Degradable Plastics Are Vulnerable to Cracks

A plastic may degrade in response to a trigger.The kinetics of degradation have long been characterized by the loss of weight and strength over time.These methods of gross characterization,however,are misleading when plastic degrades heterogeneously.Here,we study heterogeneous degradation in an extreme form:the growth of a crack under the combined action of chemistry and mechanics.An applied load opens the crack,exposes the crack front to chemical attack,and causes the crack to outrun gross degradation.We studied the crack growth in polylactic acid(PLA),a polyester in which ester bonds break by hydrolysis.We cut a crack in a PLA film using scissors,tore it using an apparatus,and recorded the crack growth using a camera through a microscope.In our testing range,the crack velocity was insensitive to load but was sensitive to humidity and pH.These findings will aid the development of degradable plastics for healthcare and sustainability.

MICROMECHANICS OF MACROELECTRONICS

The advent of flat-panel displays has opened the era of macroelectronics. Enthusiasm is gathering to develop macroelectronics as a platform for many technologies, ranging from paper-like displays to thin-film solar cells, technologies that aim to address the essential societal needs for easily accessible information, renewable energy, and sustainable environment. The widespread use of these large structures will depend on their ruggedness, portability and low cost, attributes that will come from new material choices and new manufacturing processes. For example, thin-film devices on thin polymer substrates lend themselves to roll-to-roll fabrication, and impart flexibility to the products. These large structures will have diverse architectures, hybrid materials, and small features; their mechanical behavior during manufacturing and use poses significant challenges to the creation of the new technologies. This paper describes ongoing work in the emerging field of research-the mechanics of macroelectronics, with emphasis on the mechanical behavior at the scale of individual features, and over a long time.

Adhesive anastomosis for organ transplantation

The recent development of tough tissue adhesives has stimulated intense interests among material scientists and medical doctors.However,these adhesives have seldom been tested in clinically demanding surgeries.Here we demonstrate adhesive anastomosis in organ transplantation.Anastomosis is commonly conducted by dense sutures and takes a long time,during which all the vessels are occluded.Prolonged occlusion may damage organs and even cause death.We formulate a tough,biocompatible,bioabsorbable adhesive that can sustain tissue tension and pressurized flow.We expose the endothelial surface of vessels onto a gasket,press two endothelial surfaces to the adhesive using a pair of magnetic rings,and reopen the bloodstream immediately.The time for adhesive anastomosis is shortened compared to the time for sutured anastomosis.We have achieved adhesive anastomosis of a great vein in transplanting the liver of a pig.After the surgery,the adhesive is absorbed,the vein heals,and the pig lives for over one month.

A FINITE ELEMENT METHOD FOR DIELECTRIC ELASTOMER TRANSDUCERS

We present a finite element method for dielectric elastomer （DE） transducers based on the nonlinear field theory of DE. The method is implemented in the commercial finite element software ABAQUS, which provides a large library functions to describe finite elasticity. This method can be used to solve electromechanical coupling problems of DE transducers with complex configurations and under inhomogeneous deformation.

Extension limit,polarization saturation,and snap-through instability of dielectric elastomers

A dielectric elastomer is capable of large voltage-induced deformation,particularly when the voltage is applied on the verge of snap-through instability.A model is described which shows that the snap-through instability is markedly affected by both the extension limit of polymer chains and the polarization saturation of dipoles.The model may guide the search for high-performance dielectric elastomer transducers.