Cracked titanium film on an elastomeric substrate for highly flexible, transparent, and low-power strain sensors
© Noh; licensee Springer. 2013
Received: 10 September 2013
Accepted: 13 October 2013
Published: 24 October 2013
Strain-dependent cracking behaviors in thin titanium (Ti) films on polydimethylsiloxane (PDMS) substrates were systematically investigated for their application to sensitive, flexible, transparent, and portable strain sensors. When uniaxially elongated, vertical cracks were developed in the low-strain range, and beyond a critical strain, tilted cracks appeared to intersect the vertical cracks. The cracking behaviors were also dependent on Ti film thickness. The varying strain-dependent crack patterns produced a significant resistance change in response to the applied strain, particularly, in the high- and broad-strain range. For a 180-nm-thick Ti film on PDMS substrate, a gauge factor of 2 was achieved in the range of 30% to 50% strain. The operation power was extremely low. All the Ti films on PDMS substrates were transparent, highly flexible, and very easy to fabricate. These results suggest that cracked Ti films on PDMS substrates could be a viable candidate for realizing a low-cost, flexible, transparent, and portable strain sensor.
Measuring strain accurately has become much more important since new technology fields such as health monitoring, artificial skin engineering, intelligent textile engineering, motion detection, and environment monitoring have emerged [1–7]. Flexible materials are widely employed for these applications due to the diversity of body shapes to which the sensors are attached and the variability of strain in action. Recent progress on the material systems includes graphene ripples on polydimethylsiloxane (PDMS) substrates , Si/Ge nanowire matrix on polyimide substrates , Pt-coated polymer nanofibers sandwiched between PDMS sheets , Si nanoribbons on polyimide substrates , carbon nanotube ribbons embedded in PDMS , ZnO nanowire/polystyrene hybrid structure on PDMS , and graphene on PDMS . Although high gauge factors reaching 116 and the adaptability to various forms of stresses such as tension, compression, shear stress, and torsion have been demonstrated through those approaches, a few weak points still need to be addressed. For instance, sensor fabrication processes were somewhat complicated, tolerable strains were low (less than several percent) for many systems, and most sensors were not completely transparent, whereas conventional strain sensors made of metal foils also suffer from limited sensitivity and high power consumption .
From previous works on palladium (Pd) film on a PDMS substrate, it was demonstrated that the Pd film was broken into pieces under an external or internal strain and it could be applied for highly sensitive hydrogen gas sensors [15–18]. A mechanism by which nanocracks are generated in the Pd film on PDMS substrate was proposed, and a general process on how the nanocracks thus generated respond to hydrogen molecules was also provided [15, 17]. However, systematic studies on the expandability of the proposed mechanism to other metals and the crack generation behaviors dependent on the magnitude of applied strain were missing.
In this work, we investigated the effect of applied strain and film thickness on nanocrack generation using titanium (Ti) films on PDMS substrates. Ti was chosen as the film material because of its several advantages such as good adhesion to diverse materials, high strength-to-weight ratio, good resistance to corrosion, and high biocompatibility even though it is a poor conductor [19–22]. Differing patterns of cracks in the Ti film created under varying strains resulted in a change in electrical resistance that corresponded to the applied strain, providing an opportunity that the cracked Ti film on PDMS substrate could be used for a flexible strain sensor covering a wide range of strain. The suggested strain sensor is very easy to fabricate and handle, which ultimately allows for low-cost, portable strain sensors. It is also transparent, thereby expanding its potential use to monitoring deformations in various transparent bodies such as fragile structures, flexible electronics, and health-monitoring appliances.
The strain-dependent cracking behaviors of the Ti films on PDMS substrates were examined first at the microscale using an optical microscope (Olympus BX 51,Olympus Corporation, Tokyo, Japan). To stereoscopically investigate the patterns and sizes of the cracks at the smaller scale, the samples were three-dimensional (3D)-scanned using a 3D laser scanning microscope (Olympus CLS 4000). In addition, scanning electron microscopy (SEM, Hitachi S4800, Hitachi High-Tech, Tokyo, Japan) was utilized to closely observe individual cracks. The resistances of the cracked Ti films on PDMS substrates were measured by a simple two-probe method, using a probe station connected to a high-resolution, multi-purpose electrical characterization system (Keithley 4200-SCS, Keithley Instruments Inc., Cleveland, OH, USA). The extremely high-resolution system enabled to detect a femto-ampere-level current and to measure a resistance of more than 1 TΩ. The resistance was monitored not only under normal tension, but it also measured under non-planar straining along a curved surface.
Results and discussion
Although optical microscopy revealed the overall cracking behaviors of the Ti film on PDMS substrate, its resolution is limited and the data is two-dimensional. To overcome these shortcomings, laser scanning microscopy (LSM) was utilized. LSM images for a 180-nm-thick Ti film subjected to 30% and 50% strains, respectively, are presented in Figure 2g,h. Now, both cracks and buckling are seen much more clearly, and inter-crack distances are found to range from 1 to 4 μm, which are shorter than the average value estimated from the optical images. Comparing crack patterns created by the respective strains, the average crack width (1.09 μm) at 50% strain is larger than that (0.72 μm) at 30% strain, and the buckling density is also larger at a higher strain state. The inter-crack spacings are similar for both strain states.
The cracked Ti film on PDMS substrate can also endure a mixed stress state since it is very flexible. Figure 5c shows a resistance versus strain plot obtained from the 180-nm Ti film on PDMS substrate wrapped around a cylinder with a radius of curvature of 11 mm (see Figure 4b). In this case, strains were applied along the curved surface, forcing a combination of bending and stretching to the sample. Like the simple stretching case, the resistance-changing trend is divided into a steady region (low-strain region) and a sharp-changing region (high-strain region). In the high-strain region, the ∆R/∆ϵ is approximately 2.0 TΩ/%, which is far smaller than under simple stretching. When measured again after relaxation of the applied strain, the resistance at each strain was reproducible as shown by the blue square symbols in Figure 5c. It is not clear at this moment why the resistance-changing trends are divided into two regions for both simple stretching and more complex straining of bending and stretching. A clue, however, can be deduced from the cracking behavior of the sample. The border between the two regions exists around a 30% strain for the 180-nm-thick Ti/PDMS sample, coinciding with the initiation point of the tilted secondary cracks (ϵ c ≈ 30%). It is inferred that below this strain, the vertical cracks are not fully developed and there is still a connected current path, and then all the current paths are severed with the advent of the secondary cracks above the critical strain, which causes a steep resistance increase with a small increase in strain. This was supported by the fact that no significant resistance variation was observed in the strain range of 0% to 50% for a 250-nm-thick Ti film on PDMS substrate, where only weak vertical cracks appear. Despite many advantages of the cracked Ti film on PDMS substrate as a strain sensor, there still remain issues to be further addressed, including the effects of irregular crack patterns and surface oxide and how to widen the strain-sensing range more, particularly toward the lower strains.
Thin Ti films with thicknesses of 80 to 250 nm were sputter-deposited on elastomeric PDMS substrates. All the samples were transparent and highly flexible. Cracks were introduced in the Ti films by both planar and non-planar stretching, but the cracking behaviors differed depending on the applied strain and the Ti film thickness. Vertical cracks were developed at low strains below a critical strain, and beyond it, secondary cracks tilted from the straining direction appeared to intersect the earlier formed vertical cracks. The strain-dependent crack patterns led to the strain-dependent resistance. For a 180-nm Ti film on PDMS substrate, a sharp-resistance-changing region appeared over a tensile strain range of 20% above a critical strain of 30%, where a gauge factor of 2 was achieved. It also showed extremely low-power consumption and endured a mixed strain of bending and stretching. These attributes of cracked Ti films on PDMS substrates provide a pathway for the embodiment of an advanced strain sensor with low-cost manufacturability, high transparency and flexibility, and good portability.
This work was supported by the Gachon University research fund of 2013 (GCU-2013-R291). The author thanks Professor Kwang S. Suh of Korea University for his assistance.
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