Enhanced multimaterial 4D printing with active hinges

Despite great progress in four-dimensional (4D) printing, i.e. three-dimensional (3D) printing of active (stimuli-responsive) materials, the relatively low actuation force of the 4D printed structures often impedes their engineering applications. In this study, we use multimaterial inkjet 3D printing technology to fabricate shape memory structures, including a morphing wing flap and a deployable structure, which consist of active and flexible hinges joining rigid (non-active) parts. The active hinges, printed from a shape memory polymer (SMP), lock the structure into a second temporary shape during a thermomechanical programming process, while the flexible hinges, printed from an elastomer, effectively increase the actuation force and the load-bearing capacity of the printed structure as reflected in the recovery ratio. A broad range of mechanical properties such as modulus and failure strain can be achieved for both active and flexible hinges by varying the composition of the two base materials, i.e. the SMP and the elastomer, to accommodate large deformation induced during programming step, and enhance the recovery in the actuating step. To find the important design parameters, including local deformation, shape fixity and recovery ratio, we conduct high fidelity finite element simulations, which are able to accurately predict the nonlinear deformation of the printed structures. In addition, a coupled thermal-electrical finite element analysis was performed to model the heat transfer within the active hinges during the localized Joule heating process. The model predictions showed good agreement with the measured temperature data and were used to find the major parameters affecting temperature distribution including the applied voltage and the convection rate

One-year prevalence of antimicrobial susceptibility pattern of methicillin-resistant Staphylococcus aureus recovered from raw meat

Food with animal origins and especially meat may play an important role in transmission of methicillin-resistant strains of Staphylococcus aureus to humans. The present investigation was carried out to determine the prevalence of MRSA in various types of Iranian meat samples as well as study their antimicrobial resistance properties. Nine-hundred raw meat samples were collected during various months of the year. Samples were cultured and those that were MRSA-positive were subjected to the disk diffusion method to study the antibiotic resistance pattern. One-hundred and sixty out of 900 raw meat samples (17.7%) were positive for MRSA. Raw sheep meat samples had the highest (24.0%), while raw camel meat samples had the lowest (10%) prevalence of MRSA. Samples which were collected in June, July, August, September and June months had the highest prevalence of MRSA. Bacterial strains were also resistant to ampicillin (100%), penicillin G (100%), gatifloxacin (96.8%), ceftriaxone (80%) and oxacillin (76.2%) antibiotics. We found that only one isolate was resistant to all tested antimicrobial agents. Contaminated meat samples are potential risk factor for transmission of MRSA. Thoughtful antibiotics prescription, control the hygienic quality of meat inspections and increase the hygienic status of butchers and slaughterhouses can decrease the prevalence of MRSA in meat