascicle strains resulting from coordinated muscle excursion in AB subjects (P < 0.05). AB subjects reported significantly greater phantom range of motion postamputation (AB: 12.47 ± 2.41, TB: 10.14 ± 1.45 degrees) when compared to TB subjects (P < 0.05). Furthermore, AB subjects also reported less pain (12.25 ± 5.37) than TB subjects (17.29 ± 10.22) and a significant reduction when compared to their preoperative baseline (P < 0.05). Compared with traditional amputation, the construction of AMIs during amputation confers the benefits of enhanced physiological neuromuscular dynamics, proprioception, and phantom limb perception. Subjects’ activation of the AMIs produces more differentiable electromyography (EMG) for myoelectric prosthesis control and demonstrates more positive clinical outcomes.

The standard-of-care surgical approach to amputation has not seen considerable innovation since its conception in the mid-1800s (1), despite significant progress in biomechatronics and advanced reconstructive techniques. The typical amputation procedure neglects neurological substrates and disrupts key neuromuscular relationships responsible for bidirectional (efferent, afferent) signaling. The deafferentation of lower motor neurons triggers central reorganization of motor circuits, which negatively impacts motor imagery and motor coordination (2, 3) and results in significant neuroma pain (4–6), phantom pain (7), and maladaptive or diminishing phantom sensation (8–10). Phantom sensation, the perception of one’s phantom limb while at rest and in motion, is an important component of motor imagery utilized in the preparation of motor control commands and can be a source of chronic irritation, if unpleasant. The haphazard arrangement and myodesis of residual musculature further constrain the ability for individual muscles to dynamically excurse, causing cocontraction of muscles and changing gait patterns (11–14). Together, these peripheral and central modifications result in the poor production of efferent signals for direct myoelectric control (15, 16). To compensate for these shortcomings, considerable effort has been spent on developing and deploying pattern-recognition–based myoelectric control strategies (17–19). However, even with these advanced myoelectric devices and controllers, end users find their operation cumbersome and time consuming (20). Motor control is also challenged by the lack of proprioceptive sensory feedback from prostheses (12, 21–23). Together, the limitations of the current amputation approaches significantly lower the quality of life for persons with amputation (24–27).In recognition of these shortcomings, surgical researchers have recently begun to explore new strategies to modify standard amputation procedures. Targeted muscle reinnervation (TMR) (28, 29) and regenerative peripheral nerve interfaces (RPNIs) (30, 31) represent approaches that are designed to provide greater efferent motor signals for myoelectric control and mitigate neuroma pain. However, neither approach offers muscle–tendon afferent proprioceptive signaling, which is physiologically mediated by agonist–antagonist muscle dynamics and critical for trajectory planning, fine motor control, and reflexes (32, 33).The agonist–antagonist myoneural interface (AMI) is a more recent surgical approach and neural interfacing strategy designed to augment volitional motor control and restore muscle–tendon proprioception (11, 12, 34–37) by surgically coapting agonist and antagonist muscles to restore natural physiological muscle pairing and dynamics. When the agonist muscle contracts, the antagonist muscle stretches (or vice versa), giving rise to musculotendinous afferent feedback from muscle spindle fibers (length and velocity) and Golgi tendon organs (force). For each joint in a bionic limb, one AMI is surgically constructed in the residuum. Functional electrical stimulation (FES) applied to the antagonist muscle of the AMI can provide force or position feedback onto the agonist (or vice versa) from a bionic prosthesis to inform the user of prosthetic torque or position, respectively (11). In Clites et al. (11), an early human subject with an AMI amputation demonstrated dynamic muscle excursions, individualized contraction of each AMI muscle, and graded proprioceptive muscle–tendon feedback in response to muscle activation. This subject additionally demonstrated greater control of joint position, impedance, and FES-based torque feedback from a bionic prosthesis when compared to subjects with a traditional amputation. This pilot study demonstrated the potential of the AMI and paved the way for further implementation and investigation of the physiological properties, phantom limb perceptions, pain, and motor control of the AMI neuromuscular constructs.In this study, we characterize the physiological outcomes of subjects with an AMI below-knee amputation (AB) (n = 15) and compare them against those of matched control subjects with a traditional below-knee amputation (TB) (n = 7). Given the emphasis placed on the reconstruction of peripheral neuromusculature with AMIs, we hypothesize that the AB cohort will experience an enhancement in phantom sensation and range of motion (ROM) percepts compared with the TB cohort. As a result of the dynamically coupled agonist–antagonist muscles comprising the AMI constructs, we also hypothesize that the AB cohort will demonstrate greater muscle excursion and fascicle strains compared to the TB population. With these improvements in residual limb muscle dynamics, motor capabilities, and perception, we further anticipate that AB subjects will demonstrate greater accuracy and precision of performance on ankle and subtalar intended movements compared to matched TB participants. These hypotheses are evaluated through a combination of electromyography (EMG), goniometry, ultrasonography, and surveys.     

From the Cover: Retrograde bilin signaling enables Chlamydomonas greening and phototrophic survival

The maintenance of functional chloroplasts in photosynthetic eukaryotes requires real-time coordination of the nuclear and plastid genomes. Tetrapyrroles play a significant role in plastid-to-nucleus retrograde signaling in plants to ensure that nuclear gene expression is attuned to the needs of the chloroplast. Well-known sites of synthesis of chlorophyll for photosynthesis, plant chloroplasts also export heme and heme-derived linear tetrapyrroles (bilins), two critical metabolites respectively required for essential cellular activities and for light sensing by phytochromes. Here we establish that Chlamydomonas reinhardtii, one of many chlorophyte species that lack phytochromes, can synthesize bilins in both plastid and cytosol compartments. Genetic analyses show that both pathways contribute to iron acquisition from extracellular heme, whereas the plastid-localized pathway is essential for light-dependent greening and phototrophic growth. Our discovery of a bilin-dependent nuclear gene network implicates a widespread use of bilins as retrograde signals in oxygenic photosynthetic species. Our studies also suggest that bilins trigger critical metabolic pathways to detoxify molecular oxygen produced by photosynthesis, thereby permitting survival and phototrophic growth during the light period.     

From the Cover: Patterning droplets with durotaxis

Numerous cell types have shown a remarkable ability to detect and move along gradients in stiffness of an underlying substrate—a process known as durotaxis. The mechanisms underlying durotaxis are still unresolved, but generally believed to involve active sensing and locomotion. Here, we show that simple liquid droplets also undergo durotaxis. By modulating substrate stiffness, we obtain fine control of droplet position on soft, flat substrates. Unlike other control mechanisms, droplet durotaxis works without imposing chemical, thermal, electrical, or topographical gradients. We show that droplet durotaxis can be used to create large-scale droplet patterns and is potentially useful for many applications, such as microfluidics, thermal control, and microfabrication.     

From the Cover: Fiber networks amplify active stress

Large-scale force generation is essential for biological functions such as cell motility, embryonic development, and muscle contraction. In these processes, forces generated at the molecular level by motor proteins are transmitted by disordered fiber networks, resulting in large-scale active stresses. Although these fiber networks are well characterized macroscopically, this stress generation by microscopic active units is not well understood. Here we theoretically study force transmission in these networks. We find that collective fiber buckling in the vicinity of a local active unit results in a rectification of stress towards strongly amplified isotropic contraction. This stress amplification is reinforced by the networks’ disordered nature, but saturates for high densities of active units. Our predictions are quantitatively consistent with experiments on reconstituted tissues and actomyosin networks and shed light on the role of the network microstructure in shaping active stresses in cells and tissue.Living systems constantly convert biochemical energy into forces and motion. In cells, forces are largely generated internally by molecular motors acting on the cytoskeleton, a scaffold of protein fibers (Fig. 1A). Forces from multiple motors are propagated along this fiber network, driving numerous processes such as mitosis and cell motility (1) and allowing the cell as a whole to exert stresses on its surroundings. At the larger scale of connective tissue, many such stress-exerting cells act on another type of fiber network known as the extracellular matrix (Fig. 1B). This network propagates cellular forces to the scale of the whole tissue, powering processes such as wound healing and morphogenesis. Despite important differences in molecular details and length scales, a common physical principle thus governs stress generation in biological matter: Internal forces from multiple localized “active units”—motors or cells—are propagated by a fiber network to generate large-scale stresses. However, a theoretical framework relating microscopic internal active forces to macroscopic stresses in these networks is lacking. Here we propose such a theory for elastic networks.Open in a separate windowFig. 1.Biological fiber networks (green) transmit forces generated by localized active units (red). (A) Myosin molecular motors exert forces on the actin cytoskeleton. (B) Contractile cells exert forces on the extracellular matrix. (C) The large nonlinear deformations induced by a model active unit in the surrounding fiber network result in stress amplification, as shown in this paper. Fiber color code is shown in D. (D) Each bond in the network comprises two rigid segments hinged together to allow buckling.This generic stress generation problem is confounded by the interplay of network disorder and nonlinear elasticity. Active units generate forces at the scale of the network mesh size, and force transmission to larger scales thus sensitively depends on local network heterogeneities. In the special case of linear elastic networks, the macroscopic active stress is simply given by the density of active force dipoles, irrespective of network characteristics (2). Importantly, however, this relationship is not applicable to most biological systems, because typical active forces are amply sufficient to probe the nonlinear properties of their constitutive fibers, which stiffen under tension and buckle under compression (3). Indeed, recent experiments on reconstituted biopolymer gels have shown that individual active units induce widespread buckling and stiffening (4, 5), and theory suggests that such fiber nonlinearities can enhance the range of force propagation (6, 7).Fiber networks also exhibit complex, nonlinear mechanical properties arising at larger scales, owing to collective deformations favored by the networks’ weak connectivity (3, 8). The role of connectivity in elasticity was famously investigated by Maxwell, who noticed that a spring network in dimension d becomes mechanically unstable for connectivities z < 2d. Interestingly, most biological fiber networks exhibit connectivities well below this threshold and therefore cannot be stabilized solely by the longitudinal stretching rigidity of their fibers. Instead, their macroscopic mechanical properties are typically controlled by the fiber bending rigidity (9). In contrast to stretching-dominated networks with connectivities above the Maxwell threshold, such weakly connected, bending-dominated networks are soft and extremely sensitive to mechanical perturbations (9–11). In these networks, stresses generated by active units propagate along intricate force chains (12, 13) whose effects on force transmission remain unexplored. Collections of such active units generate large stresses, with dramatic effects such as macroscopic network stiffening (14–16) and network remodeling (5, 17).Here we study the theoretical principles underlying stress generation by localized active units embedded in disordered fiber networks (Fig. 1C). We find that arbitrary local force distributions generically induce large isotropic, contractile stress fields at the network level, provided that the active forces are large enough to induce buckling in the network. In this case, the stress generated in a biopolymer network dramatically exceeds the stress level that would be produced in a linear elastic medium (2), implying a striking network-induced amplification of active stress. Our findings elucidate the origins and magnitude of stress amplification observed in experiments on reconstituted tissues (4, 18) and actomyosin networks (14, 17). We thus provide a conceptual framework for stress generation in biological fiber networks.     

From the Cover: Species interactions and the structure of complex communication networks

A universal challenge faced by animal species is the need to communicate effectively against a backdrop of heterospecific signals. It is often assumed that this need results in signal divergence to minimize interference among community members, yet previous support for this idea is mixed, and few studies have tested the opposing hypothesis that interactions among competing species promote widespread convergence in signaling regimes. Using a null model approach to analyze acoustic signaling in 307 species of Amazonian birds, we show that closely related lineages signal together in time and space and that acoustic signals given in temporal or spatial proximity are more similar in design than expected by chance. These results challenge the view that multispecies choruses are structured by temporal, spatial, or acoustic partitioning and instead suggest that social communication between competing species can fundamentally organize signaling assemblages, leading to the opposite pattern of clustering in signals and signaling behavior.One of the core principles of animal communication is that signals should be detectable and convey an accurate message against a noisy background (1–3). This background can involve direct overlap of sounds, as in the case of masking by simultaneous signals (4, 5), or simply the co-occurrence of different species using confusingly similar signals at the same location (6–8). As most animals communicate within assemblages of related species, the problem of signal interference is widespread and may have far-reaching implications for the evolution of signals and signaling behavior. This concept—variously termed the “noisy neighbors” hypothesis (9) or “cocktail party problem” (10)—has attracted much attention over recent years. However, the extent to which it provides a general explanation for patterns of signaling in animal communities remains contentious (6, 8).The traditional view is that the signaling strategies of animals are shaped by limiting similarity among competitors, much as competition for ecological resources is thought to promote partitioning of niche space (11–13). Partitioning of signal space may occur if species compete for position near overcrowded transmission optima, and, concurrently, if overlap in signal design impairs the detection or discrimination of signals mediating mate choice and resource competition (14). Under these conditions, mechanisms of selection against misdirected aggression (e.g., character displacement) or the production of unfit hybrids (e.g., reinforcement) are predicted to drive phenotypic divergence (9), whereas similar mechanisms may lead to related species signaling at different times or in different locations (13). These pathways theoretically lead to structural, temporal, and spatial partitioning of signals and signalers in animal assemblages, but tests of these patterns have produced mixed results (6, 11, 15).A contrasting possibility is that selection for signal divergence is weak and that co-occurring species instead show the opposite pattern of signal clustering (16). One potential driver of this pattern is that shared habitats can exert convergent selection on signals (17). Another is that signals often have dual function in mate attraction and resource defense (18), potentially mediating competition among closely related species for ecological resources (19). Thus, multispecies choruses may operate to some degree as extended communication networks, not only within species (20) but between species. The effect of such a network would be to increase the likelihood of interspecific communication involving closely related species with similar signals. A pattern of signal clustering caused by communication among similar congeners may be further exaggerated when competitive interactions among species promote signal similarity (16). This process may occur when individuals with convergent agonistic signals have higher fitness because they are better at defending resources against both conspecific and heterospecific competitors, driving convergent evolution (21, 22). Taken together, these alternative views suggest that the most pervasive effect of species interactions on animal communication systems may not be partitioning, as generally proposed, but synchrony and stereotypy among competing species.Progress in resolving these opposing viewpoints has been limited because most studies of signaling assemblages have compiled lists of species co-occurring at particular localities and then compared multiple assemblages across regional scales (6, 15). This approach may be misleading because of spatial biases in phylogenetic relationships and habitat. On the one hand, sympatric species tend to be significantly older than allopatric species, at least within radiations (23, 24), and thus the signals of co-occurring lineages may be more divergent than expected by chance simply because they have had more time to diverge, exaggerating the evidence for partitioning. Conversely, species co-occurring at local scales may be less divergent because they are segregated by habitat across a study site and therefore are unlikely to signal together. Although some studies (7, 11) partially overcome these issues by sampling assemblages from single points in space, none has considered the effects of habitat and the potential role of competitive interactions among related species (16). Moreover, previous studies have generally assessed partitioning in relatively small assemblages (<30 species), reducing both the likelihood of competition over transmission optima and the power of statistical tests.Here, we sample >90 signaling assemblages (Fig. S1) containing a combined total of >300 species (Dataset S1) to assess the role of species interactions in structuring and organizing the dawn chorus of Amazonian rainforest birds. Each assemblage comprised species producing acoustic signals, identified from standardized 120-min sound recordings taken at points distributed across a single study locality. We also restricted analyses to 10-min time blocks and assumed that assemblages of species signaling in these blocks were forced to discriminate among each other (i.e., they were each other’s background noise) and also that they had an increased likelihood of signaling simultaneously (i.e., directly masking each other’s signals). We use the term cosignaling to describe pairs of species signaling during the same 10- or 120-min time block and thus not necessarily signaling simultaneously. We coded all assemblages for habitat and time of day, calculated the acoustic similarity of cosignaling species using spectrographic analyses of voice recordings, and estimated the evolutionary relatedness of cosignaling species using a hierarchical taxonomic framework.Our null hypothesis states that species interactions have no effect on chorus structure and thus that species with similar signals are randomly distributed in space and time (Fig. 1A). The distance between signals in observed choruses should not differ significantly from that expected by chance, accounting for habitat and evolutionary relationships. We envisage two scenarios that may falsify the null. The partitioning hypothesis predicts that signal design is evenly spaced across communities, with a larger distance between co-occurring signals than predicted by chance (Fig. 1B). The network hypothesis predicts that competing species interact using phylogenetically conserved signals and thus that signals are clustered in distribution, with a smaller distance between co-occurring signals than predicted by chance (Fig. 1C). The partitioning and network hypotheses involve different forms of species interaction with opposing effects on chorus structure. Although we do not measure species interactions directly, we follow standard approaches in assuming that such interactions predict patterns in the trait structure of assemblages (25).Open in a separate windowFig. 1.Predictions of three hypotheses proposed to structure multispecies choruses, illustrated using hypothetical seven-species choruses with signal design plotted in multivariate signaling space. The null hypothesis that species interactions have no effect predicts that signal structure is random (A), generating an intermediate mean nearest-neighbor distance d. The partitioning hypothesis predicts an evenly spaced signal structure (B) reflected in larger values for d. The network hypothesis predicts that related species will signal together, causing signals to be clustered around optima (C), and generating small values for d. We test these predictions by assessing whether d, viewed across a sample of communities, is higher or lower than expected by chance. We calculated d in two ways: d₁ (Upper) is the mean nearest neighbor distance [nnd] across all community members; and d₂ (Lower) is the mean nnd across the three pairs of species with most similar signals.Our aims were to (i) quantify acoustic properties of signals transmitted in the dawn chorus; (ii) estimate the degree of signal similarity among cosignaling species; and (iii) compare the observed distribution of signal properties with that expected by chance. We also consider spatial explanations for chorus structure, including the reduced cosignaling of species with similar signals through spatial partitioning. This form of segregation may occur because ecological competition is elevated in tropical bird communities (26), causing parapatric (27) or “checkerboard” distributions (28) among closely related species, thus potentially leading to apparent signal partitioning by competitive exclusion. The network hypothesis predicts the opposite pattern as closely related species should synchronize their signaling activity using shared territorial signals. We test these predictions by comparing 120-min (spatially segregated) and 10-min (nonsegregated) choruses and using taxonomic relatedness to estimate the degree of cosignaling between close relatives.The Amazonian dawn chorus provides one of the world’s most diverse multispecies signaling assemblages and an ideal system for exploring the effects of competition on signaling strategies for three reasons. First, visibility is hampered by dense vegetation, and thus long-distance signaling is forced into one modality (acoustic communication). Second, background noise levels are extremely high as a result of other organisms, including insects, amphibians, and primates, suggesting that selection for partitioning of acoustic signals should be maximized (12). Finally, many tropical species are permanently resident and apparently interspecifically territorial, using acoustic signals to mediate competitive interactions with heterospecifics (18, 26, 29). In combination, these factors imply that large numbers of species compete both for ecological resources and a narrow window of optimal signaling space (7, 30), providing a context in which to test the relative importance of acoustic partitioning and interspecific communication networks.     

From the Cover: Modulation of orthogonal body waves enables high maneuverability in sidewinding locomotion

Many organisms move using traveling waves of body undulation, and most work has focused on single-plane undulations in fluids. Less attention has been paid to multiplane undulations, which are particularly important in terrestrial environments where vertical undulations can regulate substrate contact. A seemingly complex mode of snake locomotion, sidewinding, can be described by the superposition of two waves: horizontal and vertical body waves with a phase difference of ±90°. We demonstrate that the high maneuverability displayed by sidewinder rattlesnakes (Crotalus cerastes) emerges from the animal’s ability to independently modulate these waves. Sidewinder rattlesnakes used two distinct turning methods, which we term differential turning (26° change in orientation per wave cycle) and reversal turning (89°). Observations of the snakes suggested that during differential turning the animals imposed an amplitude modulation in the horizontal wave whereas in reversal turning they shifted the phase of the vertical wave by 180°. We tested these mechanisms using a multimodule snake robot as a physical model, successfully generating differential and reversal turning with performance comparable to that of the organisms. Further manipulations of the two-wave system revealed a third turning mode, frequency turning, not observed in biological snakes, which produced large (127°) in-place turns. The two-wave system thus functions as a template (a targeted motor pattern) that enables complex behaviors in a high-degree-of-freedom system to emerge from relatively simple modulations to a basic pattern. Our study reveals the utility of templates in understanding the control of biological movement as well as in developing control schemes for limbless robots.Propagating waves of flexion along the axis of a long, slender body (henceforth “axial waves”) to produce propulsion is common in biological locomotion in aquatic and terrestrial environments. The majority of biological studies of axial wave propulsion at different scales have occurred in aquatic environments (1, 2). Understanding the efficacy of given wave patterns—which are often assumed to act in a single plane (e.g., mediolateral axial bending)—can be gained through full solution of the equations of hydrodynamics (3) or approximations (4). Terrestrial environments such as sand, mud, and cluttered heterogeneous substrates encountered by limbless axial undulators such as snakes can display similar (if not greater) complexity, yet far less attention has been paid to such locomotion (5, 6).Snake axial propulsion in terrestrial environments differs from fluid locomotion in two key ways. First, most substrates are not yet described at the level of fluids (7), making it a challenge to understand how substrate–body interactions affect locomotor performance, and therefore requiring robotic physical models. Second, the body may be both laterally and/or dorsoventrally flexed (5) to allow different elements of the body to contact (or clear) the substrate and thereby control friction, drag, and substrate reaction forces. Recently progress has been made in understanding how multiplane control allows effective limbless terrestrial locomotion. In studies of laterally undulating snakes (8), a single-plane frictional force model (with drag anisotropy assumed to result from the frictional anisotropy in snake scales) proved insufficient to predict the lateral undulation locomotion performance of these snakes. The authors proposed, modeled, and visualized a mechanism of “dynamic balancing” in which regions of the body with small curvature were preferentially loaded; the addition of this mechanism to their model improved agreement with experiment.A peculiar gait called sidewinding provides an excellent example of the importance of multiplane wave movement in certain snakes (5, 9). During sidewinding alternating sections of the body are cyclically lifted from the substrate, moved forward via lateral axial waves, and placed into static contact with the substrate at a new location (Fig. 1 A and B). Owing to the vertical undulatory wave, which controls lifting and contact, the snake can minimize or eliminate drag forces on nonpropulsive portions of the body (Fig. 1B), thereby enabling minimal slip at other segments and potentially contributing to a low cost of transport (10). This makes sidewinding particularly attractive to explore aspects of multiplane undulation because lifting perpendicular to the plane of the main axial wave is clearly observable and fundamental to this mode of locomotion (5); failure to lift results in locomotor failure in other vipers (9). In addition, field observations show that sidewinders are remarkably maneuverable, capable of rapidly making large direction changes to elude capture.Open in a separate windowFig. 1.Sidewinding in C. cerastes. (A) A sidewinder rattlesnake (C. cerastes) performs sidewinding locomotion on sand. (Inset) A 1- × 2-m fluidized bed trackway filled with sand from the capture locality (Yuma, Arizona); gray arrows indicate airflow used to reset the granular surface using a fluidized bed (see Materials and Methods). (B) A diagram of sidewinding in a snake. Gray regions on the snake’s body indicate regions of static contact with the ground, whereas white regions are lifted and moving. Tracks are shown in gray rectangles. Points on the final snake indicate approximate marker locations used in our experiments. The red arrow indicates direction of motion of the estimated center of mass. (C) Horizontal and vertical body waves during straight sidewinding with the head to the right, as seen in A and B, offset by a phase difference (ϕ) of −90°. Gray regions indicate static contact. The arrow depicts the posterior propagation of waves down the body. Although depicted as sinusoidal waves here, the waves may (and often do) have other forms.Another advantage to studying sidewinding is the existence of a snake-like robot capable of performing effective sidewinding locomotion; our previous robot experiments using this “physical model” (9) revealed that despite the anatomical complexity associated with hundreds of body elements and thousands of muscles (11) biological sidewinding could be simply modeled as a combination of two axial waves: horizontal and vertical axial waves with identical spatial and temporal frequency but different amplitudes offset by a phase difference (?) of ±90° (Fig. 1C) (9). Using this scheme, we showed that sidewinders ascend granular inclines by modulating the vertical wave amplitude to control contact length, and implementing this strategy in a snake robot allowed the device to ascend similar inclines (9). We proposed more broadly that the appropriate modulation of the two waves was in fact a control “template,” defined as a behavior that contains the smallest number of variables and parameters that exhibits a behavior of interest (12). Templates provide relatively simple targets for motion control and feedback responses (13) and have proved useful to understand locomotion of legged runners (14), climbers (15), and undulatory swimmers (16).In this paper, we make the first systematic observations of turning behaviors in sidewinder rattlesnakes then show that the seemingly complex turning behaviors can be explained as modulations of the template by applying these modulations of our robotic physical model. This reveals both that the two-wave template describes the motions of the animal and that maneuvers can be achieved through independent amplitude and phase modulation of the two waves. We also go beyond biological observations to generate maneuvers on the robot that are not observed in the animals.     

From the Cover: Cognitive control in media multitaskers

Chronic media multitasking is quickly becoming ubiquitous, although processing multiple incoming streams of information is considered a challenge for human cognition. A series of experiments addressed whether there are systematic differences in information processing styles between chronically heavy and light media multitaskers. A trait media multitasking index was developed to identify groups of heavy and light media multitaskers. These two groups were then compared along established cognitive control dimensions. Results showed that heavy media multitaskers are more susceptible to interference from irrelevant environmental stimuli and from irrelevant representations in memory. This led to the surprising result that heavy media multitaskers performed worse on a test of task-switching ability, likely due to reduced ability to filter out interference from the irrelevant task set. These results demonstrate that media multitasking, a rapidly growing societal trend, is associated with a distinct approach to fundamental information processing.     

From the Cover: Conflict translates environmental and social risk into business costs

Sustainability science has grown as a field of inquiry, but has said little about the role of large-scale private sector actors in socio-ecological systems change. However, the shaping of global trends and transitions depends greatly on the private sector and its development impact. Market-based and command-and-control policy instruments have, along with corporate citizenship, been the predominant means for bringing sustainable development priorities into private sector decision-making. This research identifies conflict as a further means through which environmental and social risks are translated into business costs and decision making. Through in-depth interviews with finance, legal, and sustainability professionals in the extractive industries, and empirical case analysis of 50 projects worldwide, this research reports on the financial value at stake when conflict erupts with local communities. Over the past decade, high commodity prices have fueled the expansion of mining and hydrocarbon extraction. These developments profoundly transform environments, communities, and economies, and frequently generate social conflict. Our analysis shows that mining and hydrocarbon companies fail to factor in the full scale of the costs of conflict. For example, as a result of conflict, a major, world-class mining project with capital expenditure of between US$3 and US$5 billion was reported to suffer roughly US$20 million per week of delayed production in net present value terms. Clear analysis of the costs of conflict provides sustainability professionals with a strengthened basis to influence corporate decision making, particularly when linked to corporate values. Perverse outcomes of overemphasizing a cost analysis are also discussed.Large-scale natural resource extraction projects (including exploration and processing activities) profoundly transform environments, communities, and economies, and often generate social conflict (2, 3). Previous studies of resource extraction and conflict have highlighted the relationship between mining and hydrocarbon resources and broader civil conflict (4, 5) and individual cases of project level conflict (6, 7). In this study, we investigate the importance of company–community conflict in the context of regulation of the sustainability performance of mining and hydrocarbon companies. We estimate the cost of social conflict to companies, determine how companies interpret this conflict, and explain how they respond to conflict. Costs were understood broadly as the negative impacts of company–community conflict on a company’s tangible and intangible assets, including value erosion. Conflict is defined as the coexistence of aspirations, interests, and world views that cannot be met simultaneously, or that actors do not perceive as being subject to simultaneous satisfaction, and is viewed in this assessment as ranging from low-level tension to escalated situations involving a complete relationship breakdown or violence (8).There is growing appreciation that unmitigated environmental and social risks have the potential to negatively influence the financial success of large-scale developments in the extractive industries. A 2008 study of 190 projects operated by the major international oil companies showed that the time taken for projects to come on-line nearly doubled in the preceding decade, causing significant increases in costs (9), although this increase reflects project remoteness, scale, technical difficulty, and input price, as well as social conflict. A follow-up of a subset of those projects found that nontechnical risks accounted for nearly one-half of the total risks faced by these companies, and that risks related to company relationships with other social actors constituted the single largest category (10). A separate empirical study of 19 publicly traded junior gold-mining companies found two-thirds of the market capitalization of these firms was a function of the firm’s stakeholder engagement practices, whereas only one-third was a function of the value of gold in the ground (11).In its analysis of socio-ecological systems (SESs), the sustainability science literature has said little about the large-scale private sector as an important actor within, and regulator of, SES behavior. A review of the 450 sustainability science articles published in PNAS, for example, finds just 23 referring to “corporate,” “industry,” “private sector,” or “company” in their texts. An extensive word cloud produced by a historical review of 20,000 papers related to sustainability science (12) notes just five terms implying a focus on the private sector (“corporate social,” “corporate sustainability,” “social responsibility,” “industrial ecology,” and “supply chain”), with none of these terms invoking core company decision making, culture, or calculations. However, large-scale corporate actors are obviously of central importance to the “major questions” for research in sustainability science (13), and perhaps especially the questions: “What shapes the long-term trends and transitions that provide the major directions for this century?” and “What determines the adaptability, vulnerability, and resilience of human–environment systems?” (13).The relevance of private sector actors is particularly clear in the extractive industries where, given the evolution of technology and industrial structure in these sectors, large enterprises have become highly influential actors in SES dynamics. Dramatic events and disasters, such as the Deepwater Horizon in the Gulf of Mexico, make this clear. Such enterprises can also be critical actors in slower processes of SES change, such as those mediating the relationships among water, agriculture, livelihoods, mining, and climate change (14, 15). Companies in the extractive industries have, to greater or lesser extent, developed policies for sustainable development and used sustainability professionals to respond to the changes induced by their activities on SESs. It is therefore important to understand the drivers of company behavior to build adequate models of socio-ecological change.This study addresses one potential driver of company behavior: conflicts motivated by the social and environmental risks created by, and the impacts of, corporate activities. More specifically, the study understands social conflict as a means through which populations communicate perceptions of risk and which generate costs for companies. The study refers to risk from the perspective of the entity experiencing the risk (i.e., environmental risks are risks to the environment; social risks are risks to society, social groups, or individuals; and business risks are the risks to the business). We ask about the significance of the costs associated with community conflict to companies, how far companies are prepared to respond to these costs by seeking strategies to reduce the environmental and social risk that they generate within SESs, and the conditions that can induce regulatory and strategic change within the corporate sector itself such that it reduces any negative environmental and social impacts.Although the report addresses just one dimension of large-scale private sector activity, the purpose is to suggest the importance of paying far more attention to corporate behavior in studies of socio-ecological dynamics. Emerging research on large-scale land acquisitions, or “land grabs” (16), and the implications for land-change science (17) suggests the same need to attend to corporate actors in sustainability science. In addressing this theme, our primary purpose is to map out, explore, and identify (rather than test) particular relationships between large-scale business and SES dynamics. The intent of the research is to build SES theory in ways that treat corporate behavior as endogenous to these systems.Through in-depth confidential interviews with corporate finance, legal, and sustainability professionals, and empirical case analysis, we investigate the extent to which recognition of the costs of conflict has the potential to change the ways in which companies address the environmental and social risks of mining and hydrocarbon development. Case studies combined desk-based analysis of secondary materials with key informant interviews to confirm or supplement the analysis. Case studies were used to characterize the types of company–community conflicts affecting mining projects, the point at which conflict took effect within the project cycle, and the types of effects that conflict appeared to have on projects. Key informant interviews were used to address how large-scale mining and hydrocarbon companies interpret these conflicts, how they respond to them, the factors determining different types of company response, and the extent to which calculations of the financial costs of conflict change the ways in which companies respond.     

From the Cover: Inferring influenza dynamics and control in households

Household-based interventions are the mainstay of public health policy against epidemic respiratory pathogens when vaccination is not available. Although the efficacy of these interventions has traditionally been measured by their ability to reduce the proportion of household contacts who exhibit symptoms [household secondary attack rate (hSAR)], this metric is difficult to interpret and makes only partial use of data collected by modern field studies. Here, we use Bayesian transmission model inference to analyze jointly both symptom reporting and viral shedding data from a three-armed study of influenza interventions. The reduction in hazard of infection in the increased hand hygiene intervention arm was 37.0% [8.3%, 57.8%], whereas the equivalent reduction in the other intervention arm was 27.2% [−0.46%, 52.3%] (increased hand hygiene and face masks). By imputing the presence and timing of unobserved infection, we estimated that only 61.7% [43.1%, 76.9%] of infections met the case criteria and were thus detected by the study design. An assessment of interventions using inferred infections produced more intuitively consistent attack rates when households were stratified by the speed of intervention, compared with the crude hSAR. Compared with adults, children were 2.29 [1.66, 3.23] times as infectious and 3.36 [2.31, 4.82] times as susceptible. The mean generation time was 3.39 d [3.06, 3.70]. Laboratory confirmation of infections by RT-PCR was only able to detect 79.6% [76.5%, 83.0%] of symptomatic infections, even at the peak of shedding. Our results highlight the potential use of robust inference with well-designed mechanistic transmission models to improve the design of intervention studies.The household offers an ideal setting to study the transmission dynamics of viral respiratory pathogens (1–5) and, during periods of severe epidemics, to intervene and reduce the number of infections (6). Therefore, it is also the ideal setting in which to conduct trials of interventions designed to reduce infectivity and susceptibility. The known-index trial design has been used to measure the efficacy of different types of intervention in recent years, including nonpharmaceutical interventions (7–9), antivirals (10), and vaccines (11–13). In these studies, symptomatic individuals are recruited at a health care facility and asked if they—and potentially other members of their household—may want to participate in the trial. If the index agrees, biological samples are taken at that time in the clinic. Follow-ups normally occur in the household, with the first visit as soon after the recruitment of the index as possible. If other members of the household agree to participate, samples are taken at regular intervals after that first follow-up from the index and additional participating household members. Biological samples used in these studies include nasal or throat swabs, nasopharyngeal aspirates, and blood samples. Many different assays can be conducted on the samples (depending to some extent on the sample handling protocol), for example, rapid tests (14), RT-PCR (7, 15), and B-cell assays (16). Participants may also be asked to record symptoms in a diary or to report them over the phone.The primary outcome measure for these trials is the household secondary attack rate (hSAR) (sometimes called secondary infection risk). The hSAR is most commonly defined as the proportion of nonindex household members who become cases, according to prespecified criteria, during the period of the study. Cases are usually defined in terms of either symptoms or virological outcome (e.g., PCR-confirmed infection), or sometimes both (7). Although significant reductions in hSAR between study arms are indicative of an effect, the amplitude of differences in hSAR can be difficult to interpret, partly because the statistic itself is dependent on the assays used and on the precise follow-up protocol. For example, criteria based on symptoms may fail to capture asymptomatic infections, and RT-PCR tests are sensitive to the frequency and timing of sampling. Also, the observed value of the hSAR in any specific household must be sensitive to the number of household members who participate, the precise timing of follow-up samples, and the pattern of any dropout.Previous studies have analyzed the transmission dynamics of influenza in households by using household models and symptomatic data (3, 17), and also symptomatic data in conjunction with RT-PCR laboratory results (18). We defined a stochastic household transmission model, building on these works, that described the effect of interventions in reducing the daily hazard of infection, and estimated parameters of the model using Markov chain Monte Carlo (McMC) techniques (see Materials and Methods and SI Text).     

From the Cover: Amplifying the response of soft actuators by harnessing snap-through instabilities

Soft, inflatable segments are the active elements responsible for the actuation of soft machines and robots. Although current designs of fluidic actuators achieve motion with large amplitudes, they require large amounts of supplied volume, limiting their speed and compactness. To circumvent these limitations, here we embrace instabilities and show that they can be exploited to amplify the response of the system. By combining experimental and numerical tools we design and construct fluidic actuators in which snap-through instabilities are harnessed to generate large motion, high forces, and fast actuation at constant volume. Our study opens avenues for the design of the next generation of soft actuators and robots in which small amounts of volume are sufficient to achieve significant ranges of motion.The ability of elastomeric materials to undergo large deformation has recently enabled the design of actuators that are inexpensive, easy to fabricate, and only require a single source of pressure for their actuation, and still achieve complex motion (1–5). These unique characteristics have allowed for a variety of innovative applications in areas as diverse as medical devices (6, 7), search and rescue systems (8), and adaptive robots (9–11). However, existing fluidic soft actuators typically show a continuous, quasi-monotonic relation between input and output, so they rely on large amounts of fluid to generate large deformations or exert high forces.By contrast, it is well known that a variety of elastic instabilities can be triggered in elastomeric films, resulting in sudden and significant geometric changes (12, 13). Such instabilities have traditionally been avoided as they often represent mechanical failure. However, a new trend is emerging in which instabilities are harnessed to enable new functionalities. For example, it has been reported that buckling can be instrumental in the design of stretchable soft electronics (14, 15), and tunable metamaterials (16–18). Moreover, snap-through transitions have been shown to result in instantaneous giant voltage-triggered deformation (19, 20).Here, we introduce a class of soft actuators comprised of interconnected fluidic segments, and show that snap-through instabilities in these systems can be harnessed to instantaneously trigger large changes in internal pressure, extension, shape, and exerted force. By combining experiments and numerical tools, we developed an approach that enables the design of customizable fluidic actuators for which a small increment in supplied volume (input) is sufficient to trigger large deformations or high forces (output).Our work is inspired by the well-known two-balloon experiment, in which two identical balloons, inflated to different diameters, are connected to freely exchange air. Instead of the balloons becoming equal in size, for most cases the smaller balloon becomes even smaller and the balloon with the larger diameter further increases in volume (Movie S1). This unexpected behavior originates from the balloons’ nonlinear relation between pressure and volume, characterized by a pronounced pressure peak (21, 22). Interestingly, for certain combinations of interconnected balloons, such nonlinear response can result in snap-through instabilities at constant volume, which lead to significant and sudden changes of the membranes’ diameters (Figs. S1 and ​andS2).S2). It is straightforward to show analytically that these instabilities can be triggered only if the pressure–volume relation of at least one of the membranes is characterized by (i) a pronounced initial peak in pressure, (ii) subsequent softening, and (iii) a final steep increase in pressure (Analytical Exploration: Response of Interconnected Spherical Membranes Upon Inflation).Open in a separate windowFig. S1.Relation between the pressure and volume for three different hyperelastic spherical membranes upon inflation.Open in a separate windowFig. S2.Response of two interconnected spherical membranes upon inflation. The response of the individual membranes is shown in Fig. S1. The equilibrium states and their stability have been obtained numerically. The pressure–volume relation and the relation between the volume of the individual membranes are shown for systems comprising (A) membranes a and b, (B) membranes b and c, and (C) membranes a and c.     

From the Cover: Habitual control of goal selection in humans

Humans choose actions based on both habit and planning. Habitual control is computationally frugal but adapts slowly to novel circumstances, whereas planning is computationally expensive but can adapt swiftly. Current research emphasizes the competition between habits and plans for behavioral control, yet many complex tasks instead favor their integration. We consider a hierarchical architecture that exploits the computational efficiency of habitual control to select goals while preserving the flexibility of planning to achieve those goals. We formalize this mechanism in a reinforcement learning setting, illustrate its costs and benefits, and experimentally demonstrate its spontaneous application in a sequential decision-making task.The distinction between habitual and planned action is fundamental to behavioral research (1–4). Habits enable computationally efficient decision making, but at the cost of behavioral flexibility. They form as stimulus–response pairings are “stamped in” following reward, as in Thorndike’s law of effect (3). Planning, in contrast, enables more flexible and productive decision making. It is accomplished by first searching over a causal model linking candidate actions to their expected outcomes and then selecting actions based on their anticipated rewards. Planning imposes a severe computational cost, however, as the size and complexity of a model grows.Past research emphasizes the competition between habitual and planned control of behavior (5, 6). Habitual control is favored when an individual has extensive experience with a task and when the optimal behavior policy is relatively consistent across time; meanwhile, planning is favored for novel tasks and when the optimal policy is variable, provided that an agent represents an adequate model of their task (7).Methods of integrating habitual and planned control have received less attention (8–10), yet real-world tasks often favor elements of each. Consider, for instance, a seasoned journalist who reports on new events each day. At a high level of abstraction, her reporting is structured around a repetitive series of goal-directed actions: follow leads, interview sources, evade meddling editors, etc. Because these actions are reliably valuable for any news event, their selection is an excellent candidate for habitual control. The concrete steps necessary to carry out any individual action will be highly variable, however—optimal behavior when interviewing a pop star may be suboptimal when interviewing the Pope. Thus, the implementation of the abstract actions is an excellent candidate for planning. This example illustrates the utility of nesting elements of both habits and plans in a hierarchy of behavioral control (11–13).Indeed, it is widely recognized that humans mentally organize their behavior around hierarchically organized goals and subgoals (3, 14, 15). In principle, hierarchical organization can be implemented exclusively by habitual control (16), or exclusively by planning (13, 17). However, these homogenous mechanisms foreclose the possibility of tailoring the means of control (habit vs. planning) to the affordances of a particular level of behavioral abstraction. Our aim is to show that humans solve this dilemma by exerting habitual control over the process of goal selection, while using planning to attain the goal once selected.Traditionally, habits are modeled as a learned association between a perceptual stimulus and motor response. Our proposal entails an extension of habit learning to the relation between superordinate and subordinate goals: a superordinate goal can serve as the internally represented stimulus triggering a cognitive response of subordinate goal selection. Thus, for instance, the goal of getting an interview with a key source might be stamped in due to the history of reward associated with selecting this goal in past news-reporting episodes.Colloquially, this captures the idea of a “habit of thought”: habitual control can contribute to the effective deployment of cognitive routines that facilitate productive and flexible cognition. This proposal is consonant with recent research emphasizing the pervasive role of model-free control in related elements of higher-level cognition (18, 19), including the gating of working memory (20) and the construction of hierarchical task representations (21). These models offer an appealing functional explanation for the neuronal connections between striatum and frontal cortex (22).     

From the Cover: Femtosecond quantum control of molecular bond formation

Ultrafast lasers are versatile tools used in many scientific areas, from welding to eye surgery. They are also used to coherently manipulate light–matter interactions such as chemical reactions, but so far control experiments have concentrated on cleavage or rearrangement of existing molecular bonds. Here we demonstrate the synthesis of several molecular species starting from small reactant molecules in laser-induced catalytic surface reactions, and even the increase of the relative reaction efficiency by feedback-optimized laser pulses. We show that the control mechanism is nontrivial and sensitive to the relative proportion of the reactants. The control experiments open up a pathway towards photocatalysis and are relevant for research in physics, chemistry, and biology where light-induced bond formation is important.     

From the Cover: Disentangling the role of environmental and human pressures on biological invasions across Europe

The accelerating rates of international trade, travel, and transport in the latter half of the twentieth century have led to the progressive mixing of biota from across the world and the number of species introduced to new regions continues to increase. The importance of biogeographic, climatic, economic, and demographic factors as drivers of this trend is increasingly being realized but as yet there is no consensus regarding their relative importance. Whereas little may be done to mitigate the effects of geography and climate on invasions, a wider range of options may exist to moderate the impacts of economic and demographic drivers. Here we use the most recent data available from Europe to partition between macroecological, economic, and demographic variables the variation in alien species richness of bryophytes, fungi, vascular plants, terrestrial insects, aquatic invertebrates, fish, amphibians, reptiles, birds, and mammals. Only national wealth and human population density were statistically significant predictors in the majority of models when analyzed jointly with climate, geography, and land cover. The economic and demographic variables reflect the intensity of human activities and integrate the effect of factors that directly determine the outcome of invasion such as propagule pressure, pathways of introduction, eutrophication, and the intensity of anthropogenic disturbance. The strong influence of economic and demographic variables on the levels of invasion by alien species demonstrates that future solutions to the problem of biological invasions at a national scale lie in mitigating the negative environmental consequences of human activities that generate wealth and by promoting more sustainable population growth.     

From the Cover: Solar anti-icing surface with enhanced condensate self-removing at extreme environmental conditions

The inhibition of condensation freezing under extreme conditions (i.e., ultra-low temperature and high humidity) remains a daunting challenge in the field of anti-icing. As water vapor easily condensates or desublimates and melted water refreezes instantly, these cause significant performance decrease of most anti-icing surfaces at such extreme conditions. Herein, inspired by wheat leaves, an effective condensate self-removing solar anti-icing/frosting surface (CR-SAS) is fabricated using ultrafast pulsed laser deposition technology, which exhibits synergistic effects of enhanced condensate self-removal and efficient solar anti-icing. The superblack CR-SAS displays superior anti-reflection and photothermal conversion performance, benefiting from the light trapping effect in the micro/nano hierarchical structures and the thermoplasmonic effect of the iron oxide nanoparticles. Meanwhile, the CR-SAS displays superhydrophobicity to condensed water, which can be instantly shed off from the surface before freezing through self-propelled droplet jumping, thus leading to a continuously refreshed dry area available for sunlight absorption and photothermal conversion. Under one-sun illumination, the CR-SAS can be maintained ice free even under an ambient environment of −50 °C ultra-low temperature and extremely high humidity (ice supersaturation degree of ∼260). The excellent environmental versatility, mechanical durability, and material adaptability make CR-SAS a promising anti-icing candidate for broad practical applications even in harsh environments.

Condensation freezing/frosting on solid surfaces causes severe economic and safety issues. Thus, highly efficient anti-icing/frosting approaches are vital in many engineering applications, ranging from air conditioners to power transmission systems (1–4). Tremendous efforts have been invested into designing active and passive anti-icing surfaces. Active anti-icing strategies including mechanical, chemical, and thermal methods often consume high amounts of energy and require specific facilities for deicing, which limit their practical applications (5, 6). Passive anti-icing surfaces involve strategies to delay and inhibit ice formation, such as hydrated surfaces, lubricant-infused surfaces, bioinspired anti-freezing surfaces, and superhydrophobic surfaces (SHSs) (3, 7–13). Although these passive icephobic materials offer numerous advantages to prevent ice accretion, each comes with its own limitations (14).At extreme environmental conditions where condensation and desublimation are strongly promoted, an optimal passive anti-icing surface should immediately remove condensed water droplets and leave no water for freezing. To prepare such a kind of surface, lots of efforts have been made to study the abilities of SHSs for removing condensed water. However, regular SHSs are incapable of removing microscale condensed water droplets under humid conditions due to the wetting of surface micronanostructures, which leads to the formation of highly pinned Wenzel droplets and the loss of superhydrophobicity (15–17). Tremendous investigations have shown that SHSs with specially designed structures can retain superhydrophobicity to condensed water droplets, and coalesced droplets can be spontaneously removed from the surface via self-propelled jumping (18–20). The self-propelled jumping of condensed droplets is driven by the released surface energy during droplet coalescence after overcoming solid–liquid adhesion (21–25). However, these surfaces inevitably lose their water repellency at low temperatures (i.e., < −15 °C) because of freezing (9, 21, 26). Thus, it is highly desirable to design new anti-icing surfaces that can maintain capability for removing the condensed water at extreme environmental conditions, which is highly important for anti-icing applications in many scenarios (i.e., aircrafts flying through clouds, wind turbines operating in winter, and power transmission facilities working in extremely cold and humid regions) (27).Recently, intensive research efforts have been dedicated to solar anti-icing/frosting surfaces (SASs), which can absorb sunlight efficiently and convert solar energy to heat, thereby delaying or preventing ice formation (28–30). Because of its effective utilization of clean and renewable solar energy, SASs are environmentally friendly and energy efficient. Notably, a number of photothermal conversion materials including carbon materials, conjugated polymers, two-dimensional nanostructural materials, and metallic particles have also been applied as SASs (28, 31–34). Although remarkable improvements have been made, some challenges have yet to be tackled. For instance, most of the reported SASs cannot remove the condensed water effectively, particularly in cold and humid conditions (35, 36). The accumulation of the condensed water would significantly enhance reflectance leading to reduced photothermal efficiency (37) and decreased temperature. As a result, frost formation through the freezing of condensed water (condensation frosting) will prevent sunlight from reaching the SAS, resulting in the complete loss of its photothermal capability: as light harvesting becomes less efficient, the temperature of the SAS decreases, resulting in a vicious compounding cycle of condensation freezing. Moreover, the contaminants on the SAS can also inhibit sunlight absorption (38). Therefore, it is desirable to develop highly photothermal–efficient SASs with the abilities of removing condensed water to maintain high temperature and self-cleaning for avoiding blockage of sunlight by contaminants.Herein, we present a proof-of-concept SAS with synergistically binary effects of enhanced condensate self-removing and efficient solar anti-icing. We fabricated hierarchically structured materials using ultrafast pulsed laser deposition (PLD) technology. Low-effective refractive index and multilayered iron oxide structures endow the material with broadband ultralow reflectance and high solar-to-heat conversation efficiency. The hierarchically structured SHS demonstrated the capability of removing condensates before freezing via self-propelled droplet jumping induced by droplet coalescence and evaporation flux under heating. The sustained shedding of condensed water droplets resulted in a continuously refreshed, dry and clean area available for efficient sunlight absorption and photothermal conversion; the temperature of the condensate self-removing solar anti-icing/frosting surface (CR-SAS) could be constantly maintained above the freezing temperature, which in turn ensured high-performance water repellency. With such synergistic mutual-benefitting effects of condensate self-removal and photothermal heating, under one-sun illumination, the CR-SAS remained ice-free even at an ambient temperature (T_a) of −50 °C and ultra-high humidity with a supersaturation degree (SSD) of ∼260, demonstrating superior anti-icing performances under extremely harsh operating conditions.     

From the Cover: Stabilized detonation for hypersonic propulsion

Future terrestrial and interplanetary travel will require high-speed flight and reentry in planetary atmospheres by way of robust, controllable means. This, in large part, hinges on having reliable propulsion systems for hypersonic and supersonic flight. Given the availability of fuels as propellants, we likely will rely on some form of chemical or nuclear propulsion, which means using various forms of exothermic reactions and therefore combustion waves. Such waves may be deflagrations, which are subsonic reaction waves, or detonations, which are ultrahigh-speed supersonic reaction waves. Detonations are an extremely efficient, highly energetic mode of reaction generally associated with intense blast explosions and supernovas. Detonation-based propulsion systems are now of considerable interest because of their potential use for greater propulsion power compared to deflagration-based systems. An understanding of the ignition, propagation, and stability of detonation waves is critical to harnessing their propulsive potential and depends on our ability to study them in a laboratory setting. Here we present a unique experimental configuration, a hypersonic high-enthalpy reaction facility that produces a detonation that is fixed in space, which is crucial for controlling and harnessing the reaction power. A standing oblique detonation wave, stabilized on a ramp, is created in a hypersonic flow of hydrogen and air. Flow diagnostics, such as high-speed shadowgraph and chemiluminescence imaging, show detonation initiation and stabilization and are corroborated through comparison to simulations. This breakthrough in experimental analysis allows for a possible pathway to develop and integrate ultra-high-speed detonation technology enabling hypersonic propulsion and advanced power systems.

Achieving high-speed flight at supersonic and hypersonic speeds is now a national priority and an international focus. To achieve this ultimate goal, highly energetic propulsion modes are needed to drive the vehicles (1). One set of new concepts, detonation-based engines, could play an important role in making space exploration and intercontinental travel as routine as intercity travel is today (2).Detonation-based propulsion systems are a transformational technology for maintaining the technological superiority of high-speed propulsion and power systems (3). These systems include gas turbine engines, afterburning jet engines, ramjets, scramjets, and ram accelerators. Detonation is an innovative scheme for hypersonic propulsion that considerably increases thermodynamic cycle efficiencies (∼10 to 20%) as compared to traditional deflagration based cycles (4, 5). Even for applications where there are no additional thermodynamic benefits, detonation-based cycles have shown to provide enhanced combustion efficiency like ram rotating detonation engines (6). Research advancement in ultrahigh-speed detonation systems will help to realize and develop this technological advantage over existing propulsion and power systems.A detonation is a supersonic combustion wave that consists of a shock wave driven by energy release from closely coupled chemical reactions. These waves travel at many times the speed of sound, often reaching speeds of Mach 5, as in the case of a hydrogen–air fuel mixture. An engine operating with a Mach 5 flow path corresponds to a vehicle flight Mach number of 6 to 17 (7–9). That is comparable to a half-hour flight from New York to London and is 5 times faster than the average time it took the legendary Concorde to complete the same journey. The idea of using detonation waves for propulsion and energy generation is not new (3), although the implementation of this concept has been difficult. Three main categories of detonation engine concepts have received significant research attention: pulse detonation engines (5, 10–12), rotating detonation engines (13–15), and standing and oblique detonation wave engines (ODWE) (3, 7, 16–18). The ODWE is of particular interest here for its theoretical ability to propel hypersonic aircraft to the speeds needed for spaceplanes and other reusable space launch vehicles. Fig. 1 shows a conceptual hypersonic vehicle powered by an ODWE and illustrates the relation to the experimental and computational results of this study. The challenge in developing these engine concepts is finding reliable mechanisms for detonation initiation and robust stabilization of these waves in the high-speed, high-enthalpy conditions that would be expected of these engine concepts.Open in a separate windowFig. 1.Schematic of oblique detonation engine concept. The experimental and computational ODW domains are highlighted along with their location in the engine flow path.Laboratory experiments and numerical simulations have shown a number of modes of detonation initiation, and numerical simulations have elucidated important underlying concepts in their stabilization (19–25). Despite these advances, the problem is compounded by the historical difficulty in achieving a stabilized detonation in an experimental facility that produces realistic flight conditions which can be adapted for use in an actual engine. Previous experimental studies were unable to show a stabilized oblique detonation wave (ODW) for an extended period, due to their use of shock/expansion tubes or projectiles (7, 22, 26–28). These types of facilities have limited run times, on the order of microseconds or milliseconds. Another major difficulty in stabilizing the detonation wave is upstream wave propagation through the boundary layer leading to unstart with recent experiments showing deflagration-to-detonation transition in a hypersonic flow and an unstable detonation that propagated upstream (24). Several numerical studies have shown potentially steady ODW but lack experimental verification (21, 23, 29, 30). These leave uncertainty about the stability of ODW, which must be addressed through experiments capable of creating the appropriate conditions and maintaining them for an extended period.This paper reports results from a study demonstrating experimentally controlled detonation initiation and stabilization in a hypersonic flow for a situation similar to proposed flight conditions for these vehicle concepts with an active run time of several seconds. The experimental results capture the stabilized detonation, as shown in the shadowgraph and chemiluminescence images, and are further confirmed and explained by the theory and numerical simulations of the system. A 30° angle ramp is used in the high-enthalpy hypersonic reaction facility to ignite and stabilize an ODW, shown schematically in Fig. 2A. The shock-laden, high-Mach number flow induces a temperature rise to ignite and stabilize a detonation in the incoming hydrogen–air mixture. The combination of matching the flow Mach number to the MCJ conditions and low boundary layer fueling result in the stabilized detonation. Static pressure measurements confirm a pressure rise induced by the detonation wave. High-fidelity computational fluid dynamics simulations have been used to provide additional detailed insight into the detonation initiation and stabilization process.Open in a separate windowFig. 2.(A) HyperReact. (B) Nonreacting flow field and (C) stabilized ODW.