Liquid crystal elastomers (LCEs) combine the elasticity of polymer networks with the fluidity and responsiveness of liquid crystals. LCEs can respond to a variety of external stimuli - heat, light, electric and magnetic fields - with large and reversible shape-changes. However, the response is typically slow and requires large external fields. Here, we present our recent work with LCE bilayers and LCE composite materials that respond quickly, reversibly, and with 3-D shape changes to various externa stimuli, including heat, light, and electric fields. LCE nanocomposites are prepared by adding conductive carbon black nanoparticles to polysiloxane-LCEs in two steps. While conventional nematic LCEs are relatively unresponsive to electric fields, conductive LCE composites reversibly contract and expand in response to a 40 V electric field. The response time ( 0.1 - 10 Hz) and amplitude of shape change (1 - 20 %) can be tuned through variation of the external field and carbon black content. Next, we demonstrate expoxy-based LCEs with a reversibly crosslinked network. LCEs synthesized through transesterification reaction can be re-aligned and re-shaped by heating above the reversible crosslink temperature. Reversible LCEs are typically stiffer than polysiloxane-based LCEs and exhibit significantly higher glass-transition and isotropic transition temperatures, but through tailoring of the network composition and crosslink density, reversible LCEs with a glass-transition below room temperature are achieved. The liquid crystal phase and the hydrophilicity of the network can be tuned by incorporating functionalized chains of varying composition. We demonstrate that LCEs can be operated in biological media and can be used as responsive substrates for dynamic cell culture. Neonatal rat ventricular myocytes remain viable on LCE-carbon black bilayer substrates, and aligned myocyte cell sheets were successfully grown on LCE-composite bilayers by stimulating the LCE substrate continuously during cell culture. 5% reversible extensional strains were applied during cell culture through stimulation with an electric field or through direct local heating. The resulting cells showed improved alignment when grown on responsive LCE substrates. This work demonstrates fast and reversible shape changes in LCE nanocomposites and the potential of this class of materials for mechanical assist devices, dynamic substrates, or tissue scaffolds.

Shape can be a functional property of devices. Shape is derived topology and is designed evident in the planar fold lines of a cardboard box or the mechanical anisotropy of certain natural structures. Here, we report on the use of light to pattern the local alignment of liquid crystal elastomers. Building upon recently developed chemistries conducive to photoalignment we report on the demonstration of adaptive shape and surface control within topologically blueprinted liquid crystal elastomers.

Surfaces with stimuli-responsive microstructures can enable dynamic modulation of properties such as adhesion, light manipulation, and wetting. In order to realize these applications, microstructures with tunable geometry and mechanics need to be fabricated in a scalable fashion. Liquid crystalline (LC) polymers have been studied widely for thermomechanical and photomechanical actuators; and control of preferential alignment of mesogenic units within LC polymers enables programmability of actuation upon applied stimuli. For example, introduction of the light responsive azobenzene moiety within the networks enables remote triggering of mechanical deformation. Until now, most studies of photomechanical actuation of LC polymers have used millimeter-scale and larger shapes, and only a few studies reported thermally responsive LC polymer microstructures. Here, we study the fabrication and thermomechanical and photomechanical responses of microstructured glassy and elastomeric azobenzene-functionalized LC polymers. Large-arrays of uniform LC polymer microstructures are fabricated by a replica molding technique using polydimethylsiloxane (PDMS) molds made from a silicon master template. We find that a vacuum environment is crucial for high fidelity molding with low microstructure surface roughness by using a custom-built vacuum chamber. Micropillars as small as 4 µm diameter with aspect ratio up to 20 are fabricated, with varied cross-sectional shapes. The molecular orientation of the network is controlled by applying a magnetic field in-situ during solidification of the network in the mold. We study how the formation of the LC polymer microstructure is influenced by the constraint (e.g., size and geometry of the mold) and develop an understanding of how the local (e.g., surface chemistry) and global (e.g. magnetic field) constraints control the formation and order of the LC network. By controlling these aspects, along with the microstructure geometry, we create surfaces exhibiting complex and anisotropic deformation patterns upon thermal or optical stimulation. To conclude, we reflect on future designs of potential LC polymer based microstructured active surfaces with programmable shape change and dynamically tunable surface properties.

Ubiquitous as they are, it sometimes escapes our attention that liquid crystals (LCs)
are the original nanomaterial. The manipulation of these nanometer-size molecules
into coherent, centimeter-scale structures is now routine, as demonstrated in LC
displays used everywhere. However for another type of functional LC material-
liquid crystal elastomers (LCEs), which are liquid crystalline materials with
crosslinked polymer network, limited applications have been proven, as control of
LC alignment in LCEs is difficult.
In this work, we demonstrated a new approach and molecular design to locally
control alignment of liquid crystal monomers (LCMs) in LCEs and thus spatially
control LCE actuation. A new type of LCM bearing crosslinkable groups was
designed and synthesized. Such LCMs were found to have stable LC phase and to be
well-oriented at both particle surfaces and patterned substrates, including micro-
size colloids, porous membranes, pillar arrays, channels and etc. After carefully
aligned LCM molecules through a series of boundary conditions, we crosslinked the
LCM to lock the molecular orientation and position, and actuation behavior of the
resulting LCEs was studied.
LCE micro-pillar arrays and porous membranes: LCE pillars and porous membranes
were fabricated through soft-lithography. With correct boundary conditions, LCMs
showed uniform alignment through the whole sample and thus the final LCE pillars
and porous membranes showed uniform actuations, for instance LCE pillars
expanded upon heating and LCE porous membranes showed compression leading to
pattern transformation above LCE transition temperature.
LCE membranes with hierarchical LC alignment: Hierarchical LC alignment was
achieved on chemically patterned surfaces with micro-size patterns providing either
planar or homeotropic anchoring of LCMs at boundaries. Upon phase transition,
such LCE membranes are expected to have origami-like shape change. Our final goal
is to apply LC anchoring theory and control technique to locally and spatially align
LCMs at micro- or even nano-scale, and to finely tune the actuation of resulting
LCEs.

In order to achieve locomotion, many biological microorganisms actively deform their whole body or appendages such as cilia and flagella. On the contrary, most mobile artificial micro-devices, also known as microrobots, consist of rigid undeforming micro-structures that either are pushed [1] or rotated by means of external fields (e.g. magnetic fields) [2], or propel themselves by means of catalytic chemical reactions [3].
Here we address, both theoretically and experimentally, the development of novel soft and actively-deforming microrobots. The microrobots consist of liquid crystal elastomers (LCEs) that undergo a light-driven reversible shape change [4]. The alignment of the liquid crystal mesogenic units, which is programmed into the material during the fabrication process, determines its response to light. However, the microrobots&’ behavior is directed by the intensity profile of the incident light.
Finite element (FE) computer simulations were used to model the deformation of LCE-based microrobots in response to different light-intensity profiles. The obtained results show that periodic shape changes can be induced and that the deformation is determined by the combination of programmed shape change and excitation. Additional simulations were performed in order to investigate the action of the actively-deforming material on a surrounding fluid. The obtained results show that the shape change effects locomotion of the microrobots in a viscous fluid.
Experiments were conducted with micro-fabricated structures made of light-responsive LCEs immersed in viscous fluids and actuated by an intensity-modulated light profile from a laser source. Careful tuning of the spatial and temporal distribution of the light profile induces periodic body shape changes that result in controlled self-propulsion of the microrobots through the fluid.
Our results pave the way towards new soft microrobots and microdevices that can exploit light-driven deformation for mechanical actuation at the micro-scale. These results will enable novel advanced microrobotic applications as well as wireless micro-actuators.
[1] T. Qiu et al., Nat Commun, 5: 5119, (2014).
[2] D. Schamel et al., ACS Nano, 8, 8794-8801, (2014).
[3] T.-C. Lee et al., Nano Lett, 14(5), 2407-2412, (2014).
[4] H. Zeng et al., Adv Mater, 26(15), 2319-2322, (2014).

Many biological materials are organic-inorganic composites where spatial and hierarchical control of structure and composition lead to a variety of functions. Polymer composites with aligned anisotropic fillers exhibit many of the same properties, but often lack the complexity and stimuli-responsive behavior of natural materials. In this work, we leverage recent work in surface-aligned liquid crystal elastomers to demonstrate that mixtures of liquid crystal monomers can be used to spatially align anisotropic microparticles. These ordered liquids are then polymerized yielding anisotropic, ordered polymers composites. These materials can be designed to reversibly change shape with over 50% strain in response to stimuli such as heat or solvent. After polymerization, the elastomeric composites exhibit anisotropic properties, including shape change, due to the polymer matrix and aligned fillers. The effect of model fillers on shape change is explored. Furthermore strategies to enable shape change in response to physiological conditions, namely moisture, will be described. These liquid crystal elastomer composites provide a strategy towards mimicking the multifunctionality of highly complex biological materials.

Liquid-crystalline elastomers (LCEs) are a class of stimuli-responsive polymers that are capable of mechanical and optical function due the combination of liquid-crystalline (LC) order and rubber elasticity. These materials can demonstrate extraordinary changes in shape, soft-elasticity behavior, and tunable optical properties in response to a stimulus such as heat or light, which makes them suitable for many for potential technological applications such as artificial muscles, sensors, and actuators. The goal of this presentation is to introduce a two-stage thiol-acrylate Michael-addition photopolymerization (TAMAP) methodology for the first time in mesomorphic systems to prepare nematic main-chain LCEs from off-the-shelf starting materials. The TAMAP methodology utilizes a non-stoichiometric ratio of monomers with an excess of acrylate functional groups. The first stage reaction is used to create a polydomain LCEs via the thiol Michael-addition reaction, which is self-limited by the thiol groups. This is an intermediate LCE network that would be capable of mesogenic domain orientation by applying mechanical stress. The polydomain resulting from the first-stage Michael-addition reaction is indefinitely stable and the alignment of the monodomain does not need to occur immediately after the reaction has completed. The second-stage photopolymerization reaction between excess acrylate groups is used to permanently fix an aligned monodomain and program the LCE for reversible and stress-free (i.e. “hands free”) actuation. The facile nature of TAMAP allows tailoring the mechanical properties and the actuation of the LCE networks. First, the mechanical properties can be tailored at the first stage reaction by varying the amount of crosslinking monomer, pentaerythritol tetrakis (3-mercaptopropionate) (PETMP). We have shown the rubbery modulus E_r, glass transition temperature T_g, and strain-to-failure range from 0.18 MPa, 3°C, and 600 % to 1.9 MPa, 27°C, and 100%, respectively upon increasing the amount of the crosslinker. An example non-stoichiometric system of 15 mol% PETMP thiol groups and an excess of 15 mol% acrylate groups was used to demonstrate the robust nature of the material. The LCE formed an aligned and transparent monodomain when stretched, with a maximum failure strain over 600%. Second, we have demonstrated that the actuation can be influenced by the pre-applied strain. The samples were programed and photo polymerized at 100, 200, 300, and 400% strain, with all samples proving over 90% shape fixity when unloaded. The magnitude of total actuation increased from 35 to 115% with increased programming strain. Overall, We have been able to show the two-stage TAMAP methodology as a powerful tool to prepare LCE systems and explore structure-property-performance relationships in these fascinating stimuli-sensitive materials.

Shape-memory polymers (SMP) are a prominent example for programmable materials. The shape-memory effect (SME) is not an intrinsic property, it requires a thermomechanical treatment, which is generally named programming procedure. This process implements a deformation memory in the material by external manipulation. The deformation is elastically recovered as soon as the SME is activated in most cases by application of heat.
The classical SME in polymers has so far been limited to its one way character. Recently, first reversible shape-memory and temperature effects could be realized in semi-crystalline polymer networks. In these materials the shape changing geometry and the switching temperature are programmable. While the reversible actuation can occur many times upon heating and cooling within a suitable temperature interval, the material's memory can be erased by heating to a temperature, at which the polymer is completely amorphous. The material is then reprogrammable.
As miniaturization is a general tendency in device design, an important aspect of SMP research is the question about the lower dimensional limits of shape bodies capable of a SME. Especially in case of multiphased polymer the outer dimension might cause a confinement on the phase morphology and in this way influence the SME. Microparticles and fibres will be presented as models for studying small shape-memory devices.
References
[1] M. Behl, K. Kratz, J. Zotzmann, U. Nöchel, A. Lendlein, Adv Mater 25, 2013, 4466-4469.
[2] M. Behl, K. Kratz, U. Nöchel, T. Sauter, A. Lendlein, P Natl Acad Sci USA 110, 2013,
12555-12559.
[3] C. Wischke, M. Schossig, A. Lendlein, Small 10, 2014, 83-87.
[4] Q. Zhang, T. Sauter, L. Fang, K. Kratz, A. Lendlein, Macromol Mater Eng 300, 2015, 522-530.

Mechanical instabilities such as buckling, wrinkling, folding, creasing and crumpling are commonplace in nature and occur in a wide range of length scales (kilometers to nanometers). While they were treated as a nuisance in material systems for the longest time, recent advances demonstrate that such instabilities can be harnessed to impart novel and favorable properties and applications to otherwise traditional materials. In this work, we demonstrate the swelling-mediated massive reconstruction of an ultrathin responsive gelatinous polymer film uniformly adsorbed with plasmonic nanostructures into a network of interacting folds, resulting in bright electromagnetic hotspots within the folds. We reveal a strong correlation between the topology and near-field electromagnetic field enhancement due to the intimate contact between two plasmonic surfaces in the folded regions, resulting in strong plasmon coupling and formation of electromagnetic hotspots. Owing to the efficient trapping of the Raman reporters within the electromagnetic hotspots, the surface enhanced Raman scattering (SERS) enhancement from the morphed plasmonic gel was found to be nearly 40 times higher compared to that from the pristine plasmonic gel. Harnessing the undeterministic nature of the folds, we show that the folded plasmonic gel can serve as a multidimensional (topo-chemical) optical taggant for anti-counterfeiting applications. Such novel optical tags based on the spontaneous folding process are virtually impossible to be replicated because of the combination of nondeterministic physical and chemical patterns.

Hydrogel actuators respond to a range of stimuli to produce large strains in aqueous environments. This unique response permits these systems to be employed and operated within the human body, making them ideally suited for biomedical applications. The work presented here utilises ionoprinting, a technique which selectively prints metal cations into the hydrogel through electrolysis creating localised stiffening of the polymer network via multivalent cation crosslinking¹. This localised crosslinking induces controlled bending of the hydrogel, which can be programmed to transform flat hydrogel sheets in to complex three-dimensional structures. In this study, our hydrogel structures have been designed to globally respond to a range of stimuli including pH, temperature and solvent polarity. Our system uses a chelating monomer, ethylene glycol methacrylate phosphate and a responsive bulk polymers, N-isopropylacrylamide or hydroxyethyl acrylate. A range of metal cations and oxidation states have been explored, increased oxidation states result in stronger crosslinking and increased contraction of the hydrogel. Fe³⁺ and V³⁺ have been used to explore ionoprinting, both in terms of shape complexity (i.e. cuboid, pyramid and umbrella) and the variables effecting the angle and electrons produced have been experimentally examined. The variables studied revealed a positive relationship with duration and voltage to ionoprinting effectiveness and a negative relationship with increasing electrolyte solution concentration. However, critically, it was noted that the complexity is limited by the brittleness of the hydrogel and not by the angles achievable by ionoprinting.
Our initial studies have been extended to create programmed multi-stage deployment. To realise this dual movement, we have employed, redox chemistry to create multi-stage deployment through printing two different metals into the hydrogel and selectively varying the oxidation of one with an alternative stimuli to the bulk polymer. Our targeted aim of this approach is to develop a material system, which can exhibit primary and secondary movements, where the secondary-order movement is deployed independently of the primary movement. To achieve a dual movement system, the redox chemistry of iron, oxidation state +2 and +3, has been explored as the secondary stage deployment method due to the difference in hydrogel swelling between the two oxidation states and the range of ways of switching between the two oxidations. This latter work is still on-going and the full potential of ionoprinting is only just starting to be explored but our key target concerns its application in biomedical and soft-robotic applications and as part of 4D printing.
(1) Palleau, E.; Morales, D.; Dickey, M. D.; Velev, O. D. Nature communications2013, 4, 2257.

Hydrophilic polymer networks exhibiting shape-memory properties are potential candidate materials for applications, in which the diffusion of small molecules are important factors such as for biomedical applications.^[1] The thermally-induced shape-memory hydrogels (SMHs) reported thus far were non-porous and exhibited an almost constant material volume during the shape fixity and recovery process.^[2,3]
Here we explored, whether 3D structured SMHs can be designed by incorporating oligo(ε#8209;caprolactone) as switching segment into hydrogels. By utilizing different porogenes during thermal crosslinking 3D structured hydrogels having pore diameters between 30 and 390 µm and wall thicknesses ranging from 10 to 190 µm were synthesized. The swelling behavior, the thermal, and the thermo#8209;mechanical properties were investigated as function of the OCL content, which was varied between 20 and 40 wt%. The structured hydrogels exhibited good shape fixity and shape recovery ratios above 80% in compression experiments. In conclusion these 3D structured SMHs are interesting materials for specific applications in biomedicine with on demand directed movements.
[1] N. Annabi, J. W. Nichol, X. Zhong, C. Ji, S. Koshy, A. Khademhosseini, F. Dehghani, Tissue Eng. Part B Rev. 2010, 16, 371.
[2] Y. Osada, H. Okuzaki, H. Hori, Nature1992, 355, 242.
[3] M. Balk, M. Behl, U. Nöchel, A. Lendlein, Macromol. Mat. Eng.2012, 297, 1184.

We have sought to develop new approaches that yield soft, water-based shape memory polymers exhibiting local and macroscopic ordering. In this talk, we will present two distinct approaches toward achieving this goal, one involving local ordering by liquid crystallinity and the other involving macroscopic ordering through aligned fiber-reinforcement. For the liquid crystalline approach, we will introduce a simple synthetic approach to yield a new liquid crystalline network featuring elastomeric softness, water-swelling and shape memory characteristics. By comparing with a non-mesogenic network prepared using the same procedure, we will reveal structure-property relationships of the liquid crystalline and crystalline polymer networks. Wide angle and small angle x-ray scattering studies were used to examine the liquid crystalline ordering in both dry and hydrated states. Such ordering was found to be related to the observed shape memory and actuation (two-way shape memory) properties and these phenomena are highlighted with demonstrations of shape change in response to heat and water stimuli. For the fiber-reinforcement approach, an anisotropic hydrogel composite was developed by incorporating aligned thermoplastic fibers into an otherwise isotropic hydrogel. The resulting anisotropic composites exhibited shape memory characteristics that could be activated in response to hydration or dehydration, with a basis in anisotropic swelling. We observed that different helicoids or spirals could be formed in anisotropic hydrogel composites by varying the orientation of fiber angles and ply thickness. To further characterize these anisotropic hydrogels, the dependences of helicoids&’ pitch and radius of curvatures were studied as a function of fiber angle orientation and composite thickness. This study provides insight into the mechanisms affecting the shape evolution of water-activated anisotropic hydrogels and enables the future design of materials or devices for a variety of applications. By comparing the two approaches, we will reveal relative advantages and disadvantages for different applications, which will be attributed to both chemical composition and microstructure - a matter of scale.

The field of soft robotics encompasses actuators, sensors, devices, and machines that can deform, flex or conform in response to their environment, while performing their predesigned functions. A common approach for making polymers responsive to external stimuli is to incorporate functional nanoparticles. Moreover, different arrangements of nanoparticles within the polymer can be used to control the response of the nanocomposite. Embedding magnetic nanoparticles (MNPs) in polymers allows their actuation using magnetic fields, where the magnitude of the response is usually controlled by the particle size and concentration. When the dipoles of MNPs are aligned in an external field, they can assemble into chains along the field lines. Chaining of MNPs causes magnetic anisotropy, where the chains magnetize more easily along the chain direction. In applied magnetic fields, alignment of chains of MNPs parallel to the field is consequently favored. Actuation of polymer composites containing chained MNPs with magnetic fields allows for controlled bending, thus coupling the magnetic properties of the MNPs and the mechanical properties of the polymer.
We have demonstrated controlled and selective bending of arms of a cross-shaped elastomeric thermoplastic polyurethane film containing chained 30-nm Fe₃O₄ nanoparticles. The sample has been prepared with the chains aligned parallel to two arms (parallel arms) and perpendicular to the remaining two arms (perpendicular arms). When the sample is supported in the middle, gravity causes the arms to hang down. Application of the uniform horizontal magnetic field of an electromagnet parallel to the chains causes the parallel arms to lift and become nearly aligned with the field direction but does not lift the perpendicular arms. We have designed a simple model of the magnetic interactions (Zeeman energy and interparticle dipolar coupling) and gravity that provides an excellent match to experiment while neglecting effects of elasticity, magnetocrystalline anisotropy, and thermal energy. When rotating the sample in uniform magnetic fields, magnetic hysteresis causes mechanical hysteresis in the bending response, which is explained by the competition between the elastic energy of the polymer and the magnetostatic energy of the MNPs. When the cross-shaped sample is pinned in the center on a flat surface, the magnetic field gradient of a permanent magnet held above the sample generates a higher force on the parallel arms, causing selective lifting, while the perpendicular arms do not lift. These results highlight the potential for dipolar interactions among chained MNPs to impart controlled actuation to polymer nanocomposites, which will have applications in soft robotics, where the absence of any moving parts is desired.

Transfer printing encompasses a set of techniques for deterministic assembly of micro-and nano-objects, termed "inks," into spatially organized, functional arrangements with two and three-dimensional layouts. These processes offer a degree of flexibility in the creation of heterogeneously integrated functional systems that is unmatched by traditional microfabrication techniques, and have proven valuable in the fields of flexible electronics, curvilinear optoelectronics, and bio-integrated sensing and therapeutic devices. Printing typically involves either the transfer of a single pre-fabricated ink or pre-arranged layer of inks, with the ability to print an arbitrary pattern of inks in a scalable way yet to be demonstrated. Here, we demonstrate such a process, whereby an array of inks is printed to a receiving substrate in an arbitrary pattern using a carbon-doped, thermally-sensitive shape memory polymer (SMP) as the functional material. Pre-fabricated arrays of inks may be retrieved en masse through mechanical deformation of the SMP surface microstructure to maximize SMP-to-ink adhesion, and then selectively printed by utilizing localized heating of the SMP above its glass transition temperature to enable passive shape-reconstitution, thereby enabling release of these inks by minimizing the SMP's adhesion to them. Heat is delivered rapidly by the absorption of laser energy within the selectively carbon-doped SMP microstructure.

Amorphous polymers achieve shape memory behavior through the large change in the chain mobility above the glass transition. Above the glass transition temperature, the polymer chains are mobile and can recover quickly to equilibrium when unloaded. Below the glass transition, the chains slow to recover when unloaded. The temperature span of the glass transition has a direct effect on the shape memory behavior. Materials with broad glass transition region can store and recover multiple shapes and remember not just the deformation but also the programming temperature. Since the glass transition is the underlying mechanism of shape memory behavior in amorphous materials, shape recovery can be achieved by either increasing the temperature in the material above the glass transition temperature or by decreasing the glass transition temperature to below the temperature of the material. The latter can be achieved with the absorption of a small weight fraction of solvents. In this presentation, I will demonstrate the thermally activated temperature memory and multiple shape memory effect for both temperature and solvent driven shape recovery. Specifically, I will show the correspondence between solvent and temperature driven recovery phenomenon. For solvent-driven shape recovery, the programming temperature and solvent type both strongly influence on the shape recovery behavior. Thus, the temperature memory effect can be used to program multiple shapes for multi-staged recovery in solvent. Analogous to the temperature memory effect, different solvents can also induce different shape recovery and can also provide a mechanism for multi-staged and multiple shape memory recovery in solvent

Shape-memory polymer networks, which are capable of reversible shape changes upon heating and cooling, have been recently introduced as novel kind of thermo-sensitive actuators. [1-3] So far the reports about two-way shape-memory polymers (2W-SMP) concentrate on the characterization of macroscopic samples. One example for 2W-SMP are crosslinked poly[ethylene-co-(vinyl acetate)] networks (cPEVAs) consisting of crystallizable polyethylene (PE) domains with a broad melting transition. Here PE crystallites in the high melting temperature range act as shape determining skeleton domains, while lower melting PE crystals are responsible for the reversible temperature driven actuation. [1]
In this work, we explored whether microcubes can be prepared from cPEVA and equipped with a free standing, reversible bidirectional shape-memory effect (rbSME).
A template-based replication method was applied for preparation of cPEVA microcubes with an edge length of 10 or 25 µm from poly[ethylene-co-(vinyl acetate)] with a vinyl acetate (VA) content of 18 wt% (cPEVA18) or 40 wt% (cPEVA40) and dicumyl peroxide as crosslinking agent. For rbSME functionalization the microparticles were deformed by compression at 70 °C and 100 °C, respectively. The change in single particle height and area of the programmed microcubes during cyclic heating and cooling between 75 or 60 °C and 20 °C was monitored by optical microscopy (OM) and atomic force microscopy (AFM). A pronounced rbSME was achieved for both type of microcubes cPEVA18 and cPEVA40 with a reversible strain in the range of 5 to 7%, whereby higher compression ratios resulted improved reversible strains. The observed micro rb-SME performance is comparable to that reported for macroscopic cPEVA actuators [1].
The findings obtained for cubic model microparticles will be helpful for designing the next generation polymeric microactuators or micromanipulators.
References:
[1] Behl M, Kratz K, Noechel U, Sauter T, and Lendlein A. Proceedings of the National Academy of Sciences2013;110(31):12555-12559.
[2] Saatchi M, Behl M, Nöchel U, and Lendlein A. Macromolecular Rapid Communications2015;36(10):880-884.
[3] Gong, T., Zhao, K., Wang, W. X., Chen, H. M., Wang, L., Zhou, S. B., Thermally activated reversible shape switch of polymer particles, 2014, Journal of Materials Chemistry B, 2(39): 6855-6866

Most research on stimuli-responsive materials, in particular, shape memory polymers, has focused on the macroscopic deformation and bulk recovery mechanism. Recently, the stimuli-responsive properties of the micro- or nanostructured surfaces have garnered more attention. While studies of mechanically deformable surface pattern on polymers have been reported, there is a dearth of study on correlating geometrical designs of surface textures with pattern deformation in response to mechanical stretching. Herein, we fabricated grating and hole structures with a range of length scales, feature designs (protruding vs recessed) from shape memory thermoplastic elastomer polyurethane by nanoimprinting. The pattern deformation can be achieved through stretching the polymer film above the glass transition temperature (Tg) of the soft segments and locking the temporary shape upon cooling, followed by the removal of external stress. Upon reheating above the Tg, the deformed structure could be recovered. From the scanning electron microscopy and atomic force microscopy analysis, non-uniform deformation has been observed in all patterns investigated. Specifically, on protruding grating, there is a decreasing effective tensile stress (strain) from the base (bulk of the film) to the top part of the grating. Furthermore, variation in length scale (200nm vs 2µm line width, both at aspect ratio of 1:1 and duty cycle of 1:2) resulted in different degree of deformation, with nanoscale pattern exhibiting higher strain at the top part of the grating. The recessed hole structure was elongated along the stretching direction, with an increased anisotropy (i.e. circle to ellipse). In terms of vertical dimension, some axial pre-strain was rebounded upon the low temperature unloading process, which increased the heights of the lines. Recessed (hole) structures exhibited a more significant change in the hole depth, as compared with that in the heights of the protruding lines for grating. For micro- and nanopatterns, nearly complete shape recovery was retained even after 10 cycles. The deformed and recovered micro- and nanopatterns were investigated for the surface wettability properties. On original or recovered gratings, the water droplet displayed a Cassie-Baxter (C-B) wetting behavior. Increasing the line spacing as a result of tensile strain reduced the water contact angle (CA) on nanograting, suggesting that, in addition to the tip, the sidewall of the lines was partially wetted. In contrast, the CA on micrograting was increased at low elongation due to lower solid-air fractions (obeying C-B state), and began to decrease marginally at high elongation. The CA on the hole structure remained relatively unchanged with elongations. Our results suggest that the stimulus-responsive function in shape memory polyurethane may be tuned by varying the geometries of the micro- and nanoscale surface patterns.

Short bowel syndrome is a disorder caused by a lack of sufficient intestinal length that occurs in children at a rate of up to 25 per 100,000 births and affects near 15,000 adults in the U.S. Without sufficient digestive and absorptive capacity due to the resultant intestinal failure, patients are unable to maintain growth or weight via enteral nutrition and then must rely upon parenteral nutrition, which is associated with a 50% mortality rate and $2.3B in costs per year in the U.S. Distraction enterogenesis is an alternative treatment involving the application of mechanical force to the bowel in order to increase its length and surface area. A solvent-driven shape-memory polymer has been proposed as a material to achieve distraction enterogenesis by absorbing fluid to decrease the glass transition temperature (Tg) for activation in vivo. The objectives of this study were (1) design a solvent-driven shape-memory polymer system with tailorable recovery rates and (2) demonstrate in vitro biocompatibility and in vivo feasibility with a rat model. Isobornyl acrylate, 2-hydroxyethyl acrylate (HEA), and hexanediol diacrylate were mixed in five varying ratios and cured at 405nm with Irgacure 819 to produce polymer films. Each composition was analyzed by dynamic mechanical analysis to determine Tg. Dry Tg increased from 79.7°C to 97.1°C as isobornyl acrylate wt% increased. Water content increased from 1.8% to 5.4% as HEA wt% increased. Elastic modulus decreased from 225 MPa to 6 MPa over 7 days of immersion. Films were programmed at their respective Tg into a compact shape and recovered either in air at 37°C or in saline at 37°C without constraint. Recovery in air ranged from 6.8% to 96.9% and recovery in saline ranged from 13.5% to 100% over 7 days. Recovery amount increased as water content increased. Human intestinal fibroblasts were seeded onto discs. The cells showed continued viability throughout 14 days and showed proliferation via MTT assay, as well as, matrix deposition as demonstrated by collagen assay. The animal study was performed in accordance with the IACUC of Boston Children&’s Hospital. Young Sprague Dawley rats underwent a Roux-en-Y isolation of a small intestinal segment, which was wrapped around an extraluminal, radially expanding shape-memory polymer device. After 14 days, devices showed a 70% increase in circumference. Intestinal segments increased from 24±3mm to 50±15mm, while control segments increased from 35±9mm to 37±6mm. No difference in muscularis thickness was detected between groups, which suggests elongation is due to tissue growth, not stretching. This work demonstrates the feasibility of distraction enterogenesis with a tailorable, solvent-driven shape-memory polymer device, where high Tg polymers can be activated with saline uptake. Further studies will investigate upscaling the device size for larger animals and bowel function after distraction.

Despite their advantages, the inherent low strength and stiffness of shape-memory polymers (SMPs) can limit possible applications. A common method to improve polymer strength and stiffness is to increase the chemical crosslinking, or the addition of reinforcement such as glass/carbon fibers; however, both approaches severely limit ductility, effectively negating the purpose of an SMP. Aromatic polymers such as poly(phenylene sulfide) (PPS) and poly(ether etherketone) (PEEK) are a unique and advantageous class of polymers because they are stronger and stiffer than more common polymers and have large fracture strains relative to composites. The defining microstructural feature of these materials is the abundance of phenyl rings in the polymer backbone, which results in high strength, high stiffness, and stability at high temperatures. To date, aromatic polymers have been largely unexplored for use as SMPs.
The purpose of this study was to investigate the shape-memory behavior of poly(para-phenylene) (PPP) under varying programming temperatures, relaxation times, and recovery conditions. PPP is an inherently stiff and strong aromatic thermoplastic, not previously investigated for use as a shape-memory material. Initial characterization of PPP focused on the storage and relaxation moduli for PPP at various frequencies and temperatures, which were used to develop continuous master curves for PPP using time-temperature superposition (TTS). Shape-memory testing involved programming PPP samples to 50% tensile strain at temperatures ranging from 155°C to 205°C, with varying relaxation hold times before cooling and storage. Shape-recovery behavior ranged from nearly complete deformation recovery to poor recovery, depending heavily on the thermal and temporal conditions during programing. Straining for extended relaxation times and elevated temperatures significantly decreased the recoverable deformation in PPP during shape-memory recovery. However, PPP was shown to have nearly identical full recovery profiles when programmed with decreased and equivalent relaxation times, illustrating the application of TTS in programming of the shape-memory effect in PPP. The decreased shape recovery at extended relaxation times was attributed to time-dependent visco-plastic effects in the polymer becoming significant at longer time-scales associated with the melt/flow regime of the master curve. Under constrained-recovery, recoverable deformation in PPP was observed to have an exponentially decreasing relationship to the bias stress. This study demonstrated the effective use of PPP as a shape-memory polymer both in mechanical behavior as well as in application.

Smart shape memory polymers (SMPs) can memorize and recover their permanent shape in response to an external stimulus, such as heat, light, and solvent. They have been extensively exploited for a wide spectrum of applications ranging from biomedical devices to aerospace morphing structures. However, most of the existing SMPs are thermoresponsive and their performance is hindered by heat-demanding programming and recovery steps. Although pressure is an easily adjustable process variable like temperature, pressure-responsive SMPs are largely unexplored. By integrating scientific principles drawn from two disparate fields - the fast-growing photonic crystal and SMP technologies, here we present a new type of SMP that enables unusual "cold" programming and instantaneous shape recovery triggered by applying a contact pressure or exposing the sample to various vapors (e.g., acetone, ethanol, and toluene) at ambient conditions. This interdisciplinary integration simultaneously provides a simple and sensitive optical technique for investigating the intriguing shape memory effects at nanoscale. We have also demonstrated the reversible fabrication of reconfigurable nanooptical devices, such as photonic crystal filters and lasers, as well as tunable antireflection coatings, using these new stimuli-responsive SMPs. The striking chromogenic effects induced by the unusual shape memory behaviors of the smart polymers provide vast opportunities for a plethora of applications ranging from reconfigurable nanooptical devices to chromogenic pressure and chemical sensors to novel biometric and anti-counterfeiting materials.

Mechanically adaptive materials respond to environmental cues by converting external stimuli into motion via an internal material algorithm defined by composition and structure. Such programmable polymers include shape memory polymers (thermal stimulus), Belousov Zhabotinsky self-oscillating gels (chemical), sulfonylated polyimides (humidity) and liquid crystal networks (thermal/photo). The resultant motion, shape change or mechanical property remodeling offers unique opportunities for remote sensing, energy harvesting, robotics, and human performance technologies. Their impact can be further expanded through the design and patterning of composite adaptive materials, which contain active and inactive material domains. In addition to amplifying behavior through structural design, such as maximizing deflection through a cantilever, bi-stable plate or torsional spring geometry, arrangements of mechanically active and inactive units within a monolith can lead to communication, sensing, locomotion, or logic behaviors. These arise from the constructive or destructive superposition of long-range, transient fields, such as strain, temperature, or composition, which in turn may be modulated by the local response of the active units within the array. Recent examples of such mechanically active-inactive hybrids will be discussed, including re-writable shape memory polymers, self-oscillating hydrogels, liquid-crystal network origami, and humidity driven locomotion.

Artificial muscle materials can be employed in a range of applications including robotics, prosthetics, miniature tools and in morphing structures. Various different types of artificial muscle materials are available with different stimuli and with different outputs in terms of force and stroke. Here we discuss a range of fibre composite materials where a helical fibre orientation can induce force and stroke amplification compared with isotropic volume changes. These materials are based on the so-called “McKibben muscles” or “air muscles” in which pressurised air inflates a bladder constrained by a braid. These systems produce skeletal-muscle-like stroke, speed and force but require an external compressor for operation. As an alternative, recent reports demonstrate how a volume-expanding material can replace the pressurised air supply [1-4]. Different examples either use an external braid or a twisted yarn structure. The latter induces both torsional and tensile actuations.
Modelling the performance of these composite materials is an important step in their application. An approach to modelling the behaviour of guest-filled braided and twisted yarn artificial muscles will be presented. Expandable guest materials include paraffin wax (thermally active) and hydrogel (activated by absorption and desorption of water).
References
1. M.D. Lima, N. Li, M. Jung de Andrade, S. Fang, J. Oh, G.M. Spinks, M.E. Kozlov, C.S. Haines, D. Suh, J. Foroughi, S.J. Kim, Y. Chen, T. Ware, M.K. Shin, L.D. Machado, A.F. Fonseca, J.D.W. Madden, W.E. Voit, D.S. Galvatilde;o, R.H. Baughman, Electrically, Chemically, and Photonically Powered Torsional and Tensile Actuation of Hybrid Carbon Nanotube Yarn Muscles, Science, 338 (2012) 928-932.
2. K.-Y. Chun, S. Hyeong Kim, M. Kyoon Shin, C. Hoon Kwon, J. Park, Y. Tae Kim, G.M. Spinks, M.D. Lima, C.S. Haines, R.H. Baughman, S. Jeong Kim, Hybrid carbon nanotube yarn artificial muscle inspired by spider dragline silk, Nat Commun, 5 (2014).
3. S.M. Mirvakili, A. Pazukha, W. Sikkema, C.W. Sinclair, G.M. Spinks, R.H. Baughman, J.D.W. Madden, Niobium Nanowire Yarns and their Application as Artificial Muscles, Advanced Functional Materials, 23 (2013) 4311-4316.
4. B. Tondu, S. Mathe and R. Emirkhanian, Low pH-range control of McKibben polymeric artificial muscles, Sensors and Actuators A: Physical, 159 (2010) 73-78.

Electromechanical actuators have attracted increasing attentions due to their promising applications such as artificial muscles, micro-robots, and biomimetic devices. Among them,electrothermally, electrochemically and electrostatically driven actuators are mostly explored. However, there remain many challenges for each kind of them, e.g., slow responsiveness and weak cyclic life for electrothermally and electrochemically driven actuators, limited electrolyte media work for electrochemically driven actuators, and ultrahigh driving voltages for electrostatically driven actuators. Herein, multi-walled carbon nanotubes are assembled into hierarchically helical fibers to simultaneously generate contractive and rotary actuations based on a novel electromagnetic mechanism. They generate a large stress of more than 200 times that of typical skeletal muscle, accompanied with a high rotation output (135 revolutions per meter), rapid responsiveness (< 200 ms) and low operating voltage (< 10 V/cm). They are composed of bare aligned CNTs without any functional guest materials, endowing them with high actuation stability and adaptability to various media (such as air, water and organic solution, etc.). Additionally, due to the formation of multiple-scale capillary channels within the helical fibers, superb actuations could also be triggered by solvents and vapors besides electric. These helical assembled fibers are lightweight, flexible, mechanically robust and electrically conducting, thus could be employed to design into unique actuating, biomimetic and sensing devices with programmable and sophisticated motions.
References
[1] P. Chen, et al., Adv. Mater.2015, 27, 1042.
[2] P. Chen, et al., Adv. Mater., DOI: adma.201402867.
[3] P. Chen, et al., Nat. Nanotechnol.2015, in revision.
[4] W. Guo, et al., Adv. Mater.2012, 24, 5379.

Plants, seemingly incapable of movements like other living species, often generate movements for defense, growth and nutrition. One fascinating example is the Venus flytrap (Dionaea muscipula) leaves, which close together within a fraction of a second to capture prey. This rapid movement is achieved through snap-buckling instability and the leaves remain stable in both open and closed states. Bi-stable hemispherical domes (often referred as the elastic spherical caps) are a close analogue of the Venus flytrap leaves. Bi-stable structures have two minimum energy states, generally separated by an energy barrier. These structures are capable of achieving significant changes in shape, forming mechanically deployable and morphing structures. Research on bi-stable domes demonstrating snap-buckling instability has mostly been limited to the modeling and fabrication of structures under mechanically applied loadings. In this work, we designed bi-layer spherical domes responsive to external stimuli in the form of mismatch strain and then investigated their deformation characteristics under varying stimuli.
Finite element simulations revealed that, for certain dome geometries the strain energy of the dome initially increases with increasing mismatch strain. Then, at a critical value of the mismatch strain, the strain energy drops suddenly due to instability and the dome snaps to its inverted configuration, which is the secondary stable state. The drop in the strain energy characterizes the dome bi-stability. The dome maintains its inverted configuration with further increase in mismatch strain and then starts to roll up at the secondary critical value of the mismatch strain. Analysis of domes with different configurations revealed that, the size of the energy well, mismatch strain required for dome inversion and subsequent rolling up depend strongly on the dome geometric parameters such as the radius of curvature, arc length and total dome thickness. Furthermore, for a specified geometry, the ratio of the Young&’s moduli and thickness of the constituent layers also affect the response of the domes. For experimental observation of the dome deformation modes, we utilized the concept of PDMS (Polydimethylsiloxane) swelling in organic solvents. Bi-layer dome shaped structures consisting of strongly and weakly cross-linked layers were fabricated through a casting process. During experiment, domes with different geometries demonstrated unchanged, snap-buckled and rolled up configurations with increasing mismatch strain values. The experimental results confirmed the dependence of critical mismatch strain values on dome geometric parameters, as predicted by the simulations. The principles proposed by this work will create the foundation for shape changing structures responsive to a diverse class of external stimuli and enable the design of novel sensors and actuators, artificial muscles, smart coatings, drug delivery systems and surfaces with tunable topography.

Understanding and controlling the shape of thin, soft objects has been the focus of significant research efforts among physicists, biologists, and engineers in the last decade. These studies aim to utilize advanced materials in novel, adaptive ways such as fabricating smart actuators or mimicking living tissues. Soft mechanical structures, such as biological tissues and gels, exhibit motion, instabilities, and large morphological changes when subjected to external stimuli. Swelling a dry gel with a favorable solvent is a robust approach for inducing these structural changes. Small volumes of fluid that interact favorably with a material can cause dramatic and geometrically nonlinear deformations including beam bending and plate buckling.
In this presentation, we describe the controlled growth--like structural morphing of 2D sheets into 3D shapes by preparing geometric composite structures that deform by residual swelling. The morphing of these geometric composites is dictated by both swelling and geometry, with diffusion controlling the swelling--induced actuation, and geometric confinement dictating the structure's deformed shape. Building on a simple mechanical analog, we present an analytical model that quantitatively describes how the Gaussian and mean curvatures of a thin disk are affected by the interplay among geometry, mechanics, and swelling. This model is in excellent agreement with our experiments and numerics. We show that the dynamics of residual swelling is dictated by a competition between two characteristic diffusive length scales governed by geometry. Our results provide the first 2D analog of Timoshenko's classical formula for the thermal bending of bimetallic beams -- our generalization explains how the Gaussian curvature of a 2D geometric composite is affected by geometry and elasticity. The understanding conferred by these results suggests that the controlled shaping of geometric composites may provide a simple complement to traditional manufacturing techniques.

3D printing has become a popular method for rapid prototyping of medium-sized objects, and has found applications in creating novel mold architectures too complex to create by machining. With variety of materials available, including biodegradable ones, applications include biomedical stents, scaffolds for tissue engineering, as well as mechanical structures, some with moving parts. The mechanical response of 3D-printed structures, due to their non-uniformity, has been a somewhat neglected area, with notable exceptions, such as the 4D printing, where even functional materials were printed and objects could later be transformed to different shapes using thermal stimuli.
In this paper we use fused deposition modeling, the lowest cost and most accessible 3D printing technology, to demonstrate multifunctional geometries and structures. The geometries we create could become the basis of materials that could use novel mechanisms for changing shape and responding to different stimuli. We describe key fine-tuning to produce the complex structures from the thermoset plastics at the current scale, including appropriate tool path optimization. However we also describe theoretical considerations for scaling down these concepts to smaller dimensions and designing the structures for multifunctionality. Attention is paid to brittleness of sharp corners and other instabilities. Finite element modeling is used to visualize and inform the design of the dynamically deforming structures.

The nascent technique of 4D printing has the potential to revolutionize manufacturing in fields ranging from organs-on-a-chip to architecture to soft robotics. By expanding the pallet of 3D printable materials to include the use stimuli responsive inks, 4D printing promises precise control over patterned shape transformations. With the goal of creating a new manufacturing technique, we have recently introduced a biomimetic printing platform that enables the direct control of local anisotropy into both the elastic moduli and the swelling response of the ink. To use it as a design tool, we must solve the inverse problem of prescribing the pattern of anisotropies required to generate a given curved target structure. We show how to do this by constructing a theory of anisotropic plates and shells that can respond to local metric changes induced by anisotropic swelling. A series of experiments corroborate our model by producing a range of target shapes inspired by the morphological diverstiy of flower petals.

Weakly connected mechanical systems near the isostatic threshold are marginally stable and exhibit large deformations in response to tiny perturbations, meaning that they enter the nonlinear regime immediately. Kane and Lubensky have defined a new topological invariant of isostatic mechanical lattices which leads within linear elasticity to zero energy modes localized at boundaries akin to the edge modes studied in topological quantum matter. This invariant is defined only if the phonon spectrum is gapped away from the acoustic modes at zero momentum, and indeed some lattices are ungapped and admit zero energy bulk modes at nonzero momentum, known as Weyl modes. We present the results of theory and simulations on the dynamics and energy transport in a family of lattices where the wavelengths of these Weyl modes can be tuned via a continuous parameter. Our findings include (1) the Weyl modes provide a route to inducing large deformations in the bulk of a mechanical system via boundary activation (2) the deformation corresponding to the Weyl modes propagates via a nonlinear shock and (3) we elucidate the connection between Weyl modes and the unit-cell shape changing mechanisms that are generic to isostatic lattices.

Silicone elastomers are usually the backbone of soft bioelectronic interfaces. We have customized photosensitive silicones, silicone foams as well as metal-silicone multilayers to widen our materials palette for designing and manufacturing electronic transducers with reversible mechanical compliance.
The silicone membrane may be locally softened by (1) inhibiting the polymer cross-linking using patterned UV light and photoinhibitors preliminary mixed with the elastomer, or (2) introducing microcavities within the bulk elastomer. The engineered elastomers are next metallized and integrated into soft mechanical sensors.
The soft pressure sensors transduce standard forces exerted by a human hand and can withstand multiaxial tensile strains to tens of percent. These sensors work toward enabling generalized tactile sensing in robotic and prosthetic applications.

We demonstrate a reconfigurable electronic substrate with bidirectional motion capabilities which can grasp or conform to 3D objects triggered by temperature changes. Using this substrate we demonstrate soft, adaptive nerve cuffs for interfacing with the peripheral nervous system, which can controllably and autonomously wrap to radii below 1 mm. In recent literature, shape memory polymers and hydrogel substrates have enabled adaptive electronic devices which can change shape in vivo to enable more conformal biological interfaces. We extend on this work and have developed electronic substrates which can take on both convex and concave geometries based on a single input: temperature. We do this by exploiting both the shape changing properties of a shape memory polymer (SMP) and the lower critical solution temperature (LCST) of a hydrogel, which when combined in a laminate structure enable a reconfigurable substrate.
We pull inspiration from the erodium cicutarium seed, which reversibly curls and uncurls naturally due to changes in environmental humidity. This seed consists of a swelling and non-swelling bilayer and tightens to radii below 1 mm when the inner swelling layer is dry. Our device dimensions are motivated by structural analysis of the erodium cicutarium seed. A thiol-ene SMP which softens over two orders of magnitude when heated above a Tg of 40 C provides a structural backbone with variable stiffness dependent on temperature. A layer of poly(N-isopropylacrylamide) (PNIPAM) provides actuation via water swelling and deswelling around the LCST of 33 C. Raising the temperature above 40 C from a flat geometry both softens the SMP and deswells the PNIPAM, inducing a curvature due to volumetric changes of the PNIPAM. If then cooled below the LCST and Tg the SMP first stiffens in the deformed shape, fixing a new geometry regardless of subsequent hydrogel swelling. This can be used to autonomously reconfigure an initially flat device into a 3D shape. Furthermore, through manipulation of polymer chemistry we decrease the Tg of the SMP to 25 C and below the LCST. Thus, once heated above both the Tg and LCST, the structure become highly dependent on cooling rate due to slow deswelling kinetics of the hydrogel, which is slower than the change in modulus of the SMP. This results in a substrate which can be reconfigured multiple times by control of the heating and cooling rates.

This talk will discuss a simple approach to shape-program 2D sheets of polymer into 3D objects. Self-folding is a deterministic ‘origami&’ process that causes a predefined 2D template to fold into a desired 3D structure with high fidelity. We achieve self-folding by locally heating pre-stressed polymer films. The heat causes the polymer to locally relax (i.e., shrink) and induces a folding response. The heat can be delivered using patterned light, patterned inks that absorb light, and patterned inks that absorb microwaves. Self-folding takes advantage of the multitude of available 2D patterning techniques that can be employed to pattern these inks (e.g., lithography, inkjet printing, screen printing). Self-folding is attractive as a cost-effective 3D fabrication strategy for applications such as packaging, robotic actuators and sensors, biological devices, solar cells and reconfigurable devices. This talk will focus on our recent efforts to model the mechanics of the self-folding process using finite element modeling, to achieve sequential folding (i.e., folding individual hinges in a controlled sequence to achieve more complex geometries), and to utilize microwaves as a stimulus to induce self-folding.

A number of platforms have been developed to pattern sheets to buckle into pre-programmed three-dimensional sheets. Yet not every pattern actually folds or buckles correctly, since sometimes faces have to bend. I will describe how this leads to hysteretic folding in structures that would be rigid in a mathematically-ideal world. More generally, I will describe theoretical tools to understand how origami-like materials deform and what, in particular, determines their rigidity. Though the rigidity of an origami structure arises from some simple geometric rules, these rules can combine in an almost arbitrarily complicated way. When one accounts for face bending, typical origami structures turn out to be marginal -- having precisely as many degrees of freedom as naive constraints. Interestingly, not every rigid structure is rigid in the same way, and these differences can be exploited to design folding pathways.

Self-folding films are a unique kind of thin films, which are able to deform in response to a change of environmental conditions or internal stress and form complex 3D structures. They are very promising candidates for the design of bioscaffolds, which resemble different kinds of biological tissues. In order to be suitable for biomaterial engineering, the materials which are used for fabrication of self-folding films must fulfill the following requirements: biocompatibility, biodegradability and sensitivity to stimuli in the physiological range.
We developed simple and cheap approach for fabrication of fully biodegradable and biocompatible self-rolled tubes, whose folding can be triggered by temperature. We used a bilayer where one component is gelatin and another one is polycaprolactone. We show (i) self-folding polymer films, which fold at room temperature (22°C) and irreversibly unfold at 37°C and (ii) films, which are unfolded at room temperature (22°C), but irreversibly fold at 37°C. We also discovered new and unexpected effect of reversible actuation of ultrathin gelatin-polycaprolactone films. These films are unfolded at room temperature, fold at temperature above polycaprolactone melting point and unfold again at room temperature. We hypothesize that the origin of this unexpected behavior is the orientation of polycaprolactone chains parallel to the surface of the film, which is retained even after melting and crystallization of the polymer - “crystallization memory effect”. In this way the crystallization generates a directed force, which causes bending of the film.
Stroganov, V.; Al-Hussein, M.; Sommer, J.U.; Janke, A.; Zakharchenko, S.; Ionov, L. Reversible thermosensitive biodegradable polymeric actuators based on confined crystallization, Nano Letters 2015, 15 (3), 1786-1790.
Stroganov, V.; Zakharchenko, S.; Sperling, E.; Meyer, A.K.; Schmidt, O.G; Ionov, L. Biodegradable self-folding polymer films with controlled thermo-triggered folding, Advanced Functional Materials 2014, 24(27), 4357-4363.

Deployable and transformable structures are of broad interest for applications including satellites and space exploration, temporary shelters, packaging and transportation, robotics, and medical devices. One emerging approach to scalable fabrication of such structures involves the general concept of Origami-inspired design to build three-dimensional structures by cutting, folding, and fastening of sheet materials. However, contrasting the classical approach of modeling Origami structures as having perfect hinges and rigid panels, consideration of the finite bending and rotational stiffness of these elements is essential to understand their constituent mechanics. Moreover, meta-materials and functional structures with fundamentally new mechanical properties can be designed. We present the design, fabrication, and mechanics of a novel, deployable cellular material, which we call “flexigami”. The unit cell takes the form of two parallel regular polygons, connected by a circuit of diagonally creased panels. Upon compression, individual unit cells transform either gently or abruptly between two stable equilibrium states depending on the interplay between hinge and panel properties. The mechanical behavior of each unit cell can be deterministically designed via the geometry, dimensions, and the topology of the panels and hinges. Individual cells can be locked in a rigid state, or collapsed reversibly to less than 10% of their deployed volume. Within this transition regime, the force-displacement curve of each cell can be tuned to exhibit a smooth compression behavior or an instability followed by a self-reinforcing response. We use finite-element models complemented by analytical and computational analysis of the results to understand the importance of different mechanical properties of constituent hinges and panels, and demonstrate the fabrication of flexigami cells and mechanisms in various structural materials.

We explore and develop a simple set of rules that apply to cutting, pasting, and folding honeycomb lattices. We consider origami-like structures that are extrinsically flat away from zerodimensional sources of Gaussian curvature and one-dimensional sources of mean curvature, and our cutting and pasting rules maintain the intrinsic bond lengths on both the lattice and its dual lattice. We find that a small set of rules is allowed providing a framework for exploring and building kirigami—folding, cutting, and pasting the edges of paper.

Mechanical metamaterials are becoming attractive and fascinating due to their extraordinary characteristics which include negative Poisson&’s ratio, pattern transformation, switchable multi-stability, etc. Recently, we proposed a simple engineering scheme called “Fractal cut patterning” for creating hierarchical auxetic metamaterials; a sheet of material which have wide and desirable expandability via hierarchical cut patterns that allow materials to be stretched by rotating units. In this work, we evolve our former method of hierarchical cut patterning by adopting the concept of Kirigami, the arts of paper folding and cutting, which have been actively used to design and interpret 2-dimensional mechanical metamaterials. In our previously designed hierarchical materials, the hierarchical cut pattern was made by fractal cutting. Each inherent hierarchical pattern could be determined by their hierarchy level and cutting motif. Instead of simply cutting a number of cut motifs in a hierarchical manner on the entire 2D material, we add another design variable, ‘folding&’. Systematically repeated folding and cutting process could control the hierarchy level of fractal cut patterns such that we can significantly reduce the number of cutting to create a hierarchical auxetic metamaterial equivalent to that made by previous fractal cut patterning. In the presentation, we also provide experimental demonstration to prove our new concept.