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Strides in microrobotics, face reconstruction, minimally invasive surgery with shape shifting plastics

Strides in microrobotics, face reconstruction, minimally invasive surgery with shape shifting plastics

Shape memory polymers are smart plastic materials that can be transformed into a temporary shape and then return to their original shape, triggered by an external stimulus such as heat or pressure. These polymers can be programmed to create highly complex shapes – coils, knots, even origami-like shapes; but the transformations have conventionally been irreversible. In recent years, reversible shape memory has been enabled in certain polymers, but achieving shape switching across broad classes of polymers has been elusive. Usually, switching from one shape to another requires a persistent external mechanical force. Now, researchers have turned to elastomers – polymers with elasticity and viscosity; with carefully designed chemical structures. Shape memory materials are vital for minimally invasive surgery, hands-free packaging, the aerospace industry, and microrobotics. Reversible shape memory polymers are particularly attractive for these fields, as they allow for highly complex shape transformations on broad length scales, and in response to a vast array of external stimuli – mainly heat, but also light, electro-magnetic fields, and even acoustic waves.

A team of scientists from the University of North Carolina (UNC) at Chapel Hill, the University of Connecticut, and the US Department of Energy's Brookhaven National Laboratory, led by Sergei Sheiko (UNC), has devised a general method for enabling reversible shifting between programmable shapes. They used semicrystalline elastomers with uniform chemical compositions of various kinds and achieved shape transformations without applying a persistent external force. Their work was described in a paper published online in February 2014 in the journal Macromolecules. "We're uniquely able to achieve reversible transformations, not just one-way shape changes from one state to another, by taking advantage of interactions between crystalline polymer domains and chemical network of polymers," said Oleg Gang, group leader for Soft and Bio NanoMaterials at Brookhaven's Center for Functional Nanomaterials. Gang and his collaborators fabricated elastomers, twisted them into coiled shapes at 140°F, and then cooled them to about 40°F. Once reheated, to 100°F, the straight elastomers again took on the coiled shape spontaneously, without any applied force. The angles of the coil were repeatedly reproduced, even after many heating and cooling cycles.
Similar reversible behavior was shown in a straight polymer bending to a predetermined angle and an origami-like star or gripper folding and unfolding on its own. Gang used small-angle x-ray scattering (SAXS) at Brookhaven's Center for Functional Nanomaterials and on beamline X9 at the National Synchrotron Light Source to determine the nanoscale structural changes in the polymers during shapeshifting and the molecular mechanism that causes the reversibility. "We are only beginning to understand the molecular, nano- and meso-scale effects in this material. Many structural changes are happening on various scales simultaneously, and it is not fully clear which phenomenon dominates," Gang said. "The complexity of these materials arises from the interplay of a scaffold of small crystallites and a chemically cross-linked polymer network, which makes the study more challenging but also more stimulating. We are excited about new possibilities that NSLS-II will provide, since it will address our currently limited ability to probe structure and dynamics at multiple scales in real time when macroscopic transformations occur or mechanical stress is applied."

Injuries, birth defects (such as cleft palates) or surgery to remove a tumor can create gaps in bone that are too large to heal naturally. And when they occur in the head, face or jaw, these bone defects can dramatically alter a person's appearance. Researchers have developed a "self-fitting" material that expands with warm salt water to precisely fill bone defects, and also acts as a scaffold for new bone growth. Currently, the most common method for filling bone defects in the head, face or jaw (known as the cranio-maxillofacial area) is autografting. That is a process in which surgeons harvest bone from elsewhere in the body, such as the hip bone, and then try to shape it to fit the bone defect. "The problem is that the autograft is a rigid material that is very difficult to shape into these irregular defects," says Melissa Grunlan, Ph.D., leader of the study. Also, harvesting bone for the autograft can itself create complications at the place where the bone was taken. Another approach is to use bone putty or cement to plug gaps. However, these materials aren't ideal. They become very brittle when they harden, and they lack pores, or small holes, that would allow new bone cells to move in and rebuild the damaged tissue.
To develop a better material, Grunlan and her colleagues at Texas A&M University made a shape-memory polymer (SMP) that molds itself precisely to the shape of the bone defect without being brittle. It also supports the growth of new bone tissue. SMPs are materials whose geometry changes in response to heat. The team made a porous SMP foam by linking together molecules of poly(ε-caprolactone), an elastic, biodegradable substance that is already used in some medical implants. The resulting material resembled a stiff sponge, with many interconnected pores to allow bone cells to migrate in and grow. Upon heating to 140°F, the SMP becomes very soft and malleable. So, during surgery to repair a bone defect, a surgeon could warm the SMP to that temperature and fill in the defect with the softened material. Then, as the SMP is cooled to body temperature (98.6°F), it would resume its former stiff texture and "lock" into place. The researchers also coated the SMPs with polydopamine, a sticky substance that helps lock the polymer into place by inducing formation of a mineral that is found in bone. It may also help osteoblasts, the cells that produce bone, to adhere and spread throughout the polymer. The SMP is biodegradable, so that eventually the scaffold will disappear, leaving only new bone tissue behind.
To test whether the SMP scaffold could support bone cell growth, the researchers seeded the polymer with human osteoblasts. After three days, the polydopamine-coated SMPs had grown about five times more osteoblasts than those without a coating. Furthermore, the osteoblasts produced more of the two proteins, runX2 and osteopontin, that are critical for new bone formation. Grunlan says that the next step will be to test the SMP's ability to heal cranio-maxillofacial bone defects in animals. "The work we've done in vitro is very encouraging," she says. "Now we'd like to move this into preclinical and, hopefully, clinical studies."

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