Shape Memory Surface Research Articles

Preparation of superhydrophobic shape memory composites with uniform wettability and morphing performance

Superhydrophobic shape memory surfaces have attracted much attention in recent years, as they can intelligently control surface wettability. However, less research has focused on the development of superhydrophobic shape memory materials that can maintain uniform wettability in any morphing shape. It is common in nature that species such as lotus leaves and kingfisher show good superhydrophobicity in any movement under/on water. Based on this inspiration, this work presents a novel superhydrophobic shape memory composite (SSMC) by coupling superhydrophobic coatings and shape memory epoxy composites. The SSMC exhibits excellent water repellency and self-cleaning performance regardless of how it is bent. In addition, its good shape memory effect and variable stiffness characteristics enable it to be potentially used as deployable marine structures and morphing skin of aquatic unmanned aerial vehicles. On the other hand, the SSMC also offers good anti-icing properties and mechanical/chemical robustness that enhance long-term service performance all year round. This research provides a fresh idea for the design of new smart superhydrophobic materials and expands applications in the field of marine and aviation facilities.

Shape memory surface adhesion control based on PU-CNC/PS electrospun composite films

Wettability is a fundamental property of solid surfaces. The adhesion of liquids to solid surfaces, especially the intelligent control of surface adhesion, plays an important role in the wettability field, showing broad prospects in self-cleaning, anti-icing, oil–water separation and chemical reactions, etc. In this work, a new shape memory composite film was prepared by electrospinning, consisting of water-responsive shape memory substrate of polyurethane-cellulose nanocrystalline composite fibers and a superhydrophobic polystyrene spindle-fiber surface. Benefiting from the water-responsive shape memory performance of polyurethane-cellulose nanocrystalline substrate, the polystyrene surface spindle-fiber microstructures show sparse-dense switching during the stretching-recovery shape memory cycle, giving rise to smart control of surface adhesive behavior in superhydrophobic high-low adhesion. Given the flexibility and versatility, we demonstrate applications of the shape memory composite film in lossless droplet transfer, selective manipulation of multiple droplets, and smart detection. The composite film would provide potential applications in intelligent platforms, microliquid systems, and wearable fields, etc.

Design of Superhydrophobic Shape Memory Composites with Kirigami Structures and Uniform Wetting Property.

With the continuous increase in human demand to improve aircraft performance, intelligent aircraft technologies have become a popular research field in recent years. Among them, the deformable skin structure has become one of the key technologies to achieve excellent and reliable performance. However, during the service, deformable skin structures may encounter problems such as surface impact and adhesion of droplets in rainy weather or surface icing in low-temperature environments, which can seriously affect the flight safety of the aircraft. One way to overcome these issues is to use superhydrophobic shape memory materials in the structure. In this regard, first, shape memory composites were prepared with shape memory epoxy resin as the matrix and carbon fiber orthogonal woven fabric as the reinforcement material. Superhydrophobic shape memory composites (SSMCs) were then obtained by casting the kirigami composite with superhydrophobic carbon nanotube-polydimethylsiloxane (CNT@PDMS) mixture, and the surface was processed by laser micromachining. Shape memory performance and surface wetting performance were determined by material testing methods. The results showed that the shape memory recovery rate can reach 85.11%, the surface is superhydrophobic, the average water contact angle is 156.9 ± 4.4°, and the average rolling angle is 3 ± 0.5°. The three-point bending test of the specimens with different kirigami cell configurations showed that the shape memory composite based on the rectangular structure has the best deformability with an aspect ratio of 0.4. From the droplet impact test, it was found that the impact speed of water droplets and the curvature of the surface can greatly affect the dynamic performance of water. This work is expected to be of significant research value and importance for developing functional deformable skin materials.

Evaluation of the mechanical properties and clinical application of nickel-titanium shape memory alloy anal fistula clip.

The study investigates the mechanical properties of a nickel-titanium shape memory alloy anal fistula clip (NiTi-AFC), studies the surgical method of treating anal fistula, and evaluates its clinical efficacy. The anal fistula clip was formed in nickel-titanium alloy with a titanium content of 50.0%-51.8%. The mechanical properties and chemical properties were tested. A total of 31 patients with anal fistula were enrolled between 1 January 2020 and 1 January 2023. All patients underwent internal orifice closure surgery using NiTi-AFC, and anorectal magnetic resonance or ultrasound was performed before surgery and 6 months after surgery for diagnosis and evaluation. Fistula cure rates, length of stay, perianal pain, and Wexner incontinence scores were retrospectively compared between patients treated with NiTi-AFC and patients treated with other surgical methods. NiTi-AFC has a density of 6.44-6.50 g·cm-3, with a shape-restoring force of 63.8 N. The corrosion rate of NiTi-AFC in 0.05% hydrochloric acid solution at atmospheric pressure and 20°C is approximately 6.8 × 10-5 g·(m·h)-1. A total of 31 patients (male/female: 19/12, age: 43.7 ± 17.8 years) were included. Among them, 22.6% (7) had multiple anal fistula, 16.1% (5) had high anal fistula, and 48.3% (15) had perianal fistula Crohn's disease. In total, 12.9% (4/31) did not achieve primary healing, underwent fistula resection, and eventually recovered. A retrospective analysis showed that the fistula healing rate, length of stay, and anal pain of NiTi-AFC treatment were similar to those of other traditional surgeries, but the Wexner incontinence score was significantly lower. NiTi-AFC has shape memory properties, corrosion resistance, superelastic effect, and surface cell adhesion. It is applied to internal orifice closure surgery of anal fistula, with good therapeutic effect, and can protect the anal function.

Locomotion of an untethered, worm-inspired soft robot driven by a shape-memory alloy skeleton

Soft, worm-like robots show promise in complex and constrained environments due to their robust, yet simple movement patterns. Although many such robots have been developed, they either rely on tethered power supplies and complex designs or cannot move external loads. To address these issues, we here introduce a novel, maggot-inspired, magnetically driven “mag-bot” that utilizes shape memory alloy-induced, thermoresponsive actuation and surface pattern-induced anisotropic friction to achieve locomotion inspired by fly larvae. This simple, untethered design can carry cargo that weighs up to three times its own weight with only a 17% reduction in speed over unloaded conditions thereby demonstrating, for the first time, how soft, untethered robots may be used to carry loads in controlled environments. Given their small scale and low cost, we expect that these mag-bots may be used in remote, confined spaces for small objects handling or as components in more complex designs.

Programmable Humidity-Responsive Actuation of Polymer Films Enabled by Combining Shape Memory Property and Surface-Tunable Hygroscopicity.

Most humidity-responsive polymeric actuators can only exhibit shape transformations between a planar shape in the dry state and a bended three-dimensional (3D) shape when exposed to moisture, and it is challenging to design and prepare hygroscopic actuators with programmable actuating behaviors displayed from sophisticated 3D structures. Herein, we demonstrate that the integration of shape memory property and surface treatment enabled hygromorphic responsivity endows a single-component polymer film with programmable moisture-driven actuating behaviors. The solvent-processed polyethylene-co-acrylic acid (EAA) copolymer film is soft and stretchable at room temperature, and has a good thermal-responsive shape memory property. By surface treatment using base/acid solutions, the reversible gradient conversion between carboxyl groups and carboxylate salts along the thickness direction enables the film to exhibit designed hygroscopic actuations. The shape memory property and moisture-driven actuating behaviors can be combined to realize 3D-3D morphing by first programming the films into 3D shapes and then conducting the surface treatments. Both shape programming and surface treatment processes can be reprogrammed to make the actuation behavior readily tunable. We also show that the created surface patterns can act as moisture-sensitive conducting paths to detect human breathes, and the combination of shape memory, moisture-responsive morphing and conductivity change leads to some interesting applications such as smart switch in conducting circuit. This work provides a new and general strategy for the design of advanced humidity-responsive actuators.

Switchable shape memory wetting surface based on synergistic regulation of surface chemistry and microstructure

Smart control of surface wettability can be obtained by adjusting surface chemistry or regulating surface microstructure, however, coordinated regulation of the two factors on the same surface is still limited. Here, we report a new shape memory composited film consist of a hierarchical pillared polyurethane surface and shape memory polyurethane-cellulose substrate. Reversible switching among multiple wetting states between superhydrophobicity and superhydrophilicity can be realized by synergistic regulation of surface microstructure based on stretching-recovery shape memory effect and surface chemical composition based on O2 plasma-heating. The mild and excellent water-responsive shape memory ability of substrate enables surface microstructure and corresponding wetting to fix at desirable states without keeping continuous external force or stimuli, which is superior to elastomer substrate, responsive molecule modification, or traditional temperature/light triggered shape memory materials. Given the flexibility and versatility, we show its applications in smart platform for creating gradient wettability and patterned micro-reaction.

Light‐induced dynamically tunable micropatterned surface for the regulation of the endothelial cell alignment

The surface topography has a great effect on the behaviour of adherent cells. We report an efficient method to prepare shape memory surfaces with dynamically changed micropatterns, which can be induced by near-infrared (NIR) light by varying the power density. After polydopamine (PDA) was coated on the cross-linked polyethylene glycol-poly(e-caprolactone), the surface temperature increases by 40°C at room temperature when 808 nm light with 1.0 W/cm 2 is used because of the photothermal properties of PDA. This temperature increase is enough for the shape recovery of the pressed micropatterns. The depth of the recovered micropatterns is controllable by adjusting the power density of the 808 nm light. The NIR-induced micropatterns efficiently regulate the morphology and alignment of endothelial cells. Therefore, NIR-induced shape memory surfaces have the potential to be used in remote-controlled devices.

A Soft Shape Memory Reversible Dry Adhesive

Transfer printing is a critical procedure for manufacturing stretchable electronics. During such a procedure, stamps are utilized to transfer micro devices from silicon wafers to stretchable polymeric substrates. In addition to conventional silicone rubber stamps, epoxy resin based shape memory stamps have been developed and the transfer yield is thus significantly promoted. However, elastic modulus of the epoxy stamps is too high at both glassy and rubbery states, which may break the brittle micro devices during the adhesion process under mechanical pressure. In this work, we synthesized a copolymer of butyl acrylate (BA) and polycaprolactone diacrylate (PCLDA) as a soft reversible dry adhesive enabling a shape memory capability based on crystalline transition of polycaprolactone (PCL) segments. For the sample containing 40 wt% BA and 60 wt% PCLDA, Young’s modulus was 8.3 and 0.9 MPa respectively below and above the thermal transition temperature, which was much lower than that of the epoxy adhesive. On the other hand, the soft material still provided nearly ideal shape memory fixity and recovery ratios. Subsequently, shape memory surface with cone-shaped microstructure was prepared, which enabled a heating induced strong-to-weak adhesion transition when the microstructure recovered from a pressed temporary morphology to the permanent cone-shaped morphology. Such a soft reversible dry adhesive may contribute to large-scale and automated transfer printing processing.

Thermo-Responsive Shape-Memory Effect and Surface Features in Polycarbonate (PC)

The influence of programming strain and temperature on the shape memory effect and surface morphology in programmed polycarbonate (PC) samples via uni-axial stretching is investigated. It is found that the samples programmed at around the glass transition start temperature not only have micro-cracks on their surface, but also show a necking phenomenon. Furthermore, the surface of the necked area is concave, but the surface of the non-necked area is convex. On the other hand, despite the samples programmed at high temperatures being able to deform in a uniform manner at macroscopic scale, their surfaces are still uneven, either concave or convex. While the samples programmed at low temperatures are able to achieve full shape recovery, stretching at higher temperatures over the glass transition range to a higher strain may result in non-recoverable deformation.

Effect of indentation temperature on nickel-titanium indentation-induced two-way shape-memory surfaces

This study investigated the effect of temperature on indentation-induced one-way and two-way shape memory properties in Ti-50.3at% Ni alloy. Indentation temperatures ranged from below the martensite finish temperature (Mf) to above the austenite finish temperature (Af) with the explicit intent of varying the indented phase. Samples used in the study were characterized by differential scanning calorimetry and transmission electron microscopy (TEM). The topographical behavior of the shape memory effect was investigated through Vickers indentation and laser scanning 3D confocal measurements. The magnitudes of deformation recovery associated with the one-way and two-way shape-memory effect (OWSME, TWSME) decreased with increasing indentation temperatures, which is a reflection of the decreasing volume of material experiencing martensitic reorientation during indentation. Indented and subsequently planarized samples exhibited TWSME protrusions when thermally cycled. Laser scanning measurements were used to characterize the height of the protrusions as increasing depths of material were polished away, which provided insight into the overall affected volume beneath the indent. As indentation temperatures increased, both the height of the protrusions, and consequently the polish depth necessary to completely remove the effect, decreased. TEM investigations revealed that directly underneath a nanoindent the microstructure was very fine due to the high-strain deformation; this was contrasted with a much coarser grain size in the undeformed bulk material. Overall these results strongly imply that the deformation recovery associated with the OWSME and TWSME can be maximized by indenting at temperatures at Mf or below because the volume of deformed microstructure beneath the indent is maximized. This finding has important practical value for any potential application that utilizes indentation-induced phase transformation deformation recovery in NiTi.

Geometry- and Length Scale-Dependent Deformation and Recovery on Micro- and Nanopatterned Shape Memory Polymer Surfaces.

Micro- and nanoscale surface textures, when optimally designed, present a unique approach to improve surface functionalities. Coupling surface texture with shape memory polymers may generate reversibly tuneable surface properties. A shape memory polyetherurethane is used to prepare various surface textures including 2 μm- and 200 nm-gratings, 250 nm-pillars and 200 nm-holes. The mechanical deformation via stretching and recovery of the surface texture are investigated as a function of length scales and shapes. Results show the 200 nm-grating exhibiting more deformation than 2 μm-grating. Grating imparts anisotropic and surface area-to-volume effects, causing different degree of deformation between gratings and pillars under the same applied macroscopic strain. Full distribution of stress within the film causes the holes to deform more substantially than the pillars. In the recovery study, unlike a nearly complete recovery for the gratings after 10 transformation cycles, the high contribution of surface energy impedes the recovery of holes and pillars. The surface textures are shown to perform a switchable wetting function. This study provides insights into how geometric features of shape memory surface patterns can be designed to modulate the shape programming and recovery, and how the control of reversibly deformable surface textures can be applied to transfer microdroplets.

Developments in laser-based surface engineering processes: with particular reference to protection against cavitation erosion

With the increasing demands of economical, reliable and durable hydraulic and pneumatic systems, it is necessary to minimize the material damage from cavitation erosion (CE) when systems are handling cavitating and corrosive fluids. Cavitation erosion is a nausea for many engineering components, such as ship propellers & rudders, turbine, diesel engine, cylinder liner, pump impeller vanes, control valves, hydraulic turbines, bearings, pipes, ultrasonic cleaners and mechanical heart valves, which are exposed to the high-speed flowing or vibratory fluids. This paper reviews the rationale behind the application of laser surface modification for achieving CE resistant surfaces of fluid handling components. The problem of CE may be tackled by enhancing the surface properties of the base materials (ferrous and non-ferrous alloys) with various laser surface modification techniques including laser transformation hardening (LTH), laser surface melting (LSM), laser surface alloying (LSA), laser cladding (LC), laser dispersion (LD) and laser plasma hybrid spraying (LPHS). The CE performance of a variety of laser-surface modified layers/coatings is discussed in this review. In particular, coatings of hard-facing alloys, shape memory alloys, surface metal or intermetallic matrix composites and cermets on ferrous and non-ferrous alloys are included. The mechanisms of the enhancement in cavitation erosion resistance (Re) are discussed.

A theoretical model of reversible adhesion in shape memory surface relief structures and its application in transfer printing

Transfer printing is an important and versatile tool for deterministic assembly and integration of micro/nanomaterials on unusual substrates, with promising applications in fabrication of stretchable and flexible electronics. The shape memory polymers (SMP) with triangular surface relief structures are introduced to achieve large, reversible adhesion, thereby with potential applications in temperature-controlled transfer printing. An analytic model is established, and it identifies two mechanisms to increase the adhesion: (1) transition of contact mode from the triangular to trapezoidal configurations, and (2) explicit enhancement in the contact area. The surface relief structures are optimized to achieve reversible adhesion and transfer printing. The theoretical model and results presented can be exploited as design guidelines for future applications of SMP in reversible adhesion and stretchable electronics.

Focus on the interlude between topographic transition and cell response on shape-memory surfaces

Shape-memory surfaces with on demand, tunable nano-patterns have been developed to observe time dependent changes in fibroblast cell alignment using temperature-responsive poly(ε-caprolactone) (PCL) films. The PCL films were prepared by crosslinking tetra-branched and linear PCLs, each with acrylate end-groups. Permanent surface patterns were generated by crosslinking the PCLs in a mold. Temporary surface patterns were later embossed into the crosslinked PCL. NIH 3T3 cells cultured on the temporal nanopatterns showed marked alignment along the pattern direction, regardless of their ridge and groove widths. Then, the direction of grooves was suddenly transitioned 90° to the temporary ridges. Holographic microscope revealed that the application of heat quickly and completely transitioned temporal to permanent patterns within 30 s. However, it took more than 2 h for cells on substrate with 500 nm grooves to change their orientation, while it took more than 8 h on substrate with 2000 nm grooves. This different alignment behavior can be explained by the different adhesion strength and reorganization of cytoskeletal proteins on nano-v.s. micro-patterns. Dynamically tunable nano-structured surfaces, therefore, can be used to study the effects of surface nano-geometries on time-dependent cytoskeleton remodeling under biological relevant conditions.

Shape‐Memory Surface with Dynamically Tunable Nano‐Geometry Activated by Body Heat

Shape-memory surfaces with on-demand, tunable nanopatterns are developed to observe time dependent changes in cell alignment using temperature-responsive poly(ϵ-caprolactone) (PCL) films. Temporary grooved nanopatterns are easily programmed on the films and triggered to transition quickly to permanent surface patterns by the application of body heat. A time-dependent cytoskeleton remodeling is also observed under biologically relevant conditions.

Theoretical investigation of the interaction of oxygen with pure and K-doped NiTi shape memory surface alloys

Density functional based theories and experiments agree well on the value of the heat of formation of TiO 2 on NiTi surfaces. However, experimental studies of polycrystalline NiTi surfaces tend to indicate that Ti atoms are always abundantly available at the surface. This theoretical study indicates that whether Ti atoms are dominantly available at the surface depends on the surface index. On NiTi(001) in the B2 phase, the surface can be Ti or Ni terminated, with equal probability, while on NiTi(110) in the B2 phase and NiTi(010) in the B19ʼ phase, Ti atoms are favored to be present at the surface.

Oxidation of pure and potassium-doped NiTi shape memory surface: A density functional theory investigation

The oxidation of Ti-terminated NiTi surfaces is investigated with density functional theory. The surfaces considered are (001) and (110) of the austenite phase (B2) and (010) of the martensite phas ...

In Vitro Evaluation of Chemically Cross-Linked Shape-Memory Acrylate−Methacrylate Copolymer Networks as Ocular Implants

Acrylates have been used in ophthalmic practice as a paradigmatic implant material for decades, especially as intraocular lens for their excellent transparency. A novel polymeric shape memory system of chemically cross-linked acrylate-methacrylate copolymer networks was developed and characterized in this study. The thermomechanical properties, shape memory properties, transparency, and surface wettability as well as cytotoxicity were systematically evaluated to mimic the in vivo situation by differential scanning calorimetry (DSC), tensile tests, spectrophotometer, Abbe refractometer, contact angle measurements, and MTT assay. It was found that the chemically cross-linked copolymer network behaves as an elastomer capable of arbitrary shaping above the glass-transition temperature. Transition temperatures of the networks were tunable through the change of the composition of monomers.

Indentation-induced two-way shape memory surfaces

A method is described for the creation of surfaces with cyclically reversible topographical form. Using spherical and cylindrical indenters applied to NiTi shape-memory alloys, an indentation-planarization technique is shown to result in a two-way shape memory effect that can drive flat-to-wavy surface transitions on changing temperature. First, it is shown that deep spherical indents, made in martensitic NiTi, exhibit pronounced two-way cyclic depth changes. After planarization, these two-way cyclic depth changes are converted to reversible surface protrusions, or “exdents.” Both indent depth changes and cyclic exdent amplitudes can be related to the existence of a subsurface deformation zone in which indentation has resulted in plastic strains beyond that which can be accomplished by martensite detwinning reactions. Cylindrical indentation leads to two-way displacements that are about twice as large as that for the spherical case. This is shown to be due to the larger deformation zone under cylindrical indents, as measured by incremental grinding experiments.