Active materials are materials that exhibit different behaviour in response to external stimuli.

Materials conducting force information in a range of sizes, capable of generating time-varying forces at a single point or an array of points (surface). For the purpose of Project Lyta Materials Research, we'll focuse on distributed force I/O devices for which active materials can be used to design an 'active surface'. Depending on the scale, the distributed force transducers should be able to reproduce a fine texture on a milimetre scale, as well as a 3D relief of a shape (such as a terrain map, haptic rendering of an image…). The materials have to be able to sense the contact pressure, contact area, as well as a variety of surface shapes and textures, with good spatial resolution. However, these materials are useful for sensing and actuation of dynamic forces, static forces do not supply a continuous source of energy

Piezo or strictor materials - undergo strain deformation in response to the change in electrical, magentic or thermal field

  • piezoelectric crystal can be used as both sensors and actuators (bi-directional), by sensing the generated voltage, or can generate force by applying voltage
  • piezoceramics - (discovered 1880) - electric field applied to the piezoceramic produces an elastic strain, which can provide an external force. Some of the piezoceramics can have non-linear functional dependence of the strain - such as in electrostrictor material PMN (lead magnese niobate). They are aminly used as force actuators and oscillators, but can also be used as sensors
  • piezopolymers, such as PVDF (polyvinylfluoride) often lack the stiffness required for most actuator applications, but are often used as thin film contact sensors and acoustic transducers.
  • magnetostrictors are ferromagnetic materials that undergo elastic strains when subjected to an external magentic field. most used compound is Terfenol-D (Tb.3Dy.7Fe2), because of its range of strain (10% or more). However, in order for this actuator to work, stress has to be applied prior to actuation (non-real-time). Another class of magnetoelastic materials are 'amorphous iron alloys', known as metallic glasses, that can be used as tensile force sensors.

Shape-Memory materials

  • are metals (SMA, shape-memory alloys, such as Ni-Ti, Au-Cd, Ag-Cd, Cu-Sn, Cu-Al-Ni and Cu-Zn-Al), ceramics (SMC) and polymers (SMP - often polyurethane (PU) films with a particular transition temperature) that exhibit shape changing transformations when subjected to changes in temperature. Heat can be applied either externally, or by applying electric current. A basic overview of these materials can be found on:

The problem with these materials is the time that it takes to change the temperature, and the small range of strain. Examples:

rare-earth permanent magnetsmake compact electromagnetic actuators (such as (NdFeB?). They can be used in systems that require large displacements, as a substitute for hydraulic actuators (also for transmission of sound and vibration).

Electroactive polymers (EAP) “The recent introduction of polymers that induce large strain under electrical activation led to their consideration as potential actuators. The level of induced strain can be as high as two orders of magnitude greater than the striction-limited, rigid and fragile electroactive ceramics (EAC). Also, they are superior to shape memory alloys (SMA) in their spectral response, lower density, and resilience. Generally, EAP are electrically hard and mechanically soft. On the other hand, EAP materials reach their elastic limit at lower stress levels compared to EAC, and their actuation stress falls far shorter than EAC and SMA actuators. The most attractive feature of EAPs is their ability to emulate biological muscles offering resilience, toughness, large actuation strain and inherent vibration damping. This similarity gained them the name “Artificial Muscles” with the potential of developing biologically inspired robots.” Polymers can be used to modulate an electric current via a force-dependent electrical resistance.. Two most common physical mechanisms deployed in resistive polymer force sensors is piezo-resistance and geometric deformation. (1) deformation type of force sensor is comprised of a conducting polymer strip or foam (such as polypyrole) are relies on the deformation of the polymer to cary the conducting cross-section, thus changing the resistance. (2) piezoresistive consists of semi-insulating polymer matrixcontaining some type of conductive particles (such as graphite). Homogenous piezoresistive polymers are difficult to make and are not easily commercially available.

ferrogels Volume phase transition in response to infinitesimal change of external stimuli has been observed in various gels. We report here an abrupt shape transformation occuring in magnetic field sensitive polymer gels, called ferrogels. In a ferrogel finely distributed colloidal particles having superparamagnetic behaviour are incorporated into the swollen network. These particles couple the shape of the gel to the non-uniform external magnetic field. Shape distortion occurs instantaneously and disappears abruptly when the external field is removed. A discontinuous elongation and contraction in response to infinitesimal change in external magnetic field has been observed. The elongation induced by non-uniform magnetic field can be 40% of the initial length. The magneto-elastic properties of ferrogels could be applicable to a variety of fields as a new driving mechanism. (src:

contractile hydro polymer gels

solid conducting polymers One example of these materials are carbon nanotubes. The material consists of nanometer size tubes and it was shown to induce stains at the range of 1% along the length. The key obstacle to the commercialization of this EAP is it high cost and the difficulty to mass-produce. A carbon nanotube actuator can be constructed in about by laminating together two narrow strips cut from a carbon nanotube sheet, using an intermediate adhesive layer, which is electronically insulated. The resulting “cantilever device” is immersed in an electrolyte, such as a sodium chloride solution, and an electrical connection is made to the two formed nanotube strips. The application of about a volt bends the actuator in one direction, and reversing the potential bends the actuator in the opposite direction.

Rheological Fluids Electro-rheological fluids (ERFs) are fluids that experience dramatic changes in rheological properties, such as viscosity, in the presence of an electric field. The fluids are made from suspensions of an insulating base fluid and particles on the order of one tenth to one hundred microns in size. The electro-rheological effect, is thought to arise from the difference in the dielectric constants of the fluid and particles. In the presence of an electric field, the particles, due to an induced dipole moment, will form chains along the field lines. This induced structure changes the ERF’s viscosity, yield stress, and other properties, allowing the ERF to change consistency from that of a liquid to something that is viscoelastic, such as a gel, with response times to changes in electric fields on the order of milliseconds.

Electro-viscoelastic elastomers (These EAP materials are composites of silicone elastomer and a polar phase. Before crosslinking, in the uncured state, they behave as electro-rheological fluids. An electric field is applied during curing to orient and fix in position the polar phase in the elastomeric matrix. These materials then remain in the “solid” state but have a shear modulus (both real and imaginary parts) that changes with applied electric field (< 6 V/mm) [Shiga, 1997]. A stronger magneto-rheological effect can also be introduced in an analogous manner and as much as a 50% change in the shear modulus can be induced [Davis, 1999]. These materials may be used as alternatives to electrorheological fluids.

fletc1b.gif srcs: biomimetic-polymers-handbook.pdf ==== Shock Absorption ==== * Technogel: SMP polyurethane gel that deforms on all three axis, does not harden or deform with age: * Hydrogel: a material which contains water, electrolytes, (and humectant, if needed), etc. into hydrophilic polymer matrix. ==== Chromic Materials ==== “Other types of intelligent textiles are those which change their colour reversibly according to external environmental conditions, for this reason they are also called chameleon fibres. Chromic materials are the general term referring to materials which radiate the colour, erase the colour or just change it because its induction caused by the external stimuli, as “chromic” is a suffix that means colour. So we can classify chromic materials depending on the stimuli affecting them: * Photochromic: external stimuli energy is light. * Thermochromic: external stimuli energy is heat. * Electrochromic: external stimuli energy is electricity. * Piezorochromic: external stimuli energy is pressure. * Solvatechromic: external stimuli energy is liquid. * Carsolchromic: external stimuli energy is electron beam. === Chromic Inks === Of interest to Project Lyta: “Piezochromic Ink is consist of particles which are spherical type and able to provide the variable functions according to the properties of cell wall prepared with the capsule and capsule inner materials. Piezochromic Ink is usually used to Pressure Sensitive Paper Its colour developing mechanism can be explained that microencapsulated dyes can be emitted when the wall of capsule is broken by receiving the proper pressure.” received from: * company in the UK printing photo/thermo/piezochromic inks on a variety of materials: * chromatic technologies inc: ==== Multifunctional Textiles ==== “Empa applies chemical and physical methods to surfaces and gives textiles totally new features with the aid of plasma technology which selectively modifies the top nanometer-thin layers. Plasma technology is a surface-sensitive method that allows selective modification in the nm-range. By introducing energy into a gas, a quasi-neutral plasma can be generated consisting of neutral particles, electrically charged particles and highly reactive radicals.(…) If, for example, a fiber is metallized with a nm-thin layer of silver, its electrical conductivity increases significantly. As an additional effect this also improves the antibacterial and fungicidal properties of the fiber. (…) Another objective is to incorporate fibers in textiles as sensors by coating the fibers with nanocrystalline piezoceramic materials which Empa is developing jointly with two companies, Unaxis Balzers AG and Umicore Materials AG, as part of a TOP NANO 21 project. In this way mechanical forces exerted on the fibers can be converted into electrical signals and analyzed later. If these sensor-based textiles are worn next to the skin, bodily functions such as heart rhythm and pulse can be monitored without the need for any additional devices.”*/22276/—/l=2 Contact: Dr Armin Fischer, Laboratory for Functional Fibers and Textiles, Telephone +41 71 274 76 79, E-mail: ==== Light Modulating Materials ==== * Colour Changing Gel “Japanese researchers have developed a polymer that switches between coloured and transparent states when its environment changes. It could cut the cost of screens and sensors. Changes in ambient temperature, acidity (pH), light level or the presence of toxins or drugs can trigger the colour change.” “The Fuji Xerox team make tiny, contractible pigment bags from a polymer known as NIPAM. Its long, chainlike molecules can be crosslinked to form a soft gel. The gel's volume is controlled by temperature. At around 34 °C, the polymer molecules suddenly contract, and the gel collapses to 10% or less of its original volume. Akashi and colleagues make particles consisting of NIPAM pigment bags just 20-200 thousandths of a millimetre across when swollen. The researchers load these particles with large amounts of pigments such as carbon black (used in Indian ink) without significantly affecting their temperature-triggered shrinking. Gel particles dispersed in a solvent at room temperature are swollen, and the liquid appears dark. When warmed to 40 °C, the particles shrink and the liquid turns almost clear. The researchers have created a switchable window by sandwiching this liquid between two glass plates. Heat controls this window, just as electricity switches a liquid-crystal cell.” Reference: Akashi, R., Tsutsui, H. & Komura, A. Polymer gel light-modulation materials imitating pigment cells. Advanced Materials, 14, 1808 - 1811, (2002). Resource: Communication Polymer Gel Light-Modulation Materials Imitating Pigment Cells R. Akashi^ * , H. Tsutsui, A. Komura. Intelligent Devices Laboratory, Fuji Xerox Company Limited, 1600 Takematsu Minamiashigara, Kanagawa 250-0111, Japan Correspondence to R. Akashi, Intelligent Devices Laboratory, Fuji Xerox Company Limited, 1600 Takematsu Minamiashigara, Kanagawa 250-0111, Japan. * Iridescent ink “A new ink changes colour at the flick of a switch. It could give rise to newspapers that show shifting images, or chemical sensors that display different hues depending on what substance they detect. The substance is called P-Ink or 'photonic ink', and is being developed by Geoffrey Ozin, Ian Manners and their colleagues at the University of Toronto, Canada.” “P-Ink's iridescent colours, like those of butterfly wings or opal, depend upon a process called diffraction. The ink contains spheres of silicon dioxide about 300 millionths of a millimetre across, stacked like oranges on a greengrocer's stall. When light bounces off these, interference eliminates some wavelengths, giving the reflected light a certain colour. It is surprisingly easy to stack tiny spheres like this - they simply settle out of a liquid into an organized array called a colloidal crystal. To make the colour of the ink tunable, Ozin's team packs a polymer gel between its stacked spheres. This gel swells when it is soaked in solvent and shrinks when it dries. The nanospheres' spacing dictates the wavelength of light that they reflect, so swelling - which takes less than half a second - changes the film's colour, shifting it towards the red end of the spectrum. Moreover, the swelling gel conducts electricity. Applying a voltage makes it increasingly positively charged, which determines how much solvent it sucks up. So altering the voltage tunes the ink's colour smoothly through the spectrum.” Reference: Arsenault, A. C., Míguez, H., Kitaev, V., Ozin, G. A. & Manners, I. A polychromic, fast response metallopolymer gel photonic crystal with solvent and redox tunability: a step towards photonic ink. Advanced Materials, in press, (17 March 2003 Resource: ==== Phase Change Materials (PCMs) ==== Phase change materials (PCMs) are “latent” thermal storage materials. They use chemical bonds to store and release heat. The thermal energy transfer occurs when a material changes from a solid to a liquid, or from a liquid to a solid. This is called a change in state, or “phase.” Initially, these solid-liquid PCMs perform like conventional storage materials; their temperature rises as they absorb solar heat. Unlike conventional (sensible) storage materials, when PCMs reach the temperature at which they change phase (their melting point) they absorb large amounts of heat without getting hotter. src: * a list of available materials: ==== Workshop Notes ==== Softwear Workshop ==== Related Topics ==== * Project Lyta Materials Research * Frankfurt Textil * Second Skin * Flexible Displays

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