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The social structure of an animal group controls how members interact, and affects nearly every aspect of an individual’s behavior with others. Rodents are a model system for studying both individual and social behavior, and social status such as dominance affects access to mating opportunities and food resources. However, it is not known how individual behavioral patterns interact to generate the overall social structure of a group, and how individual social positions within a group change over time. In this study, we employ long-term tracking to demonstrate that individual social status and group social structure are emergent outcomes of interactions among group members, which are not simply predicted by individual traits or previous behavioral patterns. Using quantitative measures to describe individual behavior and group social structure, we demonstrate that although individuals maintain established behavioral patterns and social positions within a given group, behavioral routines shift when group composition is changed. A comparison demonstrates that traditional individual and pairwise tests for boldness, and social interest are inadequate for describing or predicting actual behavioral outcomes in a group setting.These results emphasize the context-dependence of social behavior as an emergent property of interactions within a group, and highlight the need to measure and quantify social behavior in naturalistic settings.

Many animal behaviours exhibit complex temporal dynamics, suggesting there are multiple timescales at which they should be studied. However, researchers often focus on behaviours that occur over relatively restricted temporal scales, typically ones that are more accessible to human observation. The situation becomes even more complex when considering multiple animals interacting, where behavioural coupling can introduce new timescales of importance. Here, we present a technique to study the time-varying nature of social influence in mobile animal groups across multiple temporal scales. As case studies, we analyse golden shiner fish and homing pigeons, which move in different media. By analysing pairwise interactions among individuals, we show that predictive power of the factors affecting social influence depends on the timescale of analysis. Over short timescales the relative position of a neighbour best predicts its influence and the distribution of influence across group members is relatively linear, with a small slope. At longer timescales, however, both relative position and kinematics are found to predict influence, and nonlinearity in the influence distribution increases, with a small number of individuals being disproportionately influential. Our results demonstrate that different interpretations of social influence arise from analysing behaviour at different timescales, highlighting the importance of considering its multiscale nature. This article is part of a discussion meeting issue ‘Collective behaviour through time’.

1AbstractEusocial insects operate as an integrated collective with tasks allocated among individuals. This applies also to reproduction, through coordinated mating flights between male and female reproductives. While in some species male sexuals take only a single mating flight and never return, in the Western honey bee,Apis mellifera, the male sexuals (drones) live in the colony throughout their lives. Prior research has focused almost exclusively on drone behavior outside of the nest (mating flights), while ignoring the majority of their life, which is spent inside the nest. To understand the in-nest behavior of drones across their lives, we used the BeesBook tracking system to track 192 individually-marked drones continuously for over 20 days, to examine how drones moved and spent time in the nest. In agreement with previous work, we find that drones spend most of their time immobile at the nest periphery. However, we also observe that drones have periods of in-nest hyperactivity, during which they become the most active individuals in the entire colony. This in-nest hyperactivity develops in drones after age 7 days, occurs daily in the afternoon, and coincides with drones taking outside trips. We find strong synchronization across the drones in the start/end of activity, such that the drones in the colony exhibit a “shared activation period”. The duration of the shared activation period depends on the weather; when conditions are suitable for mating flights, the activation period is extended. At the individual-level, we see that the activation order changes from day to day, suggesting that both the external influence of weather conditions, as well as exchange of social information, influences individual activation. Using an accumulation-to-threshold model of drone activation, we show that simulations using social information match experimental observations. These results provide new insight into the in-nest behavior of drones, and how their behavior reflects their role as the male gametes of the colony.

Objective To determine the impact of tumor characteristics and treatment approach on (1) local recurrence, (2) scoliosis development and (3) patient-reported quality of life in children with sarcoma of the chest wall. Summary background data Children with chest wall sarcoma require multimodal therapy including chemotherapy, surgery and/or radiation. Despite aggressive therapy which places them at risk for functional impairment and scoliosis, these patients are also at significant risk for local recurrence. Methods A multi-institutional review of 175 children (median age 13 years) with chest wall sarcoma treated at seventeen Pediatric Surgical Oncology Research Collaborative institutions between 2008-2017 was performed. Patient-reported quality of life was assessed prospectively using PROMIS surveys. Results The most common diagnoses were Ewing sarcoma (67%) and osteosarcoma (9%). Surgical resection was performed in 85% and radiation in 55%. A median of 2 ribs were resected (IQR = 1-3), and number of ribs resected did not correlate with margin status (p = 0.36). Local recurrence occurred in 23% and margin status was the only predictive factor (HR 2.24, p = 0.039). With a median follow-up of 5 years, 13% developed scoliosis (median Cobb angle 26) and 5% required corrective spine surgery. Scoliosis was associated with posterior rib resection (HR 8.43; p = 0.003) and increased number of ribs resected (HR 1.78; p = 0.02). Overall, patient-reported quality of life is not impaired following chest wall tumor resection. Conclusions Local recurrence occurs in one-quarter of children with chest wall sarcoma and is independent of tumor type. Scoliosis occurs in 13% of patients, but patient-reported quality of life is excellent.

Purpose Rapid sequence intubation (RSI) drugs, such as propofol, affect clinical outcomes, but this has not been examined in the pediatric population. This descriptive study compares the outcomes associated with intubation drugs used in pediatric traumatic brain injury (TBI) patients. Methods A retrospective chart review and descriptive analysis of intubated TBI patients, ages 0–17, admitted to Children's Hospital London Health Sciences Centre (LHSC) from January 2006–December 2016 was performed. Results Out of 259 patients intubated, complete data was available for 107 cases. Average injury severity score was 28; 46 were intubated at LHSC, 55 at primary care site, and 6 on scene. Intubation attempts were recorded in 87 of 107 paper charts. First-pass intubation success rate was 88.5%. Propofol (n = 21), midazolam (n = 31), etomidate (n = 13), and ketamine (n = 7) were the most commonly used intubation drugs. Paralytics were used in 50% of patients. Following use of propofol, Pediatric Adjusted Shock Index was increased as a result of worsening hypotension. Mean total hospital length of stay was 21 days with 7.5 days in ICU. Survival was 87%. Conclusion Great variability exists in the use of induction agents and paralytics for RSI. Propofol was commonly used and is potentially associated with poorer clinical outcomes. Type of Study Retrospective. Level of Evidence IV

Insect colonies are decentralized systems that employ task allocation, whereby individuals undertake different roles to fulfil colony needs, such as honey bee “nurses”, “nest workers”, and “foragers”. However, the extent to which individuals can be well-classified by discrete “roles”, how they change behavior from day-to-day, over entire lifetimes, and with environmental conditions, is poorly understood. Using long-term automated tracking of over 4,200 individually-identified bees Apis mellifera, we use behavioral metrics to quantify and compare behavior. We show that individuals exhibit behavioral variation along two dominant axes that represent nest substrate use and movement within the nest. Across lifetimes, we find that individuals differ in foraging onset, and that certain bees exhibit lifelong consistencies in their movement patterns. Furthermore, we examine a period of sudden nectar availability where the honey stores tripled over 6 days, and see that the colony exhibits a distributed shift in activity that did not require a large-scale colony reorganization. Our quantitative approach shows how collective units differ over days and lifetimes, and how sources of variation and variability contribute to the colony’s robust yet flexible response.

Nearly all animals forage to acquire energy for survival through efficient search and resource harvesting. Patch exploitation is a canonical foraging behaviour, but there is a need for more tractable and understandable mathematical models describing how foragers deal with uncertainty. To provide such a treatment, we develop a normative theory of patch foraging decisions, proposing mechanisms by which foraging behaviours emerge in the face of uncertainty. Our model foragers statistically and sequentially infer patch resource yields using Bayesian updating based on their resource encounter history. A decision to leave a patch is triggered when the certainty of the patch type or the estimated yield of the patch falls below a threshold. The time scale over which uncertainty in resource availability persists strongly impacts behavioural variables like patch residence times and decision rules determining patch departures. When patch depletion is slow, as in habitat selection, departures are characterized by a reduction of uncertainty, suggesting that the forager resides in a low-yielding patch. Uncertainty leads patch-exploiting foragers to overharvest (underharvest) patches with initially low (high) resource yields in comparison with predictions of the marginal value theorem. These results extend optimal foraging theory and motivate a variety of behavioural experiments investigating patch foraging behaviour.

Background To better characterize short-term and long-term outcomes in children with pancreatic tumors treated with pancreaticoduodenectomy (PD). Methods Patients 21 years of age or younger who underwent PD at Pediatric Surgical Oncology Collaborative (PSORC) hospitals between 1990 and 2017 were identified. Demographic, clinical information, and outcomes (operative complications, long-term pancreatic function, recurrence, and survival) were collected. Results Sixty-five patients from 18 institutions with a median age of 13 years (4 months-22 years) and a median (IQR) follow-up of 2.8 (4.3) years were analyzed. Solid pseudopapillary tumor of the pancreas (SPN) was the most common histology. Postoperative complications included pancreatic leak in 14% (n = 9), delayed gastric emptying in 9% (n = 6), marginal ulcer in one patient, and perioperative (30-day) death due to hepatic failure in one patient. Pancreatic insufficiency was observed in 32% (n = 21) of patients, with 23%, 3%, and 6% with exocrine, or endocrine insufficiencies, or both, respectively. Children with SPN and benign neoplasms all survived. Overall, there were 14 (22%) recurrences and 11 deaths (17%). Univariate analysis revealed non-SPN malignant tumor diagnosis, preoperative vascular involvement, intraoperative transfusion requirement, pathologic vascular invasion, positive margins, and need for neoadjuvant chemotherapy as risk factors for recurrence and poor survival. Multivariate analysis only revealed pathologic vascular invasion as a risk factor for recurrence and poor survival. Conclusion This is the largest series of pediatric PD patients. PD is curative for SPN and benign neoplasms. Pancreatic insufficiency is the most common postoperative complication. Outcome is primarily associated with histology.

The study of collective behavior asks how individual actions combine to produce a group response. Research questions encompassing self-organization, collective motion, and decision-making apply to study systems ranging from cellular systems to animal groups and human organizations. Recently, technological advancements have enabled an unprecedented increase in our ability to measure, quantify, and analyze collective behavior. However, despite similar inquires and the wealth of data available, it is difficult to make general statements about principles of collective behavior that apply across scales. Here we describe a hierarchical approach for comparing collective systems that uses the intermediate link of “mechanisms” to connect individual to group behavior. With this, we argue that an effective way to understand collective behavior across different systems is to ask the complementary questions of how individual behavior implements certain mechanisms, and how various mechanisms contribute to overall function of the group. We apply this framework to compare two systems at very different scales - honey bee colonies and cells of the body - and discuss how the mechanisms of specialization, distributed processing, feedback, and others contribute to function in each. Through this comparison, we demonstrate how the hierarchical approach can be used to compare and contrast different systems, and lead to new hypotheses forming a basis for common research questions of collective behavior.

OBJECTIVE Female medical students and surgical trainees are more likely to lack confidence in their clinical abilities than their male peers despite equal or superior performance. This study aims to examine the role of gender in medical student experience and confidence performing technical skills in surgical clerkship. DESIGN This was a single-center survey study conducted over 2 academic years (2016-2018). Students were surveyed on their experience and confidence performing a set of 9 technical skills during surgical clerkship and to identify skill-specific barriers to learning. SETTING This study was performed at Western University, London, Ontario, Canada. PARTICIPANTS All third-year medical students were invited to participate. RESULTS A total of 253 students participated yielding a survey response rate of 74.0%. Both male and female students reported similar levels of preclerkship interest in a surgical career, enjoyment in performing technical skills, confidence in ability to learn surgical skills and pursuit of available learning opportunities. At the conclusion of their surgical rotations, female students reported less experience and confidence performing technical skills compared to their male colleagues. Female students were more likely to cite an insufficient number of learning opportunities from consultant and resident teachers, time constraints, and lack of confidence as barriers to the achievement of technical proficiency. CONCLUSIONS Female gender was associated with less procedural experience and inferior confidence performing procedural skills. It is important for educators to be aware of this gender disparity and to actively promote equitable learning opportunities for female trainees.

Background: Image-guided percutaneous core needle biopsy (PCNB) is increasingly utilized to diagnose solid tumors. The objective of this study is to determine whether PCNB is adequate for modern biologic characterization of neuroblastoma. Procedure: A multi-institutional retrospective study was performed by the Pediatric Surgical Oncology Research Collaborative on children with neuroblastoma at 12 institutions over a 3-year period. Data collected included demographics, clinical details, biopsy technique, complications, and adequacy of biopsies for cytogenetic markers utilized by the Children's Oncology Group for risk stratification. Results: A total of 243 children were identified with a diagnosis of neuroblastoma: 79 (32.5%) tumor excision at diagnosis, 94 (38.7%) open incisional biopsy (IB), and 70 (28.8%) PCNB. Compared to IB, there was no significant difference in ability to accurately obtain a primary diagnosis by PCNB (95.7% vs 98.9%, P =.314) or determine MYCN copy number (92.4% vs 97.8%, P =.111). The yield for loss of heterozygosity and tumor ploidy was lower with PCNB versus IB (56.1% vs 90.9%, P

Foraging is a fundamental behavior as animals' search for food is crucial for their survival. Patch leaving is a canonical foraging behavior, but classic theoretical conceptions of patch leaving decisions lack some key naturalistic details. Optimal foraging theory provides general rules for when an animal should leave a patch, but does not provide mechanistic insights about how those rules change with the structure of the environment. Such a mechanistic framework would aid in designing quantitative experiments to unravel behavioral and neural underpinnings of foraging. To address these shortcomings, we develop a normative theory of patch foraging decisions. Using a Bayesian approach, we treat patch leaving behavior as a statistical inference problem. We derive the animals' optimal decision strategies in both non-depleting and depleting environments. A majority of these cases can be analyzed explicitly using methods from stochastic processes. Our behavioral predictions are expressed in terms of the optimal patch residence time and the decision rule by which an animal departs a patch. We also extend our theory to a hierarchical model in which the forager learns the environmental food resource distribution. The quantitative framework we develop will therefore help experimenters move from analyzing trial based behavior to continuous behavior without the loss of quantitative rigor. Our theoretical framework both extends optimal foraging theory and motivates a variety of behavioral and neuroscientific experiments investigating patch foraging behavior., 28 pages, 10 figures, 1 table

Summary Many animals, including humans, have evolved to live and move in groups. In humans, disrupted social interactions are a fundamental feature of many psychiatric disorders. However, we know little about how genes regulate social behavior. Zebrafish may serve as a powerful model to explore this question. By comparing the behavior of wild-type fish with 90 mutant lines, we show that mutations of genes associated with human psychiatric disorders can alter the collective behavior of adult zebrafish. We identify three categories of behavioral variation across mutants: “scattered,” in which fish show reduced cohesion; “coordinated,” in which fish swim more in aligned schools; and “huddled,” in which fish form dense but disordered groups. Changes in individual interaction rules can explain these differences. This work demonstrates how emergent patterns in animal groups can be altered by genetic changes in individuals and establishes a framework for understanding the fundamentals of social information processing., Graphical Abstract, Highlights • Genes linked to human psychiatric disorders can alter zebrafish collective behavior • Differences from wild-type lead to “scattered,” “coordinated,” and “huddled” behavior • Changes in individual interaction rules can explain emergent group level patterns, Biological Sciences; Genetics; Behavior Genetics; Behavioral Neuroscience

Parasitism is ubiquitous in the animal kingdom. Although many fundamental aspects of host-parasite relationships have been unravelled, few studies have systematically investigated how parasites affect organismal movement. Here we combine behavioural experiments ofSchistocephalus solidusinfected sticklebacks with individual-based simulations to understand how parasitism affects individual movement ability and how this in turn influences social interaction patterns. Detailed movement tracking revealed that infected fish swam slower, accelerated slower, turned more slowly, and tended to be more predictable in their movements than did non-infected fish. Importantly, the strength of these effects increased with increasing parasite load (% of body weight), with the behaviour of more heavily infected fish being more impaired. When grouped, pairs of infected fish moved more slowly, were less cohesive, less aligned, and less coordinated than healthy pairs. Mixed pairs exhibited intermediate behaviours and were primarily led by the non-infected fish. These social patterns emerged naturally in model simulations of self-organised groups composed of individuals with different speeds and turning tendency, consistent with changes in mobility and manoeuvrability due to infection. Together, our results demonstrate how infection with a complex life cycle parasite affects the movement ability of individuals and how this in turn shapes social interactions, providing important mechanistic insights into the effects of parasites on host movement dynamics.

Purpose The purpose of this study was to determine whether children with a positive ultrasound (US) for acute appendicitis but a negative clinical picture developed appendicitis requiring definitive management. Methods After obtaining IRB approval, we conducted a retrospective review of patients ≤17years who presented with possible acute appendicitis between April 1st, 2014, and December 31st, 2015. We included patients with a US suggestive of acute appendicitis based on size criteria but without concerning clinical features. Patients were discharged from the emergency department (ED) or admitted for observation. Variables included demographic data, US characteristics, clinical findings, length of follow-up, and appendectomy. Results Of the 31 patients identified, 45% were male and average age was 11.3yrs. On US, the average maximal diameter of the appendix was 6.93mm. The median length of follow-up was 16.8 months, including 10 returns to the ED by 9 patients. Three of these underwent immediate laparoscopic appendectomy, while one had interval appendectomy. There were no cases of perforated appendicitis, and only 2 cases demonstrated pathology consistent with appendicitis. Conclusion These findings demonstrate that it is safe to consider conservative measures such as observation or discharge in children with a positive US for appendicitis based on size criteria but a negative clinical picture. Level of Evidence 4

Hypertrophic pyloric stenosis (HPS) is a common neonatal condition treated with open or laparoscopic pyloromyotomy. 3D-printed organs offer realistic simulations to practice surgical techniques. The purpose of this study was to validate a 3D HPS stomach model and assess model reliability and surgical realism.Medical students, general surgery residents, and adult and pediatric general surgeons were recruited from a single center. Participants were videotaped three times performing a laparoscopic pyloromyotomy using box trainers and 3D-printed stomachs. Attempts were graded independently by three reviewers using GOALS and Task Specific Assessments (TSA). Participants were surveyed using the Index of Agreement of Assertions on Model Accuracy (IAAMA).Participants reported their experience levels as novice (22%), inexperienced (26%), intermediate (19%), and experienced (33%). Interrater reliability was similar for overall average GOALS and TSA scores. There was a significant improvement in GOALS (p0.0001) and TSA scores (p=0.03) between attempts and overall. Participants felt the model accurately simulated a laparoscopic pyloromyotomy (82%) and would be a useful tool for beginners (100%).A 3D-printed stomach model for simulated laparoscopic pyloromyotomy is a useful training tool for learners to improve laparoscopic skills. The GOALS and TSA provide reliable technical skills assessments.II.

The spatio-temporal distribution of individuals within a group (i.e its internal structure) plays a defining role in how individuals interact with their environment, make decisions, and transmit information via social interactions. Group-living organisms across taxa, including many species of fish, birds, ungulates, and insects, use vision as the predominant modality to coordinate their collective behavior. Despite this importance, there have been few quantitative studies examining visual detection capabilities of individuals within groups. We investigate key principles underlying individual, and collective, visual detection of stimuli (which could include cryptic predators, potential food items, etc.) and how this relates to the internal structure of groups. While the individual and collective detection principles are generally applicable, we employ a model experimental system of schooling golden shiner fish (Notemigonus crysoleucas) to relate theory directly to empirical data, using computational reconstruction of the visual fields of all individuals to do so. Our integrative approach allows us to reveal how the external visual information available to each group member depends on the number of individuals in the group, the position within the group, and the location of the external visually-detectable stimulus. We find that in small groups, individuals have detection capability in nearly all directions, while in large groups, occlusion by neighbors causes detection capability to vary with position within the group. We then formulate a simple, and generally applicable, model that captures how visual detection properties emerge due to geometric scaling of the space occupied by the group and occlusion caused by neighbors. We employ these insights to discuss principles that extend beyond our specific system, such as how collective detection depends on individual body shape, and the size and structure of the group.

An extracorporeal (ECA), transumbilical appendectomy has been proposed as a treatment for appendicitis. This study assessed the 30-day perioperative outcomes and cost between ECA and traditional intracorporeal (ICA) techniques for acute uncomplicated appendicitis.IRB approval was obtained for this retrospective cohort study of acute uncomplicated appendicitis in children aged 4 to 17 between April 2014 and April 2017. Patients were grouped based on ICA versus ECA. Operative time, length of stay, and complication rates were recorded.A total of 289 patients were included, and of these 217 underwent ICA, and 72 underwent ECA. Median weight-for-age percentile was the only demographic characteristic different between groups (ECA 50 [0.1-100] vs. ICA 71 [0-100]; p 0.01). Median operative time was significantly shorter in the ECA group (21.0 min [8.0-61.0] vs. 38.0 min [19.0-87.0]; p 0.0001). Length of stay and complication rates were similar between groups. The median per case cost was significantly lower in the ECA group (CAD$ 593.05 range: 499.70-900.81 vs. CAD$ 858.78 range: 490.36-1106.29; p 0.001).Extracorporeal transumbilical laparoscopic appendectomy is associated with shorter operative times and no increased risk of 30-day postoperative complications in children and adolescents. This offers a new operative approach that may reduce hospital cost and resources.III.

Objective Medical students are expected to achieve proficiency in a prescribed set of technical skills during surgical clerkship. However, available literature indicates students routinely report a lack of confidence and proficiency in these skills. Our study aims to identify barriers to technical skill development encountered by medical students during surgical clerkship with the goal of developing interventions to address these barriers. Design Three hundred and forty two medical students were surveyed about their experience learning and performing technical skills during surgical clerkship. Students reported confidence in skill performance and subjective barriers to achieving competency using forced-choice and free-text responses. Multivariate regression models identified factors independently associated with specific barriers and more frequent technical skills performance. Setting Main and satellite campuses of a Medical Council of Canada accredited Canadian academic medical center. Participants All third-year medical students. Results A total of 253 students (74%) responded to the survey. Following surgical clerkship, the only technical skills participants felt confident performing independently were sterile technique (96%) and basic suturing (52%). Interest in a surgical career, observership experience, gender, and medical campus site were independently associated with the frequency of skill performance. With respect to developing technical proficiency, commonly cited barriers included lack of suitable cases for student participation (35.0%), time constraints (33.4%), and lack of opportunities provided by both consultants (29.1%) and residents (24.7%). Female gender was independently associated with decreased resident instruction, decreased confidence in skill performance, and fewer opportunities to perform requisite skills. Students at satellite campuses reported fewer barriers. Conclusions We identified (1) lack of suitable cases, (2) time constraints, and (3) failure to provide students opportunities as the most common barriers to technical skill proficiency. Female gender increased the perception of barriers, while there were fewer barriers perceived by students at satellite campuses. Skill-specific simulation training and other interventions may improve skill development in medical students given obstacles to developing proficiency in the clinical setting.

BACKGROUND: Optimal management of neutropenic appendicitis (NA) in children undergoing cancer therapy remains undefined. Management strategies include upfront appendectomy or initial nonoperative management. We aimed to characterize the effect of management strategy on complications and length of stay (LOS) and describe implications for chemotherapy delay or alteration. METHODS: Sites from the Pediatric Surgery Oncology Research Collaborative performed a retrospective review of children with NA over a 6-year period. RESULTS: Sixty-six children, with a median age of 11 years (range 1–17), were identified with NA while undergoing cancer treatment. The most common cancer diagnoses were leukemia (62%) and brain tumor (12%). Upfront appendectomy was performed in 41% of patients; the remainder had initial nonoperative management. Rates of abscess or perforation at diagnosis were equivalent in the groups (30% vs 24%; P = .23). Of patients who had initial nonoperative management, 46% (17 of 37) underwent delayed appendectomy during the same hospitalization. Delayed appendectomy was due to failure of initial nonoperative management in 65% (n = 11) and count recovery in 35% (n = 6). Cancer therapy was delayed in 35% (n = 23). Initial nonoperative management was associated with a delay in cancer treatment (46% vs. 22%, P = .05) and longer LOS (29 vs 12 days; P = .01). Patients who had initial nonoperative management and delayed appendectomy had a higher rate of postoperative complications (P CONCLUSIONS: In pediatric patients with NA from oncologic treatment, upfront appendectomy resulted in lower complication rates, reduced LOS, and fewer alterations in chemotherapy regimens compared to initial nonoperative management.

Local interactions, when individuals meet, can regulate collective behaviour. In a system without any central control, the rate of interaction may depend simply on how the individuals move around. But interactions could in turn influence movement; individuals might seek out interactions, or their movement in response to interaction could influence further interaction rates. We develop a general framework to address these questions, using collision theory to establish a baseline expected rate of interaction based on proximity. We test the models using data from harvester ant colonies. A colony uses feedback from interactions inside the nest to regulate foraging activity. Potential foragers leave the nest in response to interactions with returning foragers with food. The time series of interactions and local density of ants show how density hotspots lead to interactions that are clustered in time. A correlated random walk null model describes the mixing of potential and returning foragers. A model from collision theory relates walking speed and spatial proximity with the probability of interaction. The results demonstrate that although ants do not mix homogeneously, trends in interaction patterns can be explained simply by the walking speed and local density of surrounding ants.

Harvester ant colonies adjust their foraging activity to day-to-day changes in food availability and hour-to-hour changes in environmental conditions. This collective behavior is regulated through interactions, in the form of brief antennal contacts, between outgoing foragers and returning foragers with food. Here we consider how an ant, waiting in the entrance chamber just inside the nest entrance, uses its accumulated experience of interactions to decide whether to leave the nest to forage. Using videos of field observations, we tracked the interactions and foraging decisions of ants in the entrance chamber. Outgoing foragers tended to interact with returning foragers at higher rates than ants that returned to the deeper nest and did not forage. To provide a mechanistic framework for interpreting these results, we develop a decision model in which ants make decisions based upon a noisy accumulation of individual contacts with returning foragers. The model can reproduce core trends and realistic distributions for individual ant interaction statistics, and suggests possible mechanisms by which foraging activity may be regulated at an individual ant level.

In this work, we construct a new physics-based model of rubber elasticity to capture the strain softening, strain hardening, and deformation-state dependent response of rubber materials undergoing finite deformations. This model is unique in its ability to capture large-stretch mechanical behavior with parameters that are connected to the polymer chemistry and can also be easily identified with the important characteristics of the macroscopic stress–stretch response. The microscopic picture consists of two components: a crosslinked network of Langevin chains and an entangled network with chains confined to a nonaffine tube. These represent, respectively, changes in entropy due to thermally averaged chain conformations and changes in entropy due to the magnitude of these conformational fluctuations. A simple analytical form for the strain energy density is obtained using Rubinstein and Panyukov's single-chain description of network behavior. The model only depends on three parameters that together define the initial modulus, extent of strain softening, and the onset of strain hardening. Fits to large stretch data for natural rubber, silicone rubber, VHB 4905 (polyacrylate rubber), and b186 rubber (a carbon black-filled rubber) are presented, and a comparison is made with other similar constitutive models of large-stretch rubber elasticity. We demonstrate that the proposed model provides a complete description of elastomers undergoing large deformations for different applied loading configurations. Moreover, since the strain energy is obtained using a clear set of physical assumptions, this model may be tested and used to interpret the results of computer simulation and experiments on polymers of known microscopic structure.

The underlying fiber architecture of soft tissues, like bat wing skin, plays an important role in the material’s overall mechanical behavior. The mesoscopic birefringent fiber architecture of the bat wing skin can be visualized directly under polarized light. This inherent property is the key to the new experimental technique developed in this work: polarized image correlation (PIC). PIC is a technique for determining full field material strains and fiber kinematics of mesoscopically resolved fibrous tissues during biaxial mechanical testing. Not only is the material birefringence used to determine fiber kinematics under finite deformations, but it is also used for image correlation and strain field computation. Pure translation tests performed with PIC verify the accuracy of the technique. A segmental image processing method was developed to solve an experimental issue of changing birefringent properties to construct accurate continuous deformation profiles. By integrating PIC with traditional digital image correlation, both surface and subsurface data give additional insight into through thickness tissue behavior. The PIC technique is applicable to semi-transparent tissues with birefringent mesosopic structures; incorporation of microscopy would resolve smaller fiber structures. Future work includes extending the techniques to three dimensions to analyze curved surfaces.

The preparation of a proton-conducting membrane from Sulponated Poly Ether Ether Ketone (SPEEK), tungsto phosphoric acid (TPA), and inorganic fillers such as titanium dioxide (TiO2), silicon dioxide (SiO2), and zirconium dioxide (ZrO2) was prepared and the electrochemical and mechanical properties were investigated with regards to its application in energy production. The role of tungsto phosphoric acid is to minimize water assistance at elevated temperatures, which led to a decrease in mechanical strength. The fabricated membranes were characterized by FT-IR, TGA, DSC, and SEM techniques. This work may efficiently serve as a basis for new projects as many aspects need to be investigated further. First, it would be interesting to devote a study to the direct comparison of the different fillers of side-chain poly ether ether ketone in order to elucidate the exact morphological features of these membranes, especially in the swollen state.

Ionic polymer transducers (IPTs), also known as ionic polymer-metal composites (IPMCs), are smart sensors and actuators which operate through a coupling of micro-scale chemical, mechanical, and electrical interactions. It is known that ion movement, when a voltage is applied, causes stresses which lead to a net bending movement of a cantilevered transducer towards the anode. However, it is not well understood how these stresses arise, and it is not known how the material microstructure affects the observed macroscopic bending response. In this work, we apply a micromechanics modeling framework to analyze how assumptions of the material microstructure of an IPT affect local interactions and the resulting boundary layer stresses which lead to actuation. In the micromechanics framework, local equilibrium consists of a balance between internal cluster pressure and the stress developed in the polymer backbone. Here, we derive a generalized expression for the electrostatic cluster pressure and show how it depends on microstructure and microstructural evolution in the boundary layers. It is proposed that the boundary layer stiffness varies locally with changes in solvent uptake and charge density; this relationship is defined from micromechanics equilibrium conditions and includes effects of the generalized electrostatic cluster pressure. By assuming a relationship between ion and solvent movement, we then examine how boundary layer stresses are affected by assumptions of the material microstructure. The results and implications of the model are compared with recent experimental observations as well as other models of IPT actuation.

The thermal shape memory effect in polymeric materials refers to the ability of a sample to retain a deformed shape when cooled below T g , and then recover its initial shape when subsequently heated. Although these properties are thought to be related to temperature-dependent changes in network structure and polymer chain mobility, a consistent picture of the molecular mechanisms which determine shape memory behavior does not exist. This, along with large differences in the shape memory cycling response for different materials, has made model development and specific property optimization difficult. In this work we use coarse-grained molecular dynamics (MD) simulations of the thermal shape memory effect to inform micro-macro relationships and systematically identify the salient features leading to desirable shape behavior. We consider a simulation test set including chains with increasing levels of the microscopic restrictions on chain motion (the freely-jointed, freely-rotating, and rotational isomeric state chain models), each simulated with both the NPT and NVT ensembles. It is found that the NPT ensemble with attractive interactions between monomers enabled is the most appropriate for simulating the temperature-dependent mechanical behavior of a polymer using coarse-grained MD. Of the different models, the freely-jointed chain system shows the most desirable shape memory characteristics; this behavior is attributed to the ability of the particles in this system to pack closely together in an energetically favorable configuration. A comparison with experimental data demonstrates that the coarse-grained simulations display all of the relevant trends in mechanical behavior during constant strain shape memory cycling. We conclude that atomistic detail is not needed to represent a shape memory polymer, and that multi-scale modeling techniques may build on the mechanisms embodied in the simple coarse-grained model.

Elastomers are polymers able to undergo large, reversible deformations, and their mechanical properties depend on the chemistry of individual chains as well as the topology of the crosslinked network. In this work we analyze the connection between micro-scale network structure and the macroscopic mechanical properties by performing molecular dynamics (MD) simulations using the Kremer & Grest bead-spring model. The chain length and the density at which crosslinking is performed are varied in order to produce systems ranging from crosslink-dominated to highly entangled, and stress-stretch results are obtained via MD in the large deformation regime. In analogy with recent work on social, technological, and biological networks, we apply mathematical graph theory to describe elastomer networks in a multi-scale modeling framework. A matrix formulation of crosslinked polymers is presented and applied in order to identify the network structure resulting from both chemical crosslinks and physical crosslinks (entanglements). We show that spectral analysis of the crosslink and chain entanglement adjacency matrices along with the corresponding degree distributions can be used to identify and differentiate between the different materials. The spectrum of the crosslink adjacency matrix resembles a sparse regular graph, and spectrum of the intermolecular chain entanglement matrix for the highly entangled systems is shown to resemble a random graph; however, deviations are noted which require further study. A comparison of the network properties with the stress-stretch response demonstrates the influence of both crosslinks and entanglements on the large deformation mechanical behavior of an elastomer material.Copyright © 2012 by ASME

Ionic polymer transducers (IPTs) are soft sensors and actuators which operate through a coupling of micro-scale chemical, electrical, and mechanical interactions. The use of an ionic liquid as solvent for an IPT has been shown to dramatically increase transducer lifetime in free-air use, while also allowing for higher applied voltages without electrolysis. In this work we model charge transport in an ionic liquid IPT by considering both the cation and anion of the ionic liquid as mobile charge carriers, a phenomenon which is unique to ionic liquid IPTs compared to their water-based counterparts. The electrochemical behavior of the large ionic liquid ions is described through a modification of the Nernst-Planck equation given by Kornyshev which accounts for steric effects in double layer packing. The method of matched asymptotic expansions is applied to solve the resulting system of equations, and analytical expressions are derived for the nonlinear charge transferred and capacitance of the IPT as a function of the applied voltage. The influence of the fraction of mobile ionic liquid ions and steric effects on the capacitance of an ionic liquid IPT is shown and compared to water-based IPTs. These results show the fundamental charge transport differences between water-based and ionic liquid IPTs and give considerations for future transducer development.

Ionic polymer transducers (IPTs) are soft sensors and actuators which operate through a coupling of micro-scale chemical, electrical, and mechanical interactions. The use of an ionic liquid as solvent for an IPT has been shown to dramatically increase transducer lifetime in free-air use, while also allowing for higher applied voltages without electrolysis. In this work we model charge transport in an ionic liquid IPT by considering both the cation and anion of the ionic liquid as mobile charge carriers, a phenomenon which is unique to ionic liquid IPTs compared to their water-based counterparts. The electrochemical behavior of the large ionic liquid ions is described by use of a modified Nernst-Planck equation which accounts for steric effects in double layer packing. The method of matched asymptotic expansions is applied to solve the resulting system of equations, and analytical expressions are derived for the nonlinear charge transferred and capacitance of the IPT as a function of the applied voltage. The boundary layer ionic concentration and charge density profiles and the leading order dynamics are also computed for the ionic liquid IPT. A simple equivalent circuit model is constructed in order to facilitate a comparison with experimental results. The implications of these model results in regards to actuation and charging performance characteristics of ionic liquid IPTs are noted.Copyright © 2010 by ASME

Ionic polymer-metal composites (IPMCs) are smart materials which function as soft sensors and actuators. When a small DC voltage (1–5 V) is applied to an IPMC in a cantilever configuration, ion and solvent transport through the thickness of the polymer membrane causes the transducer to bend towards the anode. For device development and use in engineering applications, actuation is often described at a higher level in terms of an electromechanical coupling between the ionic charge distribution and the stresses developed in the IPMC. In this work we derive a set of relationships describing the coupling response by starting with basic considerations of polymer microstructure and local interactions during actuation. A micromechanical modeling framework is employed in order to account for the material microstructure. Using a generalized expression for electrostatic cluster pressure which takes into account clusters recombining to form larger cluster upon expansion, we define an effective local stiffness which varies with both solvent uptake and charge density in the boundary layers. An equilibrium relationship between solvent uptake and charge density is determined by considering the free energy of the homogenized polymer as the sum of elastic, electrostatic, and chemical components. Stress developed in the boundary layers is then calculated from changes in local stiffness and solvent uptake with respect to charge density. The resulting relationship for electromechanical coupling is found to be in good agreement with previous empirical models, thus serving as a model validation and demonstrating why certain forms for electromechanical coupling can be used to explain a variety of experimental observations. Specifically, we see that stress developed in the boundary layers is well described as a quadratic polynomial in charge density due to the form of the electrostatic cluster pressures.Copyright © 2010 by ASME

In this work a model of ion transport in ionic liquid-based ionic polymer-metal composites (IPMC's) is formulated using Nernst-Planck/Poisson (NPP) theory and numerical simulations are performed using the finite element method. IPMC's are smart materials which act as both sensors and actuators, and the use of an ionic liquid has been shown to dramatically increase transducer lifetime in free-air use while also allowing for higher applied voltages without chemical decomposition. We consider both the cation and anion of the ionic liquid to be mobile in addition to the mobile countercation of the transducer. The results show a nonlinear dynamic response which gives insight into transduction mechanisms which are unique to ionic liquid IPMC's as compared to their water-based counterparts.

In this work we perform coarse-grained molecular dynamics (MD) simulations to study the molecular origins of the thermal shape memory effect in crosslinked polymer materials. Thermal shape memory polymers (SMPs) are materials able to hold a deformed shape when cooled below the glass transition temperature, and subsequently recover the initial shape when heated. To use SMPs in various applications requires materials which reliably hold and recover their shapes; this has sparked recent synthesis work to create new SMP materials with optimized properties. Here we use coarse-grained MD simulations with different polymer chain models to determine which parameters affect relevant SMP behavior and to investigate the molecular mechanisms at the level of individual chains during temperature cycling. The simulations show how temperature-dependent chain mobility leads to shape memory polymer behavior. In particular, we demonstrate the importance of attractive monomer interactions in leading to ‘good’ SMP behavior. The results suggest promising routes for material development. Additionally, the mechanisms identified with the simple simulation model can be used to inform multi-scale models of SMP material behavior.

In this paper we present a physics-based model for the electrochemical response of ionic liquid-ionic polymer transducers (IPTs) and show how the mobile ionic liquid ions influence the charging characteristics and actuation performance of a device. It is assumed that a certain fraction of the ionic liquid ions exist as “free,” making for a total of 3 mobile ions. This leads to predictions of distinctly different charging characteristics for ionic liquid versus water-based IPTs, since for the latter there is only a single mobile ion. The large ionic liquid ions are modeled by including steric effects in a set of modified Nernst-Planck/Poisson equations, and the resulting system of equations is solved using the method of matched asymptotic expansions (MAE). The inclusion of steric effects allows for a realistic description of boundary layer composition near actuator operating voltages (~1 V). Analytical expressions for the charge transferred and differential capacitance are derived as a function of the frac...

Ionic polymer transducers (IPTs), also known as ionic polymer-metal composites, are soft sensors and actuators which operate through a coupling of microscale chemical, electrical, and mechanical interactions. The use of an ionic liquid as solvent for an IPT has been shown to dramatically increase transducer lifetime in free-air use, while also allowing for higher applied voltages without electrolysis. In this work, we apply Nernst-Planck/Poisson theory to model charge transport in an ionic liquid IPT by considering a certain fraction of the ionic liquid ions as mobile charge carriers, a phenomenon which is unique to ionic liquid IPTs compared to their water-based counterparts. Numerical simulations are performed using the finite element method to examine how the introduction of another pair of mobile ions affects boundary layer charge dynamics, concentration, and charge density distributions in the electric double layer, and the overall charge transferred and current response of the IPT. Due to interactions with the Nafion ionomer, not all of the ionic liquid ions will function as mobile charge carriers; only a certain fraction will exist as “free” ions. The presence of mobile ionic liquid ions in the transducer will increase the overall charge transferred when a voltage is applied, and cause the current in the transducer to decay more slowly. The additional mobile ions also cause the ionic concentration profiles to exhibit a nonlinear dynamic response, characterized by nonmonotonic ionic concentration profiles in space and time. Although the presence of mobile ionic liquid ions increases the overall amount of charge transferred, this additional charge transfer occurs in a somewhat symmetric manner. Therefore, the additional charge transferred due to the ionic liquid ions does not greatly increase the net bending moment of the transducer; in fact, it is possible that ionic liquid ion movement actually decreases the observed bending response. This suggests that an optimal electromechanical conversion efficiency for bending actuation is achieved by using an ionic liquid where only a relatively small fraction of the ionic liquid ions exist as free ions. Conversely, if it is desired to increase the overall amount of charge transferred, an ionic liquid with a large fraction of free ions should be used. These theoretical considerations are found to be in good qualitative agreement with recent experimental results.