Tag: composite materials

Original paper: Printing ferromagnetic domains for untethered fast-transforming soft materials

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If we could shrink a submarine down to the microscopic scale, could we pilot it into the human body to fight infection and perform surgery? Despite suggestions from futuristic sci-fi such as “Fantastic Voyage”, “Honey, I Shrunk the Kids”, “The Magic School Bus”, “Power Rangers”, and “Rick and Morty”, we cannot survive such shrinking and our vessel would be without a pilot. But it may still be possible to “shrink” down some of our technology and control it remotely as we will see from researchers at MIT in this week’s paper.

In their letter to Nature, author Yoonho Kim and colleagues at the Zhao lab reveal a dazzling zoo of tiny transformable machines. Following recent trends in the metamaterials community, they built a series of “origami” and “kirigami” samples inspired by the cutting and folding of traditional Japanese paper art.

But, unlike traditional origami, these special sheet-like materials are able to fold themselves up and change shape on command, as shown in Figure 1. With this remarkable new technique, they assemble panels and tubes into a variety of structures which can shrink, shear, expand, and change shape, as well as pipes which can obstruct themselves on command. 

To create these machines, Kim combines two relatively simple ingredients: 3D printing and magnets. These tiny gadgets do not resemble conventional robots and are better described as 3D printed flexible materials with specialized ferromagnetic regions. So let’s break these ingredients down piece-by-piece.      

The process of 3D printing involves a nozzle depositing an “ink”. As the ink leaves the nozzle, it sticks and transitions from a fluid state to a hard or rubbery state, becoming a solid piece of the “printout”. By choosing where to put these squirts of ink, a 3D printer is able to quickly and accurately build 3D structures. 

Typically this ink is some sort of molten plastic or rubber, but by including tiny magnetic bits in the ink and applying a magnetic field as it passes through the nozzle, the magnetic bits will reorient themselves and the ink can adopt the magnetic properties of the applied field. Then, as the ink hardens, this magnetic alignment will be “frozen in”. The result is a flexible material that “remembers” the magnetic field that was present when it formed. And what’s more, by changing the direction of the applied magnetic field over the course of printing, the material can be programmed with regions of different magnetic orientations.

Now, when this printed and patterned material is exposed to a magnetic field again, all these little magnetic regions of the material will try to align themselves with the field. The result is a controlled and predictable change of shape. Careful design of these magnetic domains by Kim and colleagues is the secret behind their self-folding origami as well as complex shrinking and reshaping materials, which seem to be just the tip of the iceberg.

While all these machines are controlled remotely, the material design is permanent and raises questions of multifunctionality. As an inspiring counterexample, the researchers present a flat “soft robot” which can crawl, catch, and can even wrap and roll a small load across a distance as shown in Figure 2. The variety of moves available to this robot stem from clever dynamic variations of the external magnetic field. To really appreciate all of these devices, be sure to check out the videos in the Supplementary Section in the paper (definitely don’t miss Video 8).

Perhaps these dynamic contraptions could soon be deployed inside a living creature, opening the door to new surgical and diagnostic techniques. While an individual robot may be limited to some simple pre-designed action, many medical applications like cutting and sewing are simply repetitive, basic movements. While this possibility has inspired many researchers in the Soft Matter community and beyond to start building a remarkable variety of tiny robots, none of them have yet found their way into the human body to complete a medical task.

In medical applications, biocompatibility is crucial, for both the object itself and its control mechanism.  Fortunately, the magnetic fields used in this research can be safely applied to a human body — the field strengths used in Zhao’s lab are lower than those in standard MRIs. However, these robots currently occupy a size scale of roughly 1 centimeter across – HUGE in biological terms. To perform non-invasive surgery, a robot would need to shrink down closer to the micron scale. So it appears that the future of this game will be akin to the development of the transistor: a search for the small and the powerful.

Original paper: Jumping In and Out of the Phase Diagram Using Electric Fields:
Time Scale for Remixing of Polystyrene/Poly(vinyl methyl ether)
Blends

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Many consumer products, such as clothes and food packaging, are made of blends of polymers, long molecules consisting of repeating chemical units. The attractiveness of using blends of different polymers arises from the engineers’ desire to combine multiple unique properties of each individual polymer, such as transparency, stretchability, and breathability, into a seamless whole. However, different polymers are not necessarily miscible, a term scientists use to describe whether two materials mix at the molecular level. Miscibility isn’t a one-and-done kind of deal: scientists and engineers have known for years how to make polymer blends mix by careful temperature control. What if there were conditions other than temperature to achieve polymer blend miscibility? This may ultimately help in industrial processing of polymer blends. In this week’s paper, Professors Annika Kriisa and Connie B. Roth from Emory University in Atlanta, Georgia, explore the mixing dynamics of two polymers by using a strong electric field.

Before we dive into the meat of the paper, it’s important to know how temperature affects the miscibility of a polymer blend. The black curve in Figure 1 is a representative miscibility diagram of two blended polymers, which shows the temperature at which a polymer blend transitions from being miscible (below the black curve) to immiscible (above the black curve) as a function of the fraction of one polymer (denoted Composition ?) of the blend itself. The polymer blend is considered more miscible if the miscibility curve is shifted upwards, so that the blend turns immiscible at a higher temperature (see blue curve in Figure 1).

Kriisa and Roth wanted to explore how the application of an electric field influences the mixing dynamics of polystyrene (PS) and poly(vinyl methyl ether) (PVME) polymers. You may be quite familiar with these materials: PS is the formal name of styrofoam, the main component in plastic cups, and PVME is typically used in glues and adhesives. In the past, Kriisa and Roth studied the effect of electric fields in blends of these materials, and found that the electric field enhances polymer blend miscibility: an electric field raises the temperature at which a PS/PVME blend becomes immiscible, similarly to the blue curve in Figure 1 [1]. What interested the authors the most in this today’s paper was the dynamics of mixing; in other words, how quickly do immiscible blends remix once they are exposed to an electric field?  And what can we learn about the factors governing the remixing process?

The authors showed that the dynamics of mixing a PS/PVME blend is highly sensitive to the application of an electric field. They demonstrated this by examining a PS/PVME blend at the temperature four Kelvin higher than the temperature at which it becomes immiscible. They repeatedly switched on and off an electric field regularly, causing the blend to switch from being miscible to immiscible (see blue curve in Figure 2). To determine how well mixed the blend was, they measured the intensity of the light emitted by a fluorescing molecule, which was chemically attached to the PS molecules (see red curve in Figure 2). When PS and PVME are fully mixed, the fluorescence intensity decreases to 0. After switching on the electric field, the blend starts mixing immediately, showing a high sensitivity to the presence of the electric field.

The authors repeated this experiment for a variety of temperatures and electric field strengths. From the fluorescence curves, they extracted the remixing timescale or the time it takes for the blend to remix, as shown in Figure 3. The black symbols correspond to absence of electric field, while the red correspond to E = 12.8 MV/m and the blue to 13.0 MV/m. One may notice that the time it takes for the polymer blend to remix is largely independent of the electric field strength at a given temperature, since all remixing timescales (?) follow the same black curve. Thus, the authors concluded that the rate of remixing is not affected by the electric field.

In short, Kriisa and Roth showed that the dynamics of remixing polymer blends are sensitive to electricity. They found that immiscible blends immediately begin to remix when exposed to an electric field and that the time it takes for the blend to completely remix is independent of the field’s strength. From an industrial perspective, this shows that the miscibility of polymer blends can be influenced by factors other than temperature. An important advantage is that an electric field can be applied uniformly and instantaneously, whereas changes in temperature take time to propagate through materials. Thus, engineers may be able to instantly tune the miscibility of polymer blends using electric fields; a discovery that may lead to future technological advances in devices and materials whose properties would be quickly ‘’switched’’  through electricity.

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[1] Kriisa, A.; Roth, C. B. “Electric Fields Enhance Miscibility of Polystyrene/Poly(vinyl methyl ether) Blends.” J. Chem. Phys. 2014, 141, 134908.

Original paper: Hyperexpandable, self-healing macromolecular crystals with integrated polymer networks

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In the world of engineering, crafting a material that meets the needs of your application is challenging. Often, a given material may only provide a handful of the required properties for that application. Instead, you may choose to combine two or more materials, forming a composite with all of your desired properties. In this week’s paper, Zhang and coworkers from the University of California at San Diego took a similar approach in the world of biology by combining a biomolecular crystal with a flexible polymer. The crystal provides structure to the composite and the polymer contributes to its flexibility and expandability. They showed that the composite could reversibly expand to nearly 570% of its original volume and unexpectedly found that it was self-healing.

Before we dive into the meat of this paper, let’s look at the properties of crystalline materials.  An example is sodium chloride, also known as table salt, shown in Figure 1. You may immediately notice that the sodium (purple) and chloride (green) ions are precisely spaced apart from each other in a repeating pattern: a single sodium is surrounded by exactly six chlorides. This predictable structure is called a lattice. Many objects can form lattices if the interactions between neighboring objects can stabilize them. In the case of table salt, the crystal lattice is formed because sodium cations and chloride anions are oppositely charged, electrostatically attracting each other.

As mentioned earlier, biomolecules are also capable of forming crystals under right conditions. Ferritin is a hollow, spherical protein that is slightly negatively charged. As shown in Figure 2A, a given ferritin molecule is in direct contact with six other ferritin molecules, forming a lattice similar to table salt. You can see in Figure 2B that this lattice is held together by neighboring ferritins strongly interacting with calcium ions at the point where they come closest together. Because of the particular packing of the ferritin molecules caused by these interactions, a ferritin crystal is quite porous. Indicated by the arrow in Figure 2C, the pores between ferritin molecules are approximately 6 nanometers wide, large enough to allow water, salt solutions, and other liquids to soak into the ferritin crystal. In fact, the close contact interactions that stabilize the crystal are easily weakened when pure water is introduced into the pores, washing out calcium ions and dissolving the crystal. Instead, Zhang and coworkers wanted the crystal to expand but remain intact in water. Thus, they needed some kind of “glue.”

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Movie 1. A video of a hybrid crystal expanding when placed in pure water, followed by contraction after being placed in sodium chloride and calcium chloride solutions (by Zhang and coworkers).

They solved this problem by introducing a positively charged polymer into the pores of the crystal lattice. These polymers are known as hydrogels, as they can absorb a large amount of water and swell to many times their dry volume without dissolving away. Note that the hydrogel can’t prevent water from breaking the close contact interactions between the negatively charged ferritin molecules. Instead, the hydrogel holds the lattice in place to prevent it from dissolving due to electrostatic attraction between the hydrogel and each ferritin molecule. The close contact interactions can then be restored when a calcium salt solution is added. As shown in Movie 1, the authors demonstrated that the hybrid crystal could be expanded to nearly 570% its starting volume in the presence of pure water and returned to its original state when exposed to salt.

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Movie 2. A video showing several examples of ferritin crystal cracking and healing upon expansion (by Zhang and coworkers).

Aside from the reversible expandability of this hybrid crystal, Zhang and coworkers unexpectedly found that it can self-heal. If the crystal expands too quickly, it tends to crack, as shown in Movie 2. Despite this, the authors noticed that the cracks often healed scarlessly over time. Hydrogels cannot typically self-heal on their own, unless explicitly designed to do so. In the case of the hybrid crystal, the hydrogel and ferritin molecules work in concert to heal cracks. The hydrogel does not allow ferritin molecules on each side of the crack to drift far away. Over time, these ferritin molecules then reform the reversible close contact interactions, thereby healing the crystal. However, this process seems to be somewhat imperfect, as the crystals tend to crack in the same spots upon repeated contraction and expansion.

In short, Zhang and coworkers were able to create a self-healing material with the structure of crystalline matter and the expandability typical of polymers. Further, these hybrid materials were unexpectedly self-healing after cracking during too-rapid expansion. Many crystals formed from proteins and other biomolecules are porous like ferritin and are stabilized by similar close contact interactions. These crystals could also be infiltrated with hydrogel and similarly made expandable and resilient. As Zhang and coworkers have done, rationally combining the properties of various classes of matter will allow the engineering of novel materials for a myriad of applications and with useful, and quite unexpected, properties.