Magnetic nanoparticles are a class of nanoparticle that can be manipulated using magnetic fields. Such particles consist of two components, a magnetic material iron and cobalt, a chemical component that has functionality. While nanoparticles are smaller than 1 micrometer in diameter, the larger microbeads are 0.5–500 micrometer in diameter. Magnetic nanoparticle clusters that are composed of a number of individual magnetic nanoparticles are known as magnetic nanobeads with a diameter of 50–200 nanometers. Magnetic nanoparticle clusters are a basis for their further magnetic assembly into magnetic nanochains; the magnetic nanoparticles have been the focus of much research because they possess attractive properties which could see potential use in catalysis including nanomaterial-based catalysts and tissue specific targeting, magnetically tunable colloidal photonic crystals, magnetic resonance imaging, magnetic particle imaging, data storage, environmental remediation, optical filters, defect sensor, magnetic cooling and cation sensors.
The physical and chemical properties of magnetic nanoparticles depend on the synthesis method and chemical structure. In most cases, the particles may display superparamagnetism. Ferrite nanoparticles or iron oxide nanoparticles are the most explored magnetic nanoparticles up to date. Once the ferrite particles become smaller than 128 nm they become superparamagnetic which prevents self agglomeration since they exhibit their magnetic behavior only when an external magnetic field is applied; the magnetic moment of ferrite nanoparticles can be increased by controlled clustering of a number of individual superparamagnetic nanoparticles into superparamagnetic nanoparticle clusters, namely magnetic nanobeads. With the external magnetic field switched off, the remanence falls back to zero. Just like non-magnetic oxide nanoparticles, the surface of ferrite nanoparticles is modified by surfactants, silicones or phosphoric acid derivatives to increase their stability in solution; the surface of a maghemite or magnetite magnetic nanoparticle is inert and does not allow strong covalent bonds with functionalization molecules.
However, the reactivity of the magnetic nanoparticles can be improved by coating a layer of silica onto their surface. The silica shell can be modified with various surface functional groups via covalent bonds between organo-silane molecules and silica shell. In addition, some fluorescent dye molecules can be covalently bonded to the functionalized silica shell. Ferrite nanoparticle clusters with narrow size distribution consisting of superparamagnetic oxide nanoparticles coated with a silica shell have several advantages over metallic nanoparticles: Higher chemical stability Narrow size distribution Higher colloidal stability since they do not magnetically agglomerate Magnetic moment can be tuned with the nanoparticle cluster size Retained superparamagnetic properties Silica surface enables straightforward covalent functionalization Metallic nanoparticles may be beneficial for some technical applications due to their higher magnetic moment whereas oxides would be beneficial for biomedical applications.
This implies that for the same moment, metallic nanoparticles can be made smaller than their oxide counterparts. On the other hand, metallic nanoparticles have the great disadvantage of being pyrophoric and reactive to oxidizing agents to various degrees; this makes their handling difficult and enables unwanted side reactions which makes them less appropriate for biomedical applications. Colloid formation for metallic particles is much more challenging; the metallic core of magnetic nanoparticles may be passivated by gentle oxidation, surfactants and precious metals. In an oxygen environment, Co nanoparticles form an anti-ferromagnetic CoO layer on the surface of the Co nanoparticle. Work has explored the synthesis and exchange bias effect in these Co core CoO shell nanoparticles with a gold outer shell. Nanoparticles with a magnetic core consisting either of elementary Iron or Cobalt with a nonreactive shell made of graphene have been synthesized recently; the advantages compared to ferrite or elemental nanoparticles are: Higher magnetization Higher stability in acidic and basic solution as well as organic solvents Chemistry on the graphene surface via methods known for carbon nanotubes Several methods exist for preparing magnetic nanoparticle.
Co-precipitation is a facile and convenient way to synthesize iron oxides from aqueous Fe2+/Fe3+ salt solutions by the addition of a base under inert atmosphere at room temperature or at elevated temperature. The size and composition of the magnetic nanoparticles much depends on the type of salts used, the Fe2+/Fe3+ ratio, the reaction temperature, the pH value and ionic strength of the media, the mixing rate with the base solution used to provoke the precipitation; the co-precipitation approach has been used extensively to produce ferrite nanoparticles of controlled sizes and magnetic properties. A variety of experimental arrangements have been reported to facilitate continuous and large–scale co–precipitation of magnetic particles by rapid mixing; the growth rate of the magnetic nanoparticles was measured in real-time during the precipitation of magnetite nanoparticles by an integrated AC magnetic susceptometer within the m
Von Neumann universal constructor
John von Neumann's universal constructor is a self-replicating machine in a cellular automata environment. It was designed without the use of a computer; the fundamental details of the machine were published in von Neumann's book Theory of Self-Reproducing Automata, completed in 1966 by Arthur W. Burks after von Neumann's death. Von Neumann's goal was to specify an abstract machine. In his design, the machine consists of three parts: a'blueprint' for itself, a mechanism that can read any blueprint and construct the machine specified by that blueprint, a'copy machine' that can make copies of any blueprint. After the mechanism has been used to construct the machine specified by the blueprint, the copy machine is used to create a copy of that blueprint, this copy is placed into the new machine, resulting in a working replication of the original machine; some machines will do this backwards, copying the blueprint and building a machine. To define his machine in more detail, von Neumann invented the concept of a cellular automaton.
The one he used consists of a two-dimensional grid of cells, each of which can be in one of 29 states at any point in time. At each timestep, each cell updates its state depending on the states of the surrounding cells at the prior timestep; the rules governing these updates are identical for all cells. The universal constructor is a certain pattern of cell states in this cellular automaton, it contains one line of cells that serve as a'tape', encoding a sequence of instructions that serve as a'blueprint' for the machine. The machine performs the corresponding actions; the instructions direct the machine to use its'construction arm' to build a copy of the machine, without tape, at some other location in the cell grid. The tape can't contain instructions to build an long tape, just as a container can't contain a container of the same size. Therefore, the machine contains a separate'copy machine' which reads the tape and places a copy into the newly constructed machine; the resulting new machine and tape is identical to the old one, it proceeds to replicate again.
Von Neumann's design has traditionally been understood to be a demonstration of the logical requirements for machine self-replication. However, it is clear. Examples include trivial crystal-like growth, template replication, Langton's loops, but von Neumann was interested in something more profound: construction and evolution. This universal constructor can be seen as an abstract simulation of a physical universal assembler. Note that the simpler self-replicating CA structures cannot exist in a wide variety of forms and thus have limited evolvability. Other CA structures such as the Evoloop are somewhat evolvable but still don't support open-ended evolution. Simple replicators do not contain the machinery of construction, there being a degree to which the replicator is information copied by its surrounding environment. Although the Von Neumann design is a logical construction, it is in principle a design that could be instantiated as a physical machine; the issue of the environmental contribution to replication is somewhat open, since there are different conceptions of raw material and its availability.
The concept of a universal constructor is non-trivial because of the existence of Garden of Eden patterns. But a simple definition is that a universal constructor is able to construct any finite pattern of non-excited cells. Von Neumann's crucial insight is; this part is played by the tape of instructions in Von Neumann's combination of universal constructor plus instruction tape. The combination of a universal constructor and a tape of instructions would i) allow self-replication, ii) guarantee that the open-ended complexity growth observed in biological organisms was possible; the image below illustrates this possibility. This insight is all the more remarkable because it preceded the discovery of the structure of the DNA molecule by Watson and Crick, though it followed the Avery–MacLeod–McCarty experiment which identified DNA as the molecular carrier of genetic information in living organisms; the DNA molecule is processed by separate mechanisms that carry out its instructions and copy the DNA for insertion for the newly constructed cell.
The ability to achieve open-ended evolution lies in the fact that, just as in nature, errors in the copying of the genetic tape can lead to viable variants of the automaton, which can evolve via natural selection. Arthur Burks and others extended the work of von Neumann, giving a much clearer and complete set of details regarding the design and operation of von Neumann's self-replicator; the work of J. W. Thatcher is noteworthy, for he simplified the design. Still, their work did not yield a complete design, cell by cell, of a configuration capable of demonstrating self-replication. Renato Nobili and Umberto Pesavento published the first implemented self-reproducing cellular automaton in 1995, nearly fifty years after von Neumann's work, they used a 32-state cellular automaton instead of von Neumann's original 29-state specification, extending it to allow for easier signal-crossing, explicit memory function and a more compact design. They published an implementation of a general constructor within the original 29-state CA but not one capable of complete replication - the configuration cannot duplicate its tape, nor can it trigger its offspring.
Dry ice is the solid form of carbon dioxide. It is used as a cooling agent, its advantages include lower temperature than that of water ice and not leaving any residue. It is useful for preserving frozen foods. Dry ice sublimates at Earth atmospheric pressures; this extreme cold makes the solid dangerous to handle without protection due to burns caused by freezing. While not toxic, the outgassing from it can cause hypercapnia due to buildup in confined locations. Dry ice is the solid form of carbon dioxide, a molecule consisting of a single carbon atom bonded to two oxygen atoms. Dry ice is colorless, non-flammable, with a sour zesty odor, can lower the pH of a solution when dissolved in water, forming carbonic acid. At pressures below 5.13 atm and temperatures below −56.4 °C, CO2 changes from a solid to a gas with no intervening liquid form, through a process called sublimation. The opposite process is called deposition. At atmospheric pressure, sublimation/deposition occurs at −78.5 °C or 194.65 K.
The density of dry ice varies, but ranges between about 1.4 and 1.6 g/cm3. The low temperature and direct sublimation to a gas makes dry ice an effective coolant, since it is colder than water ice and leaves no residue as it changes state, its enthalpy of sublimation is 571 kJ/kg. Dry ice is non-polar, with a dipole moment of zero, so attractive intermolecular van der Waals forces operate; the composition results in low electrical conductivity. It is accepted that dry ice was first observed in 1835 by French inventor Adrien-Jean-Pierre Thilorier, who published the first account of the substance. In his experiments, it was noted that when opening the lid of a large cylinder containing liquid carbon dioxide, most of the liquid carbon dioxide evaporated; this left only solid dry ice in the container. In 1924, Thomas B. Slate applied for a US patent to sell dry ice commercially. Subsequently, he became the first to make dry ice successful as an industry. In 1925, this solid form of CO2 was trademarked by the DryIce Corporation of America as "Dry ice", thus leading to its common name.
That same year the DryIce Co. sold the substance commercially for the first time. Dry ice is manufactured. First, gases with a high concentration of carbon dioxide are produced; such gases can be a byproduct of another process, such as producing ammonia from nitrogen and natural gas, oil refinery activities or large-scale fermentation. Second, the carbon dioxide-rich gas is refrigerated until it liquefies. Next, the pressure is reduced; when this occurs some liquid carbon dioxide vaporizes, causing a rapid lowering of temperature of the remaining liquid. As a result, the extreme cold causes the liquid to solidify into a snow-like consistency; the snow-like solid carbon dioxide is compressed into small pellets or larger blocks of dry ice. Dry ice is produced in three standard forms: large blocks, cylindrical small pellets and cylindrical tiny, high surface to volume pellets that float on oil or water and do not stick to skin because of their high radii of curvature. Tiny dry ice pellets are used for ice blasting, quick freezing, fire fighting, oil solidifying and have been found to be safe for experimentation by middle school students wearing appropriate personal protective equipment such as gloves and safety glasses.
A standard block weighing 30 kg covered in a taped paper wrapping is most common. These are used in shipping, because they sublime slowly due to a low ratio of surface area to volume. Pellets can be bagged easily; this form is suited to small scale use, for example at grocery stores and laboratories where it is stored in a thickly insulated chest. The most common use of dry ice is to preserve food, it is used to package items that must remain cold or frozen, such as ice cream or biological samples, without the use of mechanical cooling. Dry ice can be used to flash-freeze food or laboratory biological samples, carbonate beverages, make ice cream, solidify oil spills and stop ice sculptures and ice walls from melting. Dry ice can be used to arrest and prevent insect activity in closed containers of grains and grain products, as it displaces oxygen, but does not alter the taste or quality of foods. For the same reason, it can retard food oils and fats from becoming rancid; when dry ice is placed in water, sublimation is accelerated, low-sinking, dense clouds of smoke-like fog are created.
This is used in fog machines, at theaters, haunted house attractions, nightclubs for dramatic effects. Unlike most artificial fog machines, in which fog rises like smoke, fog from dry ice hovers near the ground. Dry ice is useful in theater productions; the fog originates from the bulk water into which the dry ice is placed, not from atmospheric water vapor. It is used to freeze and remove warts. However, liquid nitrogen performs better in this role, since it is colder so requires less time to act, less pressure. Dry ice has fewer problems with storage, since it can be generated from compressed carbon dioxide gas as needed. Plumbers use equipment; the dry ic
Utility fog is a hypothetical collection of tiny robots that can replicate a physical structure. As such, it is a form of self-reconfiguring modular robotics. Hall thought of it as a nanotechnological replacement for car seatbelts; the robots would be microscopic, with extending arms reaching in several different directions, could perform three-dimensional lattice reconfiguration. Grabbers at the ends of the arms would allow the robots to mechanically link to one another and share both information and energy, enabling them to act as a continuous substance with mechanical and optical properties that could be varied over a wide range; each foglet would have substantial computing power, would be able to communicate with its neighbors. In the original application as a replacement for seatbelts, the swarm of robots would be spread out, the arms loose, allowing air flow between them. In the event of a collision the arms would lock into their current position, as if the air around the passengers had abruptly frozen solid.
The result would be to spread any impact over the entire surface of the passenger's body. While the foglets would be micro-scale, construction of the foglets would require full molecular nanotechnology. Hall suggests; each arm would have four degrees of freedom. The foglets' bodies would be made of aluminum oxide rather than combustible diamond to avoid creating a fuel air explosive. Hall and his correspondents soon realised that utility fog could be manufactured en masse to occupy the entire atmosphere of a planet and replace any physical instrumentality necessary to human life. By foglets exerting concerted force an object or human could be carried from location to location. Virtual buildings could be constructed and dismantled within moments, enabling the replacement of existing cities and roads with farms and gardens. While molecular nanotech might replace the need for biological bodies, utility fog would remain a useful peripheral with which to perform physical engineering and maintenance tasks.
Thus, utility fog came to be known as ″the machine of the future". Claytronics Grey goo Molecular machines Nanorobotics Nanotechnology Programmable matter Self-reconfiguring modular robotics Smartdust Synthetic biology The Invincible, a 1964 science fiction novel with intrigue centered on nanobotic swarms Utility Fog at Nanotech Now, many links
An electric car is a plug-in electric automobile, propelled by one or more electric motors, using energy stored in rechargeable batteries. From 2008, a renaissance in electric vehicle manufacturing occurred due to advances in batteries and deaths from air pollution, the desire to reduce greenhouse gas emissions. Several national and local governments have established tax credits and other incentives to promote the introduction and adoption in the mass market of new electric vehicles depending on battery size, their electric range and purchase price; the current maximum tax credit allowed by the US Government is US$7,500 per car. Compared with internal combustion engine cars, electric cars are quieter, have no tailpipe emissions, lower emissions in general. Charging an electric car can be done at a variety of charging stations, these charging stations can be installed in both houses and public areas; the two all-time best selling electric cars, the Nissan Leaf and the Tesla Model S, have EPA-rated ranges reaching up to 151 mi and 335 mi respectively.
The Leaf is the best-selling highway-capable electric car with more than 400,000 units sold globally by March 2019, followed by the Tesla Model S with 263,500 units sold worldwide by December 2018. As of December 2018, there were about 5.3 million light-duty all-electric and plug-in hybrid vehicles in use around the world. Despite the rapid growth experienced, the global stock of plug-in electric cars represented just about 1 out of every 250 vehicles on the world's roads by the end of 2018; the plug-in car market is shifting towards electric battery vehicles, as the global ratio between annual sales of battery BEVs and PHEVs went from 56:44 in 2012, to 60:40 in 2015, rose to 69:31 in 2018. Electric cars are a variety of electric vehicle; the term "electric vehicle" refers to any vehicle that uses electric motors for propulsion, while "electric car" refers to highway-capable automobiles powered by electricity. Low-speed electric vehicles, classified as neighborhood electric vehicles in the United States, as electric motorised quadricycles in Europe, are plug-in electric-powered microcars or city cars with limitations in terms of weight and maximum speed that are allowed to travel on public roads and city streets up to a certain posted speed limit, which varies by country.
While an electric car's power source is not explicitly an on-board battery, electric cars with motors powered by other energy sources are referred to by a different name. An electric car carrying solar panels to power it is a solar car, an electric car powered by a gasoline generator is a form of hybrid car. Thus, an electric car that derives its power from an on-board battery pack is a form of battery electric vehicle. Most the term "electric car" is used to refer to battery electric vehicles, but may refer to plug-in hybrid electric vehicles. In 1884, over 20 years before the Ford Model T, Thomas Parker built the first practical production electric car in London using his own specially designed high-capacity rechargeable batteries; the Flocken Elektrowagen of 1888 was designed by German inventor Andreas Flocken. Electric cars were among the preferred methods for automobile propulsion in the late 19th century and early 20th century, providing a level of comfort and ease of operation that could not be achieved by the gasoline cars of the time.
The electric vehicle stock peaked at 30,000 vehicles at the turn of the 20th century. In 1897, electric cars found their first commercial use in the US. Based on the design of the Electrobat II, a fleet of twelve hansom cabs and one brougham were used in New York City as part of a project funded in part by the Electric Storage Battery Company of Philadelphia. During the 20th century, the main manufacturers of electric vehicles in the US were Anthony Electric, Columbia, Edison, Milburn, Bailey Electric and others. Unlike gasoline-powered vehicles, the electric ones were less noisy, did not require gear changes. Advances in internal combustion engines in the first decade of the 20th century lessened the relative advantages of the electric car, their much quicker refueling times, cheaper production costs, made them more popular. However, a decisive moment was the introduction in 1912 of the electric starter motor which replaced other laborious, methods of starting the ICE, such as hand-cranking.
Six electric cars held the land speed record. The last of them was the rocket-shaped La Jamais Contente, driven by Camille Jenatzy, which broke the 100 km/h speed barrier by reaching a top speed of 105.88 km/h on 29 April 1899. In the early 1990s, the California Air Resources Board began a push for more fuel-efficient, lower-emissions vehicles, with the ultimate goal being a move to zero-emissions vehicles such as electric vehicles. In response, automakers developed electric models, including the Chrysler TEVan, Ford Ranger EV pickup truck, GM EV1, S10 EV pickup, Honda EV Plus hatchback, Nissan Altra EV miniwagon, Toyota RAV4 EV. Both US Electricar and Solectria produced 3-phase AC Geo-bodied electric cars with the support of GM, Delco; these early cars were withdrawn from the U. S. market. California electric automaker Tesla Motors began development in 2004 on what would become the Tesla Roadster, first delivered to customers in 2008; the Roadster was the first highway legal serial production all-electric car to use lithium-ion battery cells, the first production all-electric car to travel more than 320 km per charge.
Tesla global sales passed 250,000 units in September 2017. The Renault–Nissa
A cloaking device is a hypothetical or fictional stealth technology that can cause objects, such as spaceships or individuals, to be or wholly invisible to parts of the electromagnetic spectrum. However, over the entire spectrum, a cloaked object scatters more than an uncloaked object. Fictional cloaking devices have been used as plot devices in various media for many years. Developments in scientific research show that real-world cloaking devices can obscure objects from at least one wavelength of EM emissions. Scientists use artificial materials called metamaterials to bend light around an object. Star Trek screenwriter Paul Schneider, inspired in part by the 1958 film Run Silent, Run Deep, in part by The Enemy Below, which in turn had been released the previous year, 1957, imagined cloaking as a space-travel analog of a submarine submerging, employed it in the 1966 Star Trek episode "Balance of Terror", in which he introduced the Romulan species. Another Star Trek screenwriter, D. C. Fontana, coined the term "cloaking device" for the 1968 episode "The Enterprise Incident", which featured Romulans.
Writers and game designers have since incorporated cloaking devices into many other science-fiction narratives, including Doctor Who, Star Wars, Stargate. An operational, non-fictional cloaking device might be an extension of the basic technologies used by stealth aircraft, such as radar-absorbing dark paint, optical camouflage, cooling the outer surface to minimize electromagnetic emissions, or other techniques to minimize other EM emissions, to minimize particle emissions from the object; the use of certain devices to jam and confuse remote sensing devices would aid in this process, but is more properly referred to as "active camouflage". Alternatively, metamaterials provide the theoretical possibility of making electromagnetic radiation pass around the'cloaked' object. Optical metamaterials have featured in several recent proposals for invisibility schemes. "Metamaterials" refers to materials that owe their refractive properties to the way they are structured, rather than the substances that compose them.
Using transformation optics it is possible to design the optical parameters of a "cloak" so that it guides light around some region, rendering it invisible over a certain band of wavelengths. These spatially varying optical parameters do not correspond to any natural material, but may be implemented using metamaterials. There are several theories of giving rise to different types of invisibility. In 2014, scientists demonstrated good cloaking performance in murky water, demonstrating that an object shrouded in fog can disappear when appropriately coated with metamaterial; this is due to the random scattering of light, such as that which occurs in clouds, milk, frosted glass, etc. combined with the properties of the metamaterial coating. When light is diffused, a thin coat of metamaterial around an object can make it invisible under a range of lighting conditions. Active camouflage is a group of camouflage technologies which would allow an object to blend into its surroundings by use of panels or coatings capable of changing color or luminosity.
Active camouflage can be seen as having the potential to become the perfection of the art of camouflaging things from visual detection. Optical camouflage is a kind of active camouflage in which one wears a fabric which has an image of the scene directly behind the wearer projected onto it, so that the wearer appears invisible; the drawback to this system is that, when the cloaked wearer moves, a visible distortion is generated as the'fabric' catches up with the object's motion. The concept exists for now only in theory and in proof-of-concept prototypes, although many experts consider it technically feasible, it has been reported. Mercedes demonstrated an invisible car using LED and camera in 2012. Plasma at certain density ranges absorbs certain bandwidths of broadband waves rendering an object invisible. However, generating plasma in air is too expensive and a feasible alternative is generating plasma between thin membranes instead; the Defense Technical Information Center is following up research on plasma reducing RCS technologies.
A plasma cloaking device was patented in 1991. A prototype Metascreen is a claimed cloaking device, just few micrometers thick and to a limited extent can hide 3D objects from microwaves in their natural environment, in their natural positions, in all directions, from all of the observer's positions, it was prepared at the University of Texas, Austin by Professor Andrea Alù. The metascreen consisted of a 66 micrometre thick polycarbonate film supporting an arrangement of 20 micrometer thick copper strips that resembled a fishing net. In the experiment, when the metascreen was hit by 3.6 GHz microwaves, it re-radiated microwaves of the same frequency that were out of phase, thus cancelling out reflections from the object being hidden. The device only cancelled out the scattering of microwaves in the first order; the same researchers published a paper on "plasmonic cloaking" the previous year. University of Rochester physics professor John Howell and graduate student Joseph Choi have announced a scalable cloaking device which uses common optical lenses to achieve visible light cloaking on the macroscopic scale, known as the "Rochester Cloak".
The device consists of a series of four lenses which direct light rays around objects which would otherwise occlude the optical pathway. The concepts of cloaking are
4-dimensional printing uses the same techniques of 3D printing through computer-programmed deposition of material in successive layers to create a three-dimensional object. However, 4D printing adds the dimension of transformation over time, it is therefore a type of programmable matter, wherein after the fabrication process, the printed product reacts with parameters within the environment and changes its form accordingly. The ability to do so arises from the near infinite configurations at a micrometer resolution, creating solids with engineered molecular spatial distributions and thus allowing unprecedented multifunctional performance. 4D printing is a new advance in biofabrication technology emerging as a new paradigm in disciplines such as bioengineering, materials science and computer sciences. Stereolithography is a 3D-printing technique that uses photopolymerization to bind substrate, laid layer upon layer, creating a polymeric network; as opposed to fused-deposition modeling, where the extruded material hardens to form layers, 4D printing is fundamentally based in stereolithography, where in most cases ultraviolet light is used to cure the layered materials after the printing process has completed.
Anisotropy is vital in engineering the direction and magnitude of transformations under a given condition, by arranging the micromaterials in a way so that there is an embedded directionality to the finished print. It is possible, through 4D printing, to achieve rapid and accurate manufacturing methods for controlling spatial self-bending actuation in custom-designed soft structures. Spatial and temporal transformations can be realized through several actuation mechanisms such as liquid crystal gel phase transition, thermal expansion coefficient, thermal conductivity discrepancies, the different swelling and de-swelling ratios of bi-layer or composite beams. One approach to model 4D printing is to control 3D-printing parameters, such as different spatial patterns of hinges affecting the response time and bending angle of the 4D print products. A parametric model of physical properties of shape memory polymer panes incorporating 3D printed patterns was developed to that end; the proposed model predicts the final shape of the actuator with an excellent qualitative agreement with experimental studies.
These validated results can guide the design of functional pattern-driven 4D printings. ––– Most 4D printing systems utilize a network of fibers that vary in size and material properties. 4D printed components can be designed on the macro scale as well as the micro scale. Micro scale design is achieved through complex molecular/fiber simulations that approximate the aggregated material properties of all the materials used in the sample; the size, shape and connection pattern of these material building blocks have a direct relationship to the deformation shape under stimulus activation. Skylar Tibbits is the director of the Self-Assembly Lab at MIT, worked with the Stratasys Materials Group to produce a composite polymer composed of hydrophilic elements and non-active, rigid elements; the unique properties of these two disparate elements allowed up to 150% swelling of certain parts of the printed chain in water, while the rigid elements set structure and angle constraints for the transformed chain.
Tibbits et al. produced a chain that would spell “MIT” when submerged in water, another chain that would morph into a wireframe cube when subjected to the same conditions. Thiele et al. explored the possibilities of a cellulose-based material that could be responsive to humidity. They developed a bilayer film using cellulose steraroyl esters with different substitution degrees on either side. One ester had a substitution degree of 0.3 and the other had a substitution degree of 3 When the sample was cooled from 50 °C to 22 °C, the relative humidity increased from 5.9% to 35%, the hydrophobic side contracted and the hydrophilic side swelled, causing the sample to roll up tightly. This process is reversible, as reverting the temperature and humidity changes caused the sample to unroll again. Understanding anisotropic swelling and mapping the alignment of printed fibrils allowed A. Sydney Gladman et al. to mimic the nastic behavior of plants. Branches, stems and flowers respond to environmental stimuli such as humidity and touch by varying the internal turgor of their cell walls and tissue composition.
Taking precedent from this, the team developed a composite hydrogel architecture with local anisotropic swelling behavior that mimics the structure of a typical cell wall. Cellulose fibrils combine during the printing process into microfibrils with a high aspect ratio and an elastic modulus on the scale of 100 GPa; these microfibrils are embedded into a soft acrylamide matrix for structure. The viscoelastic ink used to print this hydrogel composite is an aqueous solution of N,N-dimethylacrylamide, glucose oxidase and nanofibrillated cellulose; the nanoclay is a rheological aid that improves liquid flow, the glucose prevents oxygen inhibition when the material is cured with ultraviolet light. Experimenting with this ink, the team created a theoretical model for a print path that dictates the orientation of cellulose fibrils, where the bottom layer of the print is parallel to the x-axis and the top layer of the print is rotated anticlockwise by an angle θ; the curvature of the sample is dependent on elastic moduli, swelling ratios, ratios of layer thickness and bilayer thickness.
Thus, the adjusted models that describe mean curvature and Gaussian curvature are H = c