H. Jerry Qi

Active materials by four-dimension printing

We advance a paradigm of printed active composite materials realized by directly printing glassy shape memory polymer fibers in an elastomeric matrix. We imbue the active composites with intelligence via a programmed lamina and laminate architecture and a subsequent thermomechanical training process. The initial configuration is created by three-dimension (3D) printing, and then the programmed action of the shape memory fibers creates time dependence of the configuration—the four-dimension (4D) aspect. We design and print laminates in thin plate form that can be thermomechanically programmed to assume complex three-dimensional configurations including bent, coiled, and twisted strips, folded shapes, and complex contoured shapes with nonuniform, spatially varying curvature. The original flat plate shape can be recovered by heating the material again. We also show how the printed active composites can be directly integrated with other printed functionalities to create devices; here we demonstrate this by cr...

Active origami by 4D printing

Recent advances in three dimensional (3D) printing technology that allow multiple materials to be printed within each layer enable the creation of materials and components with precisely controlled heterogeneous microstructures. In addition, active materials, such as shape memory polymers, can be printed to create an active microstructure within a solid. These active materials can subsequently be activated in a controlled manner to change the shape or configuration of the solid in response to an environmental stimulus. This has been termed 4D printing, with the 4th dimension being the time-dependent shape change after the printing. In this paper, we advance the 4D printing concept to the design and fabrication of active origami, where a flat sheet automatically folds into a complicated 3D component. Here we print active composites with shape memory polymer fibers precisely printed in an elastomeric matrix and use them as intelligent active hinges to enable origami folding patterns. We develop a theoretical model to provide guidance in selecting design parameters such as fiber dimensions, hinge length, and programming strains and temperature. Using the model, we design and fabricate several active origami components that assemble from flat polymer sheets, including a box, a pyramid, and two origami airplanes. In addition, we directly print a 3D box with active composite hinges and program it to assume a temporary flat shape that subsequently recovers to the 3D box shape on demand.

Photo-origami—Bending and folding polymers with light

Photo-origami uses the dynamic control of the molecular architecture of a polymer by a combination of mechanical and non-contact optical stimuli to design and program spatially and temporally variable mechanical and optical fields into a material. The fields are essentially actuators, embedded in the material at molecular resolution, designed to enable controllable, sequenced, macroscopic bending and folding to create three-dimensional material structures. Here, we demonstrate, through a combination of theory, simulation-based design, synthesis, and experiment, the operative phenomena and capabilities of photo-origami that highlight its potential as a powerful, and potentially manufacturable, approach to create three-dimensional material structures.

Heat- or water-driven malleability in a highly recyclable covalent network polymer.

DOI: 10.1002/adma.201400317 as dynamers by Lehn, [ 25,26 ] are stimuli-responsive polymers, most notably exhibiting macroscopic responses to changes in pH. [ 27,28 ] Several imine-containing polymers have been demonstrated, including pH-responsive hydrogels [ 20 ] and a working organic light-emitting diode (OLED). [ 23 ] However, the potential of polyimines as malleable, mechanically resilient polymeric materials, as well as their processability, have remained largely unexplored. We envision that imine-linked polymers can take malleability in covalent network polymers to the next level of simplicity, affordability and practicality. Herein, we present the fi rst catalyst-free malleable polyimine which fundamentally behaves like a classic thermoset at ambient conditions yet can be reprocessed by application of either heat or water. This means that green, room temperature processing conditions are accessible for this important class of functional polymers. A crosslinked polyimine network was prepared from commercially available monomers: terephthaldehyde, diethylene triamine, and triethylene tetramine ( Figure 1 a). A polyimine fi lm was obtained by simply mixing the three above components in a 3:0.9:1.4 stoichiometry in the absence of any catalyst in a mixture of organic solvents (1:1:8, v/v/v, CH 2 Cl 2 /EtOAc/EtOH), then allowing the volatiles to evaporate slowly. Alternatively, the polymer can be obtained as a powder by using ethyl acetate as the only solvent. The polymerization reaction was confi rmed by infrared spectroscopy, which revealed that aldehyde end groups were consumed while imine linkages were formed (Figure S2, Supporting Information). The resulting translucent polymer is hard and glassy at room temperature ( T g is 56 °C) (Figure S1, Supporting Information) and has a modulus of near 1 GPa with stress at break of 40 MPa (Figure S3, Supporting Information). The time and temperature dependent relaxation modulus of the polyimine fi lm was tested to characterize the heat-induced malleability. Figure 1 b depicts the results of a series of relaxation tests over a wide range of temperatures (50–127.5 °C) on a double logarithmic plot. Specifi cally, at 80 °C, the bond exchange reaction is initiated and the normalized relaxation modulus is decreased from 1 to 0.11 within 30 min, indicating an 89% release of the internal stress within the thermoset polymer. By shifting each relaxation curve horizontally with respect to a reference temperature at 80 °C, a master relaxation curve was constructed (Figure 1 c), which indicates the stress relaxation of the polyimine follows the classic time-temperature superposition (TTSP) behavior. The plot of time-temperature shift factors as a function of temperature (Figure 1 d) shows that the polyimine’s stress-relaxation behavior exhibits Arrheniuslike temperature dependence. Using the extrapolation, we calculated that while it takes 30 min for the stress to be relaxed by ca. 90% at 80 °C, the same process would take ca. 480 days at room temperature. The polyimine is thus the fi rst reported Covalent network polymers, which offer robust mechanical properties, generally lack the ability to be recycled. [ 1 ] There has been a great deal of research effort to incorporate reversible crosslinks into network polymers in order to obtain mechanically tough materials with self-healing properties. [ 2–13 ] Many have employed non-covalent crosslinks to achieve this goal. Ionic and hydrogen bonds are readily reversible and have been known to achieve effi cient self-healing performances. [ 14–17 ]

Sequential Self-Folding Structures by 3D Printed Digital Shape Memory Polymers

Folding is ubiquitous in nature with examples ranging from the formation of cellular components to winged insects. It finds technological applications including packaging of solar cells and space structures, deployable biomedical devices, and self-assembling robots and airbags. Here we demonstrate sequential self-folding structures realized by thermal activation of spatially-variable patterns that are 3D printed with digital shape memory polymers, which are digital materials with different shape memory behaviors. The time-dependent behavior of each polymer allows the temporal sequencing of activation when the structure is subjected to a uniform temperature. This is demonstrated via a series of 3D printed structures that respond rapidly to a thermal stimulus, and self-fold to specified shapes in controlled shape changing sequences. Measurements of the spatial and temporal nature of self-folding structures are in good agreement with the companion finite element simulations. A simplified reduced-order model is also developed to rapidly and accurately describe the self-folding physics. An important aspect of self-folding is the management of self-collisions, where different portions of the folding structure contact and then block further folding. A metric is developed to predict collisions and is used together with the reduced-order model to design self-folding structures that lock themselves into stable desired configurations.

Changes in the structure-function relationship of elastin and its impact on the proximal pulmonary arterial mechanics of hypertensive calves

Extracellular matrix remodeling has been proposed as one mechanism by which proximal pulmonary arteries stiffen during pulmonary arterial hypertension (PAH). Although some attention has been paid to the role of collagen and metallomatrix proteins in affecting vascular stiffness, much less work has been performed on changes in elastin structure-function relationships in PAH. Such work is warranted, given the importance of elastin as the structural protein primarily responsible for the passive elastic behavior of these conduit arteries. Here, we study structure-function relationships of fresh arterial tissue and purified arterial elastin from the main, left, and right pulmonary artery branches of normotensive and hypoxia-induced pulmonary hypertensive neonatal calves. PAH resulted in an average 81 and 72% increase in stiffness of fresh and digested tissue, respectively. Increase in stiffness appears most attributable to elevated elastic modulus, which increased 46 and 65%, respectively, for fresh and digested tissue. Comparison between fresh and digested tissues shows that, at 35% strain, a minimum of 48% of the arterial load is carried by elastin, and a minimum of 43% of the change in stiffness of arterial tissue is due to the change in elastin stiffness. Analysis of the stress-strain behavior revealed that PAH causes an increase in the strains associated with the physiological pressure range but had no effect on the strain of transition from elastin-dominant to collagen-dominant behavior. These results indicate that mechanobiological adaptations of the continuum and geometric properties of elastin, in response to PAH, significantly elevate the circumferential stiffness of proximal pulmonary arterial tissue.

Reduced time as a unified parameter determining fixity and free recovery of shape memory polymers

Shape memory polymers are at the forefront of recent materials research. Although the basic concept has been known for decades, recent advances in the research of shape memory polymers demand a unified approach to predict the shape memory performance under different thermo-temporal conditions. Here we report such an approach to predict the shape fixity and free recovery of thermo-rheologically simple shape memory polymers. The results show that the influence of programming conditions to free recovery can be unified by a reduced programming time that uniquely determines shape fixity, which consequently uniquely determines the shape recovery with a reduced recovery time. Furthermore, using the time-temperature superposition principle, shape recoveries under different thermo-temporal conditions can be extracted from the shape recovery under the reduced recovery time. Finally, a shape memory performance map is constructed based on a few simple standard polymer rheology tests to characterize the shape memory performance of the polymer.

Mechanisms of multi-shape memory effects and associated energy release in shape memory polymers

Shape memory polymers have attracted increasing research interest due to their capability of fixing a temporary shape and associated deformation energy then releasing them later on demand. Recently, it has been reported that polymers with a broad thermomechanical transition temperature range can demonstrate a multi-shape memory effect (m-SME), where shape recovery and energy release occur in a stepped manner during free recovery. This paper investigated the underlying physical mechanisms for these observed shape memory behaviors and the associated energy storage and release by using a theoretical modeling approach. A multibranch model, which is similar to the generalized standard linear solid model of viscoelasticity, was used for a quantitative analysis. In this model, individual nonequilibrium branches represent different relaxation modes of polymer chains with different relaxation times. As the temperature was increased in a staged manner, for a given temperature, different numbers of branches (or relaxation modes) became shape memory active or inactive, leading to the observed m-SME. For energy release during free recovery, under a tensile deformation of the SMP, stored energy in individual nonequilibrium branches was first transferred into a compressive deformation energy, then gradually declined to zero. Energy release during recovery was a complicated process due to the involvement of multiple relaxation modes.

Programmable, pattern-memorizing polymer surface.

However, all current SMP applications focus on harvesting the macroscopic scale deformation, i.e. employing the SMP as structural materials. An intriguing capability of all SMPs, which remains largely unexplored, is their ability to memorize and recover nanoscale patterns or structures. Here we demonstrate that SMPs can memorize and faithfully recover their lithographically fabricated, permanent or even temporary surface patterns. More signifi cantly, tunable multi-pattern memory capability can be achieved in Nafi on fi lms. Considering the prevalence of nanostructured surfaces in emerging nanotechnologies, such pattern-memorizing surfaces could potentially transform these technologies. During a typical shape memory cycle, an SMPs permanent shape is fi rst “programmed” into a temporary shape under mechanical loading at a temperature higher than the transition temperature (either glass transition temperature, T g , or melting temperature, T m ) of the SMP. At the permanent shape, the polymer chains between crosslinking points can be considered at the equilibrium state, or the lowest energy state. The mechanical loading during the programming deforms the chains into a higher energy state (with lower entropic freedom), forming the temporary shape. Without the mechanical constraints, the SMP sample will return to their permanent shape to minimize the system energy. However, this temporary shape can be “fi xed” as the temperature decreases below the T g (or T m ) of the SMP before releasing the mechanical loading and remains stable indefi nitely. The SMP softens and recovers its permanent shape when exposed to an environmental stimuli such as heat, [ 4 , 5 ] light, [ 1 ] or even solvent vapors. [ 6 ] During the recovery, strain or stress can be harvested under free or constrained conditions, respectively. [ 7 , 8 ] However, beyond such structural applications, the potential applications of SMP surfaces have yet to be explored. King et al. reported that AFM-indented holes on a SMP surface can be recovered via heating which enables the AFM-based data storage. [ 9 , 10 ] Recently, Burke et al. showed that micron scale patterns embossed on liquid crystalline elastomer can be erased

3D Printed Reversible Shape Changing Components with Stimuli Responsive Materials

The creation of reversibly-actuating components that alter their shapes in a controllable manner in response to environmental stimuli is a grand challenge in active materials, structures, and robotics. Here we demonstrate a new reversible shape-changing component design concept enabled by 3D printing two stimuli responsive polymers—shape memory polymers and hydrogels—in prescribed 3D architectures. This approach uses the swelling of a hydrogel as the driving force for the shape change, and the temperature-dependent modulus of a shape memory polymer to regulate the time of such shape change. Controlling the temperature and aqueous environment allows switching between two stable configurations – the structures are relatively stiff and can carry load in each – without any mechanical loading and unloading. Specific shape changing scenarios, e.g., based on bending, or twisting in prescribed directions, are enabled via the controlled interplay between the active materials and the 3D printed architectures. The physical phenomena are complex and nonintuitive, and so to help understand the interplay of geometric, material, and environmental stimuli parameters we develop 3D nonlinear finite element models. Finally, we create several 2D and 3D shape changing components that demonstrate the role of key parameters and illustrate the broad application potential of the proposed approach.