We have previously demonstrated that various types of patterns at different scales can be easily realized atop shape
memory polymers (SMPs) through a procedure in three steps, namely, indentation, polishing and heating (IPH). On the
other hand, by coating a thin metallic layer (e.g., a few nanometers thick of gold) atop SMPs with or without prestraining,
different types of wrinkles could be generated. In this paper, we investigate the wrinkling patterns formed on
the top of SMPs with a patterned surface, i.e., wrinkling atop a surface with a protrusion pattern. As the wrinkles are
resulted simultaneously by two phenomena upon heating, namely the shape recovery of the SMP substrate and buckling
of the elastic metallic layer, the wrinkle pattern varies from one location to another depending on the exact local strain.
Utilizing this technique, we are able to simultaneously produce a surface with complicate surface patterns at both micro
and millimeter levels. Such surfaces could significantly enhance many surface properties, e.g., bonding, adhesion and
We demonstrate a simple approach to significantly reduce the electrical resistivity of thermo-responsive shape-memory
polymers (SMPs), so that they can be easily triggered for shape recovery by Joule heating at a low electrical voltage.
After adding a small amount of Ni micro particles into a polyurethane SMP filled with carbon black (CB), the electrical
resistivity is slightly reduced. However, if these Ni particles are aligned into chains (by applying a low magnetic field on
SMP/CB/Ni solution and then drying to fix the conductive chains), the drop of electrical resistivity is significant. This
kind of SMP composites is suitable for cyclic operation as only micro/nano particles are used. A sample (40×15×1mm)
with 10vol% of CB and 0.5vol% of chained Ni can be heated to 80°C for shape recovery at 30 V (1.2 W) of power. This
approach is generic and applicable for producing other conductive polymers.
Shape memory polymers (SMPs) have a number of advantages as compared with their metal counterpart, namely shape
memory alloys (SMAs), in particular for some particular medical applications. The recent finding of the influence of
moisture on the glass transition temperature of polyurethane SMPs, which are typically thermo-responsive in nature,
enable us to realize not only the water driven feature for shape recovery, but also the recovery following a predetermined
sequence, i.e., programmed recovery. Utilizing these new features, we demonstrate a few novel applications
of this SMP, which might be of the particular interest for medical devices in, for instance, minimally invasive surgery.
The surface relief phenomenon is one of the interesting properties of shape memory alloys. In this paper, this phenomenon in a NiTi shape memory alloy rod is investigated quantitatively using a temperature controllable atomic force microscope. The surface reflection, which depends heavily on the surface roughness, is studies quantitatively with different incident angles and incident directions. It is found that the incident angle α has a slight effect on the distribution of θ, which is the angle between the real reflective direction and the ideal one. The material is almost uniform in any direction, despite that only one incident angle and four directions have been analyzed. At room temperature (martensite phase), θ mainly falls to around 3°. While at 100°C (austenite phase), a large portion of θ shifts to around 20°. In order to obtain a ratio of change over 0.5, θ is expected to be at around 3°.
It has been proved that NiTi shape memory alloy thin film is the best one for micro actuators as compared with the others, e.g., electrostatic, electromagnetic and piezoelectric thin films. If the deposition of NiTi thin films on silicon wafers is carried out at room temperature, the resultant thin films are normally amorphous without shape memory. Subsequent annealing in a high vacuum chamber is required for re-crystallization. In this paper, we present an alternative annealing approach, namely by CO2 laser. After laser annealing, optical microscope, X-ray diffraction (XRD) and atomic force microscope (AFM) were applied to characterize the NiTi thin films. Strong austenite/martensite lattice structures were observed by XRD. The relationship between the surface roughness of the annealed NiTi thin film and temperature was obtained using AFM. The results indicate that the CO2 laser annealed NiTi thin films are with shape memory.
We present a way to fabricate microgrippers that can meet the industry's needs well, i.e., low cost and large tip deflection, etc. The microgripper is fabricated by bonding two identical micro NiTi-Si cantilever beams together with a silicon spacer in between. It can be actuated by electrical current directly. We have tested the behavior of micro NiTi-Si cantilever beams of three different sizes, and compared that with our simulation results. According to our simulation, the maximum strain and the maximum stress in NiTi should enable the grippers to survive after 106 cycles. Due to the simple fabrication process, this design is very suitable for batch production at low cost, which is a significant advantage in both medical and manufacturing industries
A simple method is proposed for simulating the behavior of shape memory alloys at grain level. Different from many previous models in the literature, this model is applicable for not only proportional load at a constant temperature but also complicate non-proportional load together with temperature variation. The capability of this model is demonstrated by simulating the strain response of a set of combined tensile-shear stress loading.
NiTi Shape Memory Alloy (SMA) is with great potential for actuation in microsystems. It is particularly suitable for medical applications due to its excellent biocompatibility. In MEMS, local annealing of SMA is required in the process of fabrication. In this paper, local annealing of Ni52Ti48 SMA with excimer laser is proposed for the first time. The Ni52Ti48 thin film in a thickness of 5 μm was deposited on Si (100) wafer by sputtering at room temperature. After that, the thin film was annealed by excimer laser (248nm KrF laser) for the first time. Field-Emission Scanning Electron Microscopy (FESEM) and Atomic Force Microscopy (AFM) were used to characterize the surface profile of the deposited film after laser annealing. The phase transformation was measured by Differential Scanning Calorimeter (DSC) test. It is concluded that NiTi film sputtering on Si(100) substrate at room temperature possesses phase transformation after local laser annealing but with cracks.
Surface relief is one of the interesting properties of shape memory alloys. In this paper, we investigate the surface roughness of NiTi shape memory alloys polished in austenite and twinned martensite phases using a Wyko interferometer. The potential application of surface relief in shape memory alloys as a kind of novel micro mirror is proposed.
NiTi shape memory alloy thin film has been proved to be the best micro actuation mechanism due to the largest displacement and the highest actuation force as compared with other mechanisms. The combination of laser and traditional MEMS techniques might provide a better and cheaper solution for the fabrication of micro devices. In this paper, we present some preliminary results of cutting and annealing of NiTi shape memory alloy thin films using different types of lasers.
In this paper, we present the results of theoretical study of thin film technique based micro grippers. Three most popular actuation mechanisms, namely, piezoelectric, bimetal and shape memory alloy are investigated. First, we present the simulation results against the measured behavior of NiTi shape memory alloy thin film based micro grippers. Then we compare the performances of these three kinds of micro grippers. It shows that shape memory alloy based micro gripper is much better than the others.
In this paper, we present a way to fabricate microgripper that could well meet the industry needs, i.e. low cost, high performance etc. A medium sized microgripper of 1.6 mm in length has been fabricated, tested and simulated. This novel design, with its fabrication process, makes it possible for batch production, which results in lower production cost. Its low cost has unique advantages in both medical and manufacturing industries. For example, in the medical field, the microgripper could be disposed after every use, much like a syringe without imposing excessive costs. Our finite element simulation agrees reasonably well with the measured behavior.
In this paper, a novel micro assembly method using Shape Memory Alloy (SMA) is investigated. The principle of this method and its advantages are briefly discussed. A finite element package, ANSYS, is used to simulate the whole assembly process. A special material element is used for modeling the behavior of SMA.
Thin film shape-memory alloys have been recognized as a promising and high performance material in the field of microelectromechanical systems applications. In this investigation, TiNi films were prepared by sputtering Ti and Ni target in argon gas using a magnetron sputtering system. Chemical composition, crystallography, microstructure and phase transformation behaviors of the deposited TiNi film were studied. Differential scanning calorimeter results showed that a two-stage transformation occurs in a sequence of monoclinic martensitic phase to rhombohedral phase, then to B2 phase upon heating, and vice versa on cooling. X-ray diffraction analysis also revealed the crystalline structure changes with the change of the temperatures. Nano- indentation reveals the elastic modulus of the film is about 5.11 GPa and the film intrinsic hardness is 2.84 +/- 0.5 GPa. By depositing TiNi films on the bulk micromachined Si cantilever structures, we obtained micro-grippers exhibiting a good shape-memory effect.
In this paper, the experimental study of a NiTi shape memory alloy bar with nominal diameter of 6.5 mm is presented. First, some torsion experiments, including torsion cycling at constant temperature and thermal cycling under constant torque, were carried out. In these test, the torque was applied in both positive and negative directions. Two-way memory behavior and some unique phenomena, such as kink and easy-training, were found and hence, a series of uniaxial tension test was performed in order to understand the observations. After presenting the experimental work, the reason behind these phenomena is discussed.