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Marchetti M.C.,Syracuse Biomaterials Institute | Joanny J.F.,University Pierre and Marie Curie | Ramaswamy S.,Indian Institute of Science | Liverpool T.B.,University of Bristol | And 5 more authors.
Reviews of Modern Physics | Year: 2013

This review summarizes theoretical progress in the field of active matter, placing it in the context of recent experiments. This approach offers a unified framework for the mechanical and statistical properties of living matter: biofilaments and molecular motors in vitro or in vivo, collections of motile microorganisms, animal flocks, and chemical or mechanical imitations. A major goal of this review is to integrate several approaches proposed in the literature, from semimicroscopic to phenomenological. In particular, first considered are "dry" systems, defined as those where momentum is not conserved due to friction with a substrate or an embedding porous medium. The differences and similarities between two types of orientationally ordered states, the nematic and the polar, are clarified. Next, the active hydrodynamics of suspensions or "wet" systems is discussed and the relation with and difference from the dry case, as well as various large-scale instabilities of these nonequilibrium states of matter, are highlighted. Further highlighted are various large-scale instabilities of these nonequilibrium states of matter. Various semimicroscopic derivations of the continuum theory are discussed and connected, highlighting the unifying and generic nature of the continuum model. Throughout the review, the experimental relevance of these theories for describing bacterial swarms and suspensions, the cytoskeleton of living cells, and vibrated granular material is discussed. Promising extensions toward greater realism in specific contexts from cell biology to animal behavior are suggested, and remarks are given on some exotic active-matter analogs. Last, the outlook for a quantitative understanding of active matter, through the interplay of detailed theory with controlled experiments on simplified systems, with living or artificial constituents, is summarized. © 2013 American Physical Society.

If they aren't removed quickly using the right amount of cleaning agent, they can rapidly propagate and become resistant to some cleaners and even antibiotics. These "superbugs," which include methicillin-resistant Staphylococcus aureus (MRSA), are difficult to treat and can gain a foothold in such places as medical settings. Professor Dacheng Ren, in the Department of Biomedical and Chemical Engineering (BMCE) in the College of Engineering and Computer Science and the Syracuse Biomaterials Institute (SBI), and his team of researchers are seeking ways to stop the spread of microbes, and they may have found a solution. In a recent study, the researchers collaborated with the lab of Associate Professor James Henderson (also in BMCE and SBI) and discovered that the use of shape-shifting polymers with topographic patterns as surfaces effectively reduces the adhesion of microbes and can remove up to 99.9 percent of attached cells. The results appeared in the article "On-Demand Removal of Bacterial Biofilms via Shape Memory Activation" in the journal of ACS Applied Materials and Interfaces. The postdoctoral student researchers who also worked on the project are Huan Gu, Sang Won Lee and Shelby Lois Buffington. "Microbes like to attach to surfaces," Ren says. "Basically anywhere you have water, you could have a so-called biofilm—a cluster of microbial cells that stick together. They also produce a polymeric matrix that gives them protection." This makes them problematic for multiple reasons. "Number one is it's very hard to kill them. Once they clump together and form a biofilm, you need up to about 1,000 times more antimicrobial agents—such as antibiotics and other cleaning agents—to kill them," Ren says. The second problem relates to their closeness. "They can share genes and the DNA around them. Once a cell becomes resistant to something, another can become resistant," Ren says. "They can teach other because they are so close to each other." Biofilm is really the city of the microbes, Ren says. "The biofilm protects them and allows them to survive under stress and over time to mutate and become permanently resistant to the control agents we use, especially antibiotics. That's why we need to worry about them," he says. Researchers in the field are looking at how to engineer a material or a surface to be less vulnerable to microbial adhesion and biofilm formation—making it less welcome to them. Earlier published work by Ren's group showed that engineering surface topography could be impactful. "Just as you like to walk on certain surfaces but you don't like to walk on other surfaces, microbes are the same way," Ren says. "You understand what they prefer and engineer the surface so they don't come to it. That's one of the strategies." In that particular paper, Ren and his team used hexagon patterns on the surface to reduce biofilm by about 50 percent. However, because it does not offer total prevention, over time the microbes can slowly take over. That is the challenge in controlling them—on a static surface it will only work for a certain amount of time. "The motivation for us was to create a surface that can potentially change after a biofilm has formed," Ren says. "This approach we did something different; we changed the surface when needed so you can slide them off the surface, basically." The surface has a certain topography and then suddenly another—essentially breaking the microbes' grip on the surface in the process. Ren and his group used a material that had been created before, but they discovered a new way of using the dynamic topography for biofouling control. "We're ultimately changing topography, using an existing material that is known to be biocompatible, meaning it doesn't kill cells," Ren says. This class of polymers has a permanent shape and a temporary shape. "You begin this application with a temporary shape, so you have some success for reducing adhesion. But bacterial cells can still attach to it over time and grow biofilms and, at that point, the initial topography has lost its function," Ren says. "However, then we can trigger the change of surface topography within minutes. In front of your eyes you can see the change if you have a microscope. And then you can see the cells come off." Based on previous research that was supported by the National Science Foundation and the National Institutes of Health, the recent work reduced Pseudomonas aeruginosa biofilms by 99.9 percent compared to the static flat control. It was also successful in removing biofilms of Staphylococcus aureus and an Escherichia coli strain that can cause urinary tract infections. This area of shape memory polymers has been studied by material scientists and now the technology can be used in the field of bioengineering for control of contamination. "It's a good interdisciplinary application of polymer science and material science and microbiology," Ren says, noting the work that Henderson's lab played in the current research. "This is collaborative work. The Henderson group has more experience in shape memory polymer and he helped us characterize some of the features of the polymers." Some of the shape memory polymers can also be engineered to have more than two stages. "Eventually we want to do something that can do cycles," Ren says. "The next step would be to apply this to more novel materials with more complicated or more sophisticated ways of shape change and also to find more gentle conditions for biocompatible materials and real effective applications. We are also studying the effects of such detachment on the physiology of biofilm cells." Explore further: First evidence that bacteria get 'touchy-feely' about dangerous biofilms More information: Huan Gu et al. On-Demand Removal of Bacterial Biofilms via Shape Memory Activation, ACS Applied Materials & Interfaces (2016). DOI: 10.1021/acsami.6b06900

Banerjee S.,Syracuse University | Marchetti M.C.,Syracuse University | Marchetti M.C.,Syracuse Biomaterials Institute
Physical Review Letters | Year: 2012

Using a minimal model of cells or cohesive cell layers as continuum active elastic media, we examine the effect of substrate thickness and stiffness on traction forces exerted by strongly adhering cells. We obtain a simple expression for the length scale controlling the spatial variation of stresses in terms of cell and substrate parameters that describes the crossover between the thin and thick substrate limits. Our model is an important step towards a unified theoretical description of the dependence of traction forces on cell or colony size, acto-myosin contractility, substrate depth and stiffness, and strength of focal adhesions and makes experimentally testable predictions. © 2012 American Physical Society.

Fily Y.,Syracuse University | Marchetti M.C.,Syracuse University | Marchetti M.C.,Syracuse Biomaterials Institute
Physical Review Letters | Year: 2012

We study numerically and analytically a model of self-propelled polar disks on a substrate in two dimensions. The particles interact via isotropic repulsive forces and are subject to rotational noise, but there is no aligning interaction. As a result, the system does not exhibit an ordered state. The isotropic fluid phase separates well below close packing and exhibits the large number fluctuations and clustering found ubiquitously in active systems. Our work shows that this behavior is a generic property of systems that are driven out of equilibrium locally, as for instance by self-propulsion. © 2012 American Physical Society.

Gu X.,Syracuse Biomaterials Institute | Mather P.T.,Syracuse Biomaterials Institute
RSC Advances | Year: 2013

In this article we describe the preparation and characterization of a water-triggered shape memory polymer (SMP) family, PCL-PEG based thermoplastic polyurethanes (TPUs). Upon immersion in water, water molecules selectively swelled the hydrophilic PEG domains, resulting in durable hydrogels with strain-to-failure values greater than 700%. Dry samples fixed in a temporary shape underwent water-triggered shape recovery wherein only the oriented PEG domains recovered, causing incomplete shape recovery toward the equilibrium shape upon contact with liquid water. Addressing the limited recovery observed for dry-fixing samples that led to some PCL domain deformation, we developed a novel, "wet-fixing" SM cycle, where the temporary shape is achieved by deforming the material in the hydrogel state (wet drawing) and is later fixed via PEG recrystallization upon drying. The fixing and recovery ratios were substantially improved using this new shape memory programming method, the mechanism of which was proven by X-ray diffraction analysis. The recovery speed of this material system was studied by varying the thickness of bulk films and demonstrated that water-recovery is diffusion-limited. By processing the TPUs as a web of microfibers, rapid shape recovery was achieved in water at room temperature within 1.3 s. The controllable actuation speed, the high recoverable strain, and the simple fixing and recovery process make these materials potential candidates for applications as water responsive sensors, actuators, and medical devices. © 2013 The Royal Society of Chemistry.

Luo X.,Syracuse Biomaterials Institute | Mather P.T.,Syracuse Biomaterials Institute
ACS Macro Letters | Year: 2013

In this communication, we report the preparation and characterization of new shape memory assisted self-healing (SMASH) coatings. The coatings feature a phase-separated morphology with electrospun thermoplastic poly(ε- caprolactone) (PCL) fibers randomly distributed in a shape memory epoxy matrix. Mechanical damage to the coating can be self-healed via heating, which simultaneously triggers two events: (1) the shape recovery of the matrix to bring the crack surfaces in spatial proximity, and (2) the melting and flow of the PCL fibers to rebond the crack. In controlled healing experiments, damaged coatings not only heal structurally, but also functionally by almost completely restoring the corrosion resistance. We envision the wide applicability of the SMASH concept in designing the next-generation self-healing materials. © 2013 American Chemical Society.

Luo X.,Syracuse Biomaterials Institute | Mather P.T.,Syracuse Biomaterials Institute
Advanced Functional Materials | Year: 2010

In this paper, the fabrication and characterization of triple-shape polymeric composites (TSPCs) that, unlike traditional shape memory polymers (SMPs), are capable of fixing two temporary shapes and recovering sequentially from the first temporary shape (shape 1) to the second temporary shape (shape 2), and eventually to the permanent shape (shape 3) upon heating, are reported. This is technically achieved by incorporating non-woven thermoplastic fibers (average diameter -760 nm) of a low-Tm semicrystalline polymer into a Tg-based SMP matrix. The resulting composites display two well-separated transitions, one from the glass transition of the matrix and the other from the melting of the fibers, which are subsequently used for the fixing/recovery of two temporary shapes. Three thermomechanical programming processes with different shape fixing protocols are proposed and explored. The intrinsic versatility of this composite approach enables an unprecedented large degree of design flexibility for functional triple-shape polymers and systems. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Luo X.,Syracuse Biomaterials Institute | Mather P.T.,Syracuse Biomaterials Institute
Soft Matter | Year: 2010

A new shape memory nanocomposite that exhibits rapid electrical actuation capabilities is fabricated by incorporating continuous, non-woven carbon nanofibers (CNFs) into an epoxy based SMP matrix. The fiber morphology and nanometre size provide a percolating conductive network with a large interfacial area. This not only gives high electrical conductivity but also simultaneously enhances heat transfer and recovery stress. © 2010 The Royal Society of Chemistry.

Giomi L.,Harvard University | Marchetti M.C.,Syracuse Biomaterials Institute
Soft Matter | Year: 2012

We study the spatio-temporal dynamics of a model of polar active fluid in two dimensions. The system exhibits a transition from an isotropic to a polarized state as a function of density. The uniform polarized state is, however, unstable above a critical value of activity. Upon increasing activity, the active fluids displays increasingly complex patterns, including traveling bands, traveling vortices and chaotic behavior. The advection arising from the particles self-propulsion and unique to polar fluids yields qualitatively new behavior as compared to that obtained in active nematic, with traveling-wave structures. We show that the nonlinear hydrodynamic equations can be mapped onto a simplified diffusion-reaction-convection model, highlighting the connection between the complex dynamics of active system and that of excitable systems.

Farrell F.D.C.,University of Edinburgh | Marchetti M.C.,Syracuse Biomaterials Institute | Marenduzzo D.,University of Edinburgh | Tailleur J.,University Paris Diderot
Physical Review Letters | Year: 2012

We study the behavior of interacting self-propelled particles, whose self-propulsion speed decreases with their local density. By combining direct simulations of the microscopic model with an analysis of the hydrodynamic equations obtained by explicitly coarse graining the model, we show that interactions lead generically to the formation of a host of patterns, including moving clumps, active lanes, and asters. This general mechanism could explain many of the patterns seen in recent experiments and simulations. © 2012 American Physical Society.

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