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1 School Inquiries Based on Soft Elastomer Lithography Anssi Lindell 1, Anna-Leena Latvala 1 and Jouni Viiri 1 1 Department of Teacher Education, University of Jyväskylä, Finland anssi.lindell@jyu.fi We have used polydimethylsiloxane (PDMS) and also tested other polymers as a material for a set of school science inquiries based on soft lithography. In soft lithography, polymers are used instead of conventional rigid photo masks for lithography. These may be used as stamps, molds or masks to generate mesoscopic patterns and structures for small devices. The advantages of soft lithography over traditional photolithography make it ideal in the boundary conditions of school science. Soft lithography does not require expensive clean room processes or instruments; a standard school laboratory is enough. The materials to be processed can be chosen for safety and cost. Soft lithography allows for three-dimensional structures on three-dimensional substrates with a large variety of materials with interesting chemical, biological and physical properties to study at school. We took advantage of the mouldability, elasticity and transparency of PDMS in designing three school inquiries. First, we fabricated elastic replicas of a commercial optical grating with 1000 slits/mm. If the elastic replica is stretched parallel or perpendicular to these grooves, the spacing between them i.e. the grating constant decreases or increases respectively. The elastic gratings may be used as dynamometers, pressure gauges or strain gauges. The second inquiry we demonstrate is an elastic lens. It is a liquid-filled circular PDMS pocket whose curve radius and focal length can be adjusted by changing the volume of the liquid inside the pocket. Personally adjustable eyeglasses are an application for this. The third inquiry uses PDMS as a stamp to fabricate physically, chemically and biologically controlled active patterns on a substrate. Electrical components, etching masks and cell cultivation pads are examples of these. The learning objectives were defined for the three school inquiries and experimental setups were constructed and tested for the elastic optical grating and the liquid filled lens experiments. Introduction In the long run it will be profitable to teach nanoscience in schools. First, the world market for nanotechnological products will grow to a trillion US Dollars by 2015, and an increase of 2 million manufacturing jobs related to nanotechnology is expected [1]. Second, nanoscience offers fascinating opportunities to increase the piquancy of science lessons as it challenges traditional models of science with achievements of modern technology. However, there are barriers in adopting new contents to science education. Bamberger and Krajcik [2] reported that science teachers in USA designated the lack of knowledge, time and teaching materials as reasons which suppress the integration of nanoscience into their classes. Other barriers of integrating technology into curriculum are the price of instruments, devices and facilities needed for typical nanoscience experiments. For lowering these external barriers [3], we designed three low budget nanotechnology-oriented inquiries for physics courses at high schools and universities. The focus of the two first experiments is in applied optics and mechanics. The third introduces interdisciplinary techniques to deposit structures on surfaces using elastic stamps. The experiments take advantages of Polydimethylsiloxane (PDMS) soft lithography. Soft lithography is a technique based on replica molding and self assembly of organic molecules [4].

2 PDMS is a two-component elastic and transparent mouldable polymer that is widely used for biomedical applications, such as implants, as it is biocompatible, flexible, and can adapt itself into chemically and geometrically challenging organic environments like the human body [5]. The micro- and nanoapplications range from micro capillary channels, pumps and valves to electrophoresis, micro reactors and integrated lab-on-chips [6]. It is also an excellent material for school inquiries and demonstrations, as it is cheap and safe to use. The elasticity and transparency suggests using it for experiments combining optics and mechanics. The mouldability makes it possible to tailor individual geometries for devices or, on the other hand, make several identical replicas of one device. An elastic optical grating The first inquiry is based on diffractive optics. We used PDMS to fabricate elastic replicas of a rigid commercial optical grating with 1000 slits/mm, as described in detail in [7]. Figure 1. shows the experimental set-up used in our classes. The grating was hung from a clothespin and illuminated by a red He-Ne laser (wavelength 632 nm). A support with extra weights was hanged by another clothespin into the opposite end of the grating to stretch it by definite forces. The shift of the diffraction maxima can then be measured to determine grating constant, strain and Young s modulus of the material. Figure 1 An experimental set up for an elastic optical grating inquiry. The weight of a clothespin (10 g) and a support for extra weights (50 g) have stretched the elastic grating and thus caused the shift of the first diffraction side maximum towards the central maximum. A typical result of this experiment is plotted in Figure 2. The elasticity of the grating shows two linear regions. The Young s modulus obtained from the first linear region, about 200 kpa is

3 probably due to recoiling of molecules (not studied in this particular experiment), and that from the second part is 2 MPa. Figure 2. Plotted experimental data for stretching originally 0.5 mm thick, 11.5 mm wide and 15 mm long elastic PDMS grating. F is the stretching force, A is the cross sectional area of the grating and α 0 and α are the angles of diffraction for unstretched and stretched grating respectively [7]. The linear regions give Young s modulus of 200 kpa for low stress region and 2 MPa for high stress region. The two linear regions in the curve makes this inquiry ideal for teaching students to test and not just apply models, as students typically do in their experimental work. This was also the case with force versus strain characteristics, where students took Hooke s law granted and measured only one or two data points to determine Young s modulus for PDMS material. This is a typical process for solving text book end of chapter problems. However, it does not show any ability of testing models and doing science, which are essential parts of learning science [8, 9]. Using this inquiry with an elastic optical grating we are able to explicit instruct our students to put models to the proof and test their limits while doing experiments. Another main learning objective for this inquiry is applying diffractive optics and mechanics in an innovative way. In addition, students have an opportunity to test the different predictions of the geometrical and wave models of light. According to the geometrical model, the pattern of the slits of the optical grating should spread on the screen as the grating is stretched. However, the diffraction pattern on the screen gets narrower as the grating constant increases due to the stretching. A liquid-filled elastic lens The second inquiry deals with geometrical optics. We molded thin PDMS membranes and mounted them on both sides of an O-ring to make a lens with adjustable central reservoir. The refractive power of the lens can be tuned by pumping liquid into or out of this reservoir by a syringe (see Figure 3). When the volume of the liquid inside the lens reservoir is changed from the flat lens volume by ΔV, the PDMS membranes bulge either outwards (ΔV > 0), making a biconvex lens, or inwards (ΔV < 0), making a biconcave lens.

4 Figure 3. An elastic liquid filled lens made of two PDMS membranes and an O-ring washer. The syringe is used to pump liquid inside and out of the device to adjust the lens from biconvex to biconcave respectively. The tube on the top of the lens is for removing air. It is capped after first time filling. Assuming a spherical profile for this bulge, the lens curvature R is related to the increase of the volume by the equation V 2R r0 2R R r0 2R R r0, 3 where r 0 is the radius of the lens aperture. The focal length f of the lens can then be related to the change of the volume by lensmaker's equation, assuming a thin lens or using thick lens model: ( n 1) d n 1, f R1 R2 nr1 R2 where n is refractive index of the liquid used and d is the thickness of the lens. For the case of water (n = 1.33) and a double convex lens (where R 1 = -R 2 =R), we measured the focal length of the lens as a function of volume of water added into the reservoir of an originally flat lens. The result was also compared with the thin and thick lens models. A graphical plot of the results can be seen in Figure 4. The focusing behaviour of the flat, biconvex and biconcave lens is presented in Figure 5 a, b and c respectively. Elastic lenses have been originally designed to for self adjustable eyeglasses for developing countries [10]. The main reason for developing this inquiry was to demonstrate the awesome social impact of methods of nanotechnology. The learning goals of this inquiry are geometrical optics, fluid mechanics and scientific models. A student examining liquid filled lenses need to consider several assumptions and simplifications to be able to calculate the radius of curvature and focal length of the lens as a function of the change in lens volume, for example. We may also fabricate one or the both sides of the lens of diffractive sheet of PDMS and thereby combine the concepts of geometrical optics and wave model for light.

5 Figure 4. A measured focal lengths for a liquid filled elastic lens as a function of water added to originally flat lens. The thin and thick lens models for the device are represented by a dashed and a solid line respectively. Figure 5. Focusing a chessboard pattern with a liquid filled lens. The lens is flat (ΔV = 0) in (a), biconvex (ΔV > 0) in (b) and biconcave with a flat central region (ΔV << 0) in (c). The aperture of the lens is 50 mm and the thickness of a flat lens in (a) is 5 mm. PDMS stamp The development of the third inquiry is still in progress. The basic idea is an interdisciplinary experiment using elastomer stamps for sample fabrication. Students draw the structures they want to fabricate on an aluminum foil using a pencil or a wooden stick, for example. The resulting aluminum relief can then be used as a master for casting liquid prepolymer PDMS. The cured polymer can be used as a stamp to deposit chemically protective, so-called ''resist'', film patterns on metal film. These patterns may be lines or double lines to fabricate components such as resistors or capacitors respectively, after etching the unprotected metal away.

6 Another method that we tested is to use an edge of an eraser to make glue lines on a sheet of paper. After sprinkling aluminum powder on these lines, we obtained conductive lines with a sheet resistance of about 10 kω/square (see Figure 6). Then we sintered the lines applying a voltage of approximately 5 V/square across them reducing the resistance almost to 0 Ω as described in [11]. The two techniques described above may also be combined to demonstrate simple static memory circuits, for example. Yet another possibility is to control cell shape, growth and function by patterning proteins using PDMS stamps as described in details in [12]. The PDMS stamp may be used to deposit methylgroup terminated alkanethiol self-assembled monolayers on protein resistant coated gold film, for example. Then, proteins adsorb only on this stamped pattern and, further, cells attach only on the protein coated areas. Figure 6. Glue lines stamped on a paper by an office eraser have been sprinkled with silver powder. Applying a voltage of approximately 10 V sintered the powder causing the resistance decrease from original 30 kω to close to 0 Ω. One square in the paper is 1 cm 2. The learning objectives for inquiries by PDMS stamps range from understanding specific topics of physics (the difference between DC and AC impedance, for example), chemistry (selective etching and activation energy) and biology (the growth of biofilms and cells) to a wider understanding of interdisciplinary nature, methods and applications of nanoscience. Conclusions Nanoscience is an important but not too easy topic to include in the school science classes. Science teachers must overcome many internal and external barriers. By this article we have tried to lower external barriers (i.e. expensive equipment, insufficient time to plan instruction, and inadequate technical support) of teaching nanoscience. Safe and inexpensive methods of soft lithography can be exploited to design school inquiries in nanotechnology. We have demonstrated here three inquiries based on PDMS polymer, but the easy and versatile technique allows also numerous other materials to be processed. This is important for schools where low cost and safety are determinative criteria for material selection. The inquiries of liquid filled elastic lens and PDMS stamp techniques are under further development. We are also designing Predict-Observe-Explain [13] tests to be used in

7 investigations of students understanding and ability to apply their skills in novel and often surprising situations combining several aspects of science. References [1] C. Palmberg, H. Dernis and C. Miguet, Nanotechnology: An overview based on indicators and statistics. OECD, STI Working Paper 7 (2009). [2] Y. Bamberger and J. Krajcik, The role of teachers' barriers in integrating new ideas into the curriculum: The case of nanoscale science and technology. Paper Presented in the Annual Conference of the National Association of Research in Science Teaching, Philadelphia, PA (2010). [3] P. A. Ertmer, Addressing first- and second-order barriers to change: Strategies for technology integration. Educational Technology Research and Development, 47(4), (1999). [4] X-M. Zhao, Y. Xia and G. M. Whitesides, Soft lithographic methods for nano-fabrication. J. Mater. Chem. 7(7) (1997). [5] D. B. Weibel, W. R. DiLuzio and G. M. Whitesides, Microfabrication meets microbiology. Nature Rev. Microbiol (2007). [6] T. Fujii, PDMS-based microfluidic devices for biomedical applications. Microelectron. Eng. 61, (2002). [7] G. Planinsic, A. Lindell and M. Remskar, Themes of nanoscience for introductory physics course. Eur. J. of Phys. 30, S17-S31(2009). [8] D. Hodson, Re-thinking old ways: towards a more critical approach to practical work in school science. Studies in Science Education, 83, (1993). [9] R. Justi and J. K. Gilbert. Teachers views on the nature of models. International Journal of Science Education, 25, (2003). [10] K. Harmon, Designer Focuses on Marketing Adjustable Eyeglasses at $1 a Pair, Scientific American, 24 (2009). [11] A. Alastalo, T. Mattila, M. L. Allen, M. J. Aronniem, J. H. Leppäniemi, K. A. Ojanperä, M. P. Suhonen and H. Seppä, Rapid electrical sintering of nanoparticle structures. Mater. Res. Soc. Symp. Proc. 1113, 2-7 (2009). [12] R. S. Kane, S.Takayama, E. Ostuni, D. E. Ingber and G. M. Whitesides, Patterning cells and proteins using soft lithography. Biomaterials 20, (1999). [13] R. White and R Gunstone, Probing Understanding. (RoutledgeFalmer, London, 1992).