Author :  Tanu Suryadi Kustandi
Last Updated : 18 May 2007

Submicron Gecko Mimicking Adhesive Structures 

Background

Geckos have adopted nanoscale fibrillar structures on their feet as adhesion devices, allowing them to maneuver on vertical walls and ceilings. Despite more than 300 years of studies on hairy attachment systems, there is still a debate concerning the attachment mechanism of geckos walking on smooth and rough surfaces. Based on experimental data, some of the theories, such as sticking fluid, microsuckers, and electrostatic forces, have been rejected and adhesion has been attributed to either a combination of molecular interactions and capillary attractive force or purely van der Waals interactions. This finding has inspired the creation of novel adhesives by manufacturing small, closely packed arrays mimicking adhesive hairs. Numerous theoretical works have been presented to understand adhesion mechanisms in geckos, suggesting the best possible strategies to develop and utilize novel adhesive materials for engineering applications.

Objectives of project

1.    Measure the adhesive force of a single spatula in controlled environment to determine the true nature of geckos’ adhesion force.

2.    Develop novel methods to manufacture the artificial structures of gecko foot-hairs. Further development of manufacturing methods to mimic the hierarchical structure of gecko foot-hairs would become another focus of this research so as to achieve the aims of creating the true gecko-inspired adhesive structures. 

3. Develop a theoretical model to explain (a) the significance of hierarchical structures in gecko foot-hairs and (b) the efficient detachment mechanism despite of geckos’ excellent grip. 

Capillary Forces Hypothesis

Even under very dry conditions with relative humidity below 10%, the adsorbed water layer with a thickness ranging between a few angstrom and several nanometers is still observed. Liquids from the environment spontaneously condense from the vapor phase to the liquid state on the surface cavities of any substances. These capillary forces of water films can significantly influence the attraction between two surfaces. Dependent upon the interface conditions (surface energy, relative humidity, and liquid volumes), the capillary forces can be large as compared to weak van der Waals forces.

Adhesion force measurements of a single spatula

Two types of experiments were performed: First, hydrophobic and hydrophilic cantilevers were used to assess the capillary forces contribution in the adhesion of geckos’ setae. The second type of experiment involved controlled variation of air humidity. To control the relative humidity, the experiments were conducted in a chamber purged with nitrogen gas of known humidity.

Figure 1a shows the typical force-displacement curve of geckos’ spatulas measured by a rectangular silicon cantilever using an isolated single seta sample.

Figure 1 Force-displacement curves of geckos’ spatulas using (a) an isolated seta and (b) multi-setae samples, measured with a silicon cantilever with a spring constant of 0.1 N/m, in air with relative humidity of 70%. The black and red lines are the extending and retracting curves, respectively. [1]

Figure 2 Histograms of forces measured with (a) hydrophilic and (b) hydrophobic silicon cantilevers. [1]

As the variation in force with surface hydrophobicity is a major feature of capillary forces, modification of the cantilever hydrophobicity is a conventional way to determine its amplitude. 

The mean adhesion force derived from histograms (shown in Figure 2) was 11.8 and 4.9 nN, respectively, which suggests that the dominating component of the gecko force is the capillary force as the amplitude of van der Waals force decreases instead of increases with the increase in water between the surface and the spatula.

Force-distance measurements at different relative humidity (RH) were performed to confirm the above derived conclusion, as shown in Table 3.1 . When the sample was immersed into water, the adhesion force decreased to less than 20% of its original value (see Table 1). This confirms that the dominant force in an ambient air environment is the capillary force.

 Table 3.1 Spatula Adhesion Force [1]

Self assembled nanoparticles based fabrication of gecko foot-hairs inspired polymer nanofibers

A new paradigm that combines a self-assembly patterning technique and semiconductor technologies is proposed to efficiently construct densely packed high aspect-ratio nanostructures. The inexpensive method of colloidal nanolithography as a facile patterning protocol, a modified silicon etching to form deep columnar trenches, and nanomolding are used to form the flexible polymeric nanostructures. 

The role of the flexible membrane was further investigated in relation to the adhesion of the nanostructured surface on a macroscopic scale, and the “easy-to-clean” characteristic was associated with the role of water condensation / capillary forces formed between particulate contaminants and the substrate.

Figure 3 SEM pictures of (a) densely packed nanofibrils structure at 15˚ angle, (b) enlarged view of (a), inset shows free standing parylene nanofibrils structure. [2]

Nanoscopic Adhesive Properties

Figure 4 presents the unique saw-tooth pattern in a force-displacement AFM characterization of a nanofibrillar surface. Adhesion force measurements were conducted at 10 different locations with 10 adhesion data collected at each point and the result is plotted in Figure 4b. The mean adhesion forces of a single nanofibril or artificial “nanohair” range from 0.91 ± 0.34 nN to 1.35 ± 0.37 nN, which is about an order of magnitude lower than that of a single natural nanohair (11.8 ± 2.2 nN).

Figure 4 (a) AFM characterization of the adhesive properties of parylene nanostructures, (b) Mean adhesive force of individual nanofibril or artificial “nanohair” at 10 different locations (indicated by sample A, B, and so on). [2]

Macroscopic Adhesive Properties

In order to assess the adhesion of the synthetic surface on a macroscopic scale, a series of experiments was carried out with a 100 mm² flexible nanostructured parylene film and a smooth microscope glass, used as the substrate. The sample was pressed against the substrate with a preloading force of 1 N to initiate the attachment process. 

Adhesive property of the nanostructured parylene film was characterized by measuring its carrying capacity by attaching a weight to the adhesive pad with a string. All nanofibrils arrays attached simultaneously could theoretically generate 1 N of adhesive force. The whole 100 mm² adhesive pad was found to support an object weighing 70 g. This corresponds to about 70% of the nanofibrils arrays attached to the substrate. The adhesive force of nanofibrils arrays can be improved by changing the types of polymer material and increasing the density of fibrils arrays. By adopting the fabrication technique demonstrated here, it is possible to prepare fibrils array in a large area using a wide range of materials which can be deposited by various template-based deposition methods.

Figure 5 (a) SEM picture of modulated height and inter-fibrils distance due to a deflected supporting parylene membrane, (b) contact angle of water droplets on bare parylene film (70˚), (c) contact angle of water droplets on nanostructured parylene film (155˚). [2]

References:

[1]Wanxin Sun, Pavel Neuzil, Tanu Suryadi Kustandi, Sharon Oh, and Victor D. Samper, "The nature of the gecko lizard adhesive force", Biophysical Journal, 89: L14-L17, 2005.

[2]Tanu Suryadi Kustandi, Victor Samper, Dong Kee Yi, Wan Sing Ng, Pavel Neuzil, and Wanxin Sun,  " Self-assembled nanoparticles based fabrication of gecko foot-hairs inspired polymer nanofibers", Advanced Functional Materials, 2007 (in press).

Acknowledgements

The research group wishes to acknowledge the support of Agency for Science, Technology, and Research (A*STAR), Institute of Bioengineering and Nanotechnology (IBN), Institute of Materials Research and Engineering (IMRE), Micromachines Centre (MMC), and Nanyang Technological University (NTU). We would like to thank Dr. Victor Samper, Dr. Dong Kee Yi, Dr. Sun Wanxin, Dr. Pavel Neuzil for the constructive advice and suggestions throughout this project.

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