Multistable sheets with rewritable patterns for switchable shape-morphing.

Flat sheets patterned with folds, cuts or swelling regions can deform into complex three-dimensional shapes under external stimuli1-24. However, current strategies require prepatterning and lack intrinsic shape selection5-24. Moreover, they either rely on permanent deformations6,12-14,17,18, preventing corrections or erasure of a shape, or sustained stimulation5,7-11,25, thus yielding shapes that are unstable. Here we show that shape-morphing strategies based on mechanical multistability can overcome these limitations. We focus on undulating metasheets that store memories of mechanical stimuli in patterns of self-stabilizing scars. After removing external stimuli, scars persist and force the sheet to switch to sharply selected curved, curled and twisted shapes. These stable shapes can be erased by appropriate forcing, allowing rewritable patterns and repeated and robust actuation. Our strategy is material agnostic, extendable to other undulation patterns and instabilities, and scale-free, allowing applications from miniature to architectural scales.

Health and security risks of humanitarian aid workers during field missions: Experience of the International Red Cross.

BACKGROUND: Humanitarian disaster environments put relief workers at risk, both physically and mentally. Research looking at the growing humanitarian workforce's health and working conditions is lacking. METHODS: A questionnaire was presented to ICRC (International Committee of the Red Cross) workers during debriefing between 2016 and 2018. Participants were queried about their experience abroad including security incidents (the primary objective), health, and malaria prevention. Descriptive statistics and multivariate logistic regressions were used for data analysis. RESULTS: Security incidents were reported by 12% (95/796) of expatriates, mainly armed threats and attacks (83%, 79/95). Accidents or injuries occurred for 7.5% (60/796), of which 42% (25/60) were due to sporting activities and 22% (13/60) to road accidents. Many engaged in risky behaviours (15%, 119/795), such as driving too fast or inebriated (37%, 44/119) and/or engaging in unsafe sex, (unfamiliar partner, no condom) (71%; 84/119). Compliance to malaria prophylaxis was low 43% (113/263). Over 40% (324/795) found the mission more stressful than expected. Almost a third (28%, 219/796) reported worsened health on their return. CONCLUSIONS: Humanitarian aid workers display a worsening of their health when on mission due to violence, accidents and risky behaviours. To preserve their staff's wellbeing, humanitarian organizations should pay attention to medical history, pre-travel training specific to assignment, and continuous medical and psychological support.

Rayleigh scattering of surface plasmons by sub-wavelength holes.

We study the scattering of surface plasmons from sub-wavelength holes and find that it exhibits a stronger wavelength dependence than the traditional λ(-4) scaling found for Rayleigh scattering of light from small particles. This experimental observation is consistent with recent theoretical work and linked to the two-dimensional nature of the surface plasmon and the wavelength dependence of its spatial extent in the third dimension. The scattering cross sections are obtained with a frequency-correlation technique, which compares intensity speckle patterns observed behind various random structures of holes and recorded at different wavelengths. This powerful technique even allows us to distinguish between scattering of surface plasmons into photons and scattering into other surface plasmons.