Microelectromechanical Systems from Aligned Cellulose Nanocrystal

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Functional Nanostructured Materials (including low-D carbon)

Microelectromechanical Systems (MEMS) from Aligned Cellulose Nanocrystals Films Partha Saha, Naveed Ansari, Christopher L. Kitchens, W. Robert Ashurst, and Virginia A. Davis ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04985 • Publication Date (Web): 25 Jun 2018 Downloaded from http://pubs.acs.org on June 29, 2018

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ACS Applied Materials & Interfaces

Microelectromechanical Systems (MEMS) from Aligned Cellulose Nanocrystals Films

Partha Saha, † Naveed Ansari, † Christopher L. Kitchens, ‡ W. Robert Ashurst†,* Virginia A. Davis †,* †

Department of Chemical Engineering, Auburn University, AL 36849, USA

Correspondence to: [email protected] ‡

Department of Chemical and Biomolecular Engineering, Clemson University, SC 29634, USA

KEYWORDS. cellulose nanocrystals, films, MEMS, microfabrication, anisotropy

ABSTRACT. Microelectromechanical systems (MEMS) have become a ubiquitous part of a multitude of industries including transportation, communication, medical, and consumer products. The majority of commercial MEMS devices are produced from silicon using energy intensive and harsh chemical processing. We report that actuatable standard MEMS devices such as cantilever beam arrays, doubly clamped beams, residual strain testers, and mechanical

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strength testers can be produced via low temperature fabrication of shear aligned cellulose nanocrystal (CNC) films. The devices had feature sizes as small as 6 microns and anisotropic mechanical properties. For 4 µm thick doubly clamped beams with the CNC aligned parallel to the devices’ long axes, the Young’s moduli averaged 51 GPa and the fracture strength averaged 1.1 GPa. These mechanical properties are within one third of typical values for polysilicon devices. This new paradigm of producing MEMS devices from CNC extracted from waste biomass provides the simplicity and tunability of fluid phase processing while enabling anisotropic mechanical properties on the order of those obtained in standard silicon MEMS.

INTRODUCTION Microelectromechanical systems (MEMS) encompass complex structures and devices that are used in micrometer scale sensing and actuation.1 MEMS are an integral components of numerous products in the transportation, electronic, medical, and consumer products industries; they sense or control physical, chemical, and optical quantities through an electrical interface.2 The micromachining technology for MEMS was initially developed for integrated circuit (IC) fabrication; as a result silicon has been the predominant material choice.3 The current market for silicon MEMS is $14 billion and is expected to grow to $20 billion by 2020, with much of the growth occurring in biomedical sensors.4 However, for this rapidly growing market there are ongoing concerns about the use of silicon in MEMS in terms of cost, scope, and potential adverse environmental impacts. Silicon fabrication is energy intensive, requiring processing temperatures on the order of 1000 °C, and etching/releasing is performed with hazardous chemicals such as hydrofluoric acid. Furthermore, silicon is limited in terms of the range of


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microstructural and chemical functionalities that can be achieved; this is particularly considered a concern for BioMEMS (MEMS based sensors with biological interfaces).5,6 BioMEMS have become increasingly prevalent particularly in therapeutics, diagnostics, and tissue engineering;7,8 there is a drive to explore materials that are disposable, have readily tunable surface chemistry for binding biomolecules, and are biocompatible (for in vivo use). Polymer MEMS are emerging as an alternative to silicon for some applications. Polymer MEMS can have the advantages of lower cost, easier fabrication, rapid prototyping, biocompatibility, and greater ductility. However, they are typically limited in terms of mechanical properties and thermal stability.9 Therefore, there is a need for a material that combines silicon’s mechanical properties with polymer processability. In the last decade, there has been considerable interest in broad applications of cellulose nanocrystals (CNCs) extracted from waste cellulosic biomass.10 CNCs’ intrinsic elastic modulus of approximately 150 GPa11 is comparable to that of silicon. Unlike semi-crystalline polymers, CNC do not undergo a low temperature glass transition12; their thermal stabilities range from 190 to 300 °C depending on the surface chemistry.13 CNCs’ natural abundance, renewability, biodegradability, tunable surface chemistry, and ability to be processed in aqueous solutions at near ambient conditions make CNC a greener, lower cost alternative to silicon-based materials. In addition, CNCs’ anisotropy and ability to be processed into highly aligned materials could enable MEMS with controlled anisotropic mechanical and optical properties which are not readily achieved in MEMS made from traditional materials. However, to date, CNC research has primarily been focused on enhancing polymer composite mechanical properties14-16 or exploiting liquid crystalline phase behavior to make optical films.17,18 There have also been efforts to develop CNC based electroactive papers19 and composites (with polyvinyl acetate) for


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implantable microscale structures.6 Nonetheless, to the authors’ knowledge, functional MEMS devices produced via aligning individual CNC in a structural film followed by conventional microfabrication on a test wafer has not previously been reported. In this research, the shear alignment of aqueous cholesteric CNC dispersions enabled the formation of films with anisotropic mechanical and optical properties. These films were used as the device layer for low temperature fabrication of a range of standard free-standing functional microdevices including doubly clamped beams (DCBs), cantilever beam arrays (CBAs), residual stress testers (RSTs), and mechanical strength testers (MSTs).20-23 Testing of these devices revealed that for CNC aligned parallel to the device direction, the average elastic moduli of the 4 µm thick devices was 51 GPa, approximately one third the value of both individual CNC and polysilicon. In addition, the devices’ 1.1 GPa fracture strength was one third that of polysilicon. However, when the CNC were aligned perpendicular to the devices’ long axes the mechanical properties were reduced; the average elastic modulus for 4 µm thick devices was only 25 GPa. Similar anisotropy effects were found for 2 µm thick devices. The results of this work may provide the foundation for the development of inexpensive CNC BioMEMS and other applications requiring the combination of the simplicity and tunability of fluid phase assembly with mechanical strengths approaching those of silicon.

EXPERIMENTAL SECTION A sulfonated cellulose nanocrystal aqueous suspension (11.8 wt. %/7.7 vol. %) containing 1.2 wt. % sulfur with Na+ as the counter ion was produced by the U.S. Forest Products Laboratory (FPL), and supplied by University of Maine Process Development Center (Batch Number-2015FPL-077 CNC). Based on cross-polarized optical microscopy, the onsets of biphasic and liquid


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crystalline phases were approximately 4.8 and 7.5 wt. % (3.0 and 4.8 vol. %), respectively. Based on atomic force microscopy (AFM) measurements, the CNCs’ average height, width, and length were 5.5 nm (ߪ = 1.3 nm), 47 nm (ߪ = 8.5 nm), and 160 nm (ߪ = 47 nm), respectively. The silicon substrate (100) was subjected to dehydration bake for 15 minutes at 150 °C followed by sufficient cooling. Next, the cleaned wafer was exposed to hexamethyl disilizane (HMDS) vapor for 5 min for priming. AZ5214E, an image reversal positive photoresist was spin coated onto the substrate. Using a ramp of 500 rpms-1 to 2500 rpm, the photoresist was spin coated for 30 s followed by a hot plate drying at 110 °C for 1 min; this resulted in a 2 µm photoresist thickness. An anchoring mask (Figure S1a) was then applied and the resist layer was UV exposed (UV400) and developed with a potassium-based developer solution of AZ400K (one part developer to three parts water). The UV exposed photoresist was removed and rinsed with DI water; this was followed by drying under nitrogen. Next, the wafer with the photoresist anchor pattern was treated with air plasma at 500 mTorr and 90 watts for 1 min. This reduced the photoresists’ static water contact angle from 53° to 17° and promoted adhesion of the subsequent CNC device layer to the substrate. The CNC device layer was then created by shearing the CNC dispersion onto the anchor photoresist layer using a Gardco (Pompano Beach, FL) Microm II film applicator. This was followed by drying in an oven at 80 °C for 20 minutes, and 24 hours dessication. Electron beam plasma vapor deposition (EBPVD) was used to deposit a 10 nm layer of Ti/ TiO2 to improve the adhesion of the second photoresist layer to the CNC film. This second (AZP4620) photoresist layer was spin coated onto the Ti/TiO2 and the entire wafer was heated for 10 min in a 70 - 80 °C convection oven for curing. After sufficient cooling, the dried photoresist was UV exposed using the device mask (see Figure S1b)

and the

previously described procedure to create the device pattern. After developing and drying, the


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parts of the Ti/TiO2 coated CNC film covered with the second photoresist layer formed the actual structural profile of the MEMS devices. These windows, or uncovered areas, were then then plasma etched to create the CNC MEMS structures. First, the 10 nm Ti/TiO2 layer was etched out using CF4 (flow rate 20 sccm) inductively coupled plasma (ICP-STS AOE). Along with induction coil power (500 watts), a platen power (300 watts) was applied (at 20 mTorr, 20 s) for anisotropic etching. Next, the Ti/TiO2 free CNC film was plasma etched using O2 (40 sccm). Cross-polarized reflected light microscopy was used to confirm etching completeness. Then diced wafer pieces with devices were submerged into 99.99% pure isopropyl alcohol (IPA). This was followed by the slow addition of acetone to wash off the residual photoresist layers from the devices’ surfaces and the anchoring layer beneath them. The released and submerged devices were carefully transferred (keeping an IPA bubble on the die) to the chamber of a critical point dryer (CPD) filled with IPA (99.99%) and dried using supercritical CO2 at a pressure and temperature of 1400 psi and 32 °C, respectively. This was followed by storage in a dessicator until testing.

RESULTS AND DISCUSSION Microfabrication of CNC MEMS. As has been shown for the fluid-phase processing of other rod-like nanomaterials, processing biphasic or liquid crystalline CNC dispersions generally results in better alignment and mechanical properties in assembled solid materials such as films.24-26 However, the final structure in a film is a complex function of numerous factors including dispersion microstructure, applied shear, wet thickness, capillary effects, and microstructural relaxation time relative to the drying time. The majority of the rod-like nanomaterial dispersions investigated to date form nematic liquid crystals in which the long axes


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of the nanomaterials are aligned along a common director.27 In contrast, CNC obtained by sulfuric acid hydrolysis of cellulosic biomass form a cholesteric (chiral nematic) phase in which the director rotates in a helical fashion with a characteristic pitch (see Figure S2). Retention of this cholesteric microstructure in dried films has been the foundation for significant research into using CNC as color filters, optical devices, and security papers with the characteristic selective reflection of visible spectra.28,29 Cross-polarized optical microscopy is a standard method for understanding the microstructure of materials produced from liquid crystal dispersions. Figure 1a shows a transmitted cross-polarized microscopy image of a cholesteric fingerprint texture in a film obtained from a 6.5 wt. % (4.0 vol. %, biphasic) aqueous CNC dispersion by drop casting without applied shear. Achieving more uniform alignment requires unwinding the pitch to form a nematic microstructure and solidifying this microstructure before it can relax back to the cholesteric state. Figure 1b shows a transmitted cross-polarized image of a film produced from a 8.9 wt. % (5.6 vol. %, fully cholesteric) dispersion sheared at 1000 s-1 and dried under ambient conditions. The cholesteric texture is absent (no fingerprints), but the presence of multiple colors in a film of uniform thickness indicates a polydomain microstructure with only local ordering. Furthermore, the colloidal forces experienced during drying30 resulted in crack formation along the flow direction. Using a lower concentration and shear rate alleviated the cracking, but a periodically banded polydomain structure formed during drying (Figure 1c). This band texture is associated with long range undulation of the director orientation31 due to stress relaxation after flow cessation and contraction strain of the sheared sample.32 This issue was resolved by faster drying at an elevated temperature of 80 °C followed by 24 hours dessication to further ensure the films were completely dry. The more uniform color of the resulting film (Figure 1d) indicates that this method resulted in macroscopically uniform alignment. Scanning electron microscopy


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(SEM) of the film’s cross section (Figure 1e) shows that the alignment was uniaxial near the free surface, but a more chiral structure existed near the substrate. This variation can be attributed to a glass transition at the liquid-vapor interface during dispersion drying. Vitrified surface regimes help the retention of shear induced alignment of CNCs near the film’s free surface, but closer to the substrate the sample remains still in a liquid crystalline state.29 The formation of a glassy skin on the free surface results in decreased water transport from the bulk and more time for the shear aligned rods near the substrate to relax back into a cholesteric microstructure prior to solidification.

Figure 1. Cellulose nanocrystal (CNC) films. Transmitted cross-polarized and scanning electron microscopy (SEM) images of dried CNC films produced from aqueous dispersions. Arrows indicate the shear casting direction. (a) The signature cholesteric texture/fingerprints in a film cast without shear using 6.5 wt. % (4.0 vol. %) CNC. (b) Crack formation along the shear casting


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(flow) direction in a polydomain film made using 8.9 wt. % (5.6 vol. %) CNC and a 1000 s-1 shear rate. (c) Banded structure in a film made using 6.5 wt. % CNC and a 100 s-1 shear rate. (d) Crack free aligned film obtained using the same process of (c) followed by drying at 80 °C for 20 minutes and 24 hours desiccation. (e) SEM micrograph of the cross section of d, the inset shows better uniaxial alignment of CNCs at the drying surface compared to near the substrate. The method resulting in the films shown in Figures 1d and e was used to make 2 and 4 µm thick CNC films (root mean square (RMS) roughness 6.34 nm, see Figure S3a) onto a patterned photoresist layer on a substrate. For convenience, a 100 mm diameter Si (100) wafer was used as the substrate, but a wide range of substrates are possible. The pattern on a spin coated photoresist (AZ5214E) film was created using a photomask that was designed to shape the anchor layer of the MEMS devices using Layout Systems for Individuals (LASI). The device pattern included over 10,000 devices as shown in Figure 2a. The pattern was modified from a protocol previously established for the microfabrication of silicon devices33 based on the results of CNC film nanoindentation and tensile testing. Figures 2b-2g show representative reflective cross-polarized microscopy images of devices after each step in the fabrication process. The color change between the initial CNC film on the patterned photoresist layer (Figure 2b) and Figure 2c is due to the Ebeam-physical vapor deposition (Ebeam PVD) deposition of a 10 nm Ti/TiO2 layer. The Ti/TiO2 layer facilitated adhesion of the next photoresist layer to the CNC film; it should be noted that HMDS which is commonly used as an adhesion promoter for silicon devices was not effective for CNC. The Ti/TiO2 deposition increased the RMS roughness to 28.2 nm (see Figure S2b). This layer also protected the CNC films from disruption by the aqueous developer solution after patterning the second photoresist layer. Figure 2d shows the cured structural profile of the MSTs after spin coating, drying, UV-exposure/mask aligning, and developing the second


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photoresist layer. Figure 2e shows the same devices after the following subsequent steps needed to complete their formation. The portions of CNC film that were not covered by the second photoresist layer were plasma etching the using an inductively coupled plasma (ICP) based advanced oxide etcher. Feature sizes as low as 6 µm (see Figure S4) and high device sidewall fidelity were achieved by using platen power at the substrate in addition to coil power to get directional etching.34 First the Ti/TiO2 layer was etched with CF4 plasma, then the CNC layer was etched with O2 plasma (40 sccm). Short, low pressure plasma cycles were found to prevent micro-crack formation and photoresist rounding. Figure 2f shows the released devices; the residual and anchor photoresist layers were removed by acetone and isopropyl alcohol (IPA) rinses. CO2 critical point drying was applied as the post-release drying step in order to overcome the in-plane surface stiction associated with micrometer features and substrates,35 and to obtain a dry and ultraclean surface.


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Figure 2. Photograph and cross-polarized reflected light optical microscopy images showing the whole substrate and steps involved in photolithographic fabrication scheme for CNC MEMS. (a) Photograph of the 4” wafer with CNC MEMS devices. (b) Shear aligned and dried CNC film on a patterned photoresist layer using a Si substrate. (c) The same CNC film of after E-beam deposition of 10 nm thick Ti/TiO2 layer. (d) Device pattern created by using a second photoresist layer. (e) Etched out device profiles after inductively coupled plasma etching, white and black areas indicate the etched film and the residual photoresist on the device patterns respectively. (f) Released devices after washing off the residual photoresist layers. (g) Higher magnification image of a released MST after critical point drying. Scale bars are 100 µm.

In addition to the released and dried MST shown in Figure 2g, the fabrication resulted in freestanding actuatable cantilever beam arrays (CBAs), doubly clamped beam arrays (DCBs), and residual stress testers (RSTs). These devices are standard tools for measuring the mechanical properties in MEMS devices. Figure 3a shows representative interferograms obtained by phase shifting interferometry (PSI) which were used to determine the height profiles of suspended beams in the CBAs. Figure 3b shows a DCB; this type of device was used for elastic modulus determination. Figures 3c and d show a functioning RST before and after the release. The RST’s design was a suspended I-beam supported by two supporting beams that are equidistant from the midpoint of the central arm at a separation distance of d = 300 µm. Once the RST was released, a torque was generated at the center of the I-beam due to the residual stress induced off-axis forces. Depending on the nature of the residual stress (compressive or tensile) present in the structural film, the rotation can either be clockwise or counterclockwise. Figure 3d shows a 1.65°


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counterclockwise rotation after release; this indicates the device was freestanding and capable of detecting the compressive residual stress in the aligned CNC film.

Figure 3. Representative micrographs of freestanding CNC-MEMS after release. (a) Interferograms on a cantilever beam array (CBA). (b) A doubly clamped beam (DCB) array with 30 µm wide beams with lengths ranging from 100–300 µm. (c) An optical micrograph showing a residual stress tester (RST) with a 300 µm (d) distance between supporting beams connected to Ibeam before release. (d) The same RST after release; the counterclockwise rotation is due to the compressive residual stress in the structural CNC film and indicates the device was functional and freestanding. Scale bars are 100 µm.

Optical and Micromechanical Properties of CNC MEMS. The effects of thickness and shear alignment on the anisotropic compressive stress gradients in suspended cantilever beams were visualized using reflected microscopy (Figure 4). CNC alignment parallel to long axis of


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the beam, resulted in significantly less curvature than when the CNC were oriented perpendicular to the long axis of the beam. This effect was more pronounced for the 2 µm thick beams (Figures 4a and b) than the 4 µm thick beams (Figures 4c and d). Similarly, the DCBs exhibited little curvature when CNC were aligned parallel to the beams’ long axes (Figure 4e). However, long beams with perpendicular CNC alignment were prone to stress induced cracking (Figure 4f). These results demonstrate that fabricating MEMS from uniaxially aligned CNC enables devices with anisotropic mechanical properties including parallel or transverse elastic moduli, or compliance, which affects the creep, or stress relaxation, in microdevices.36 These directional mechanical properties can easily be tailored as a function of device thickness and the direction of applied shear. Such readily tunable anisotropic mechanical properties may not be achieved in traditional silicon and/or polymer MEMS. In addition, controlling the thickness and CNC orientation also enables tunable optical properties. Figure 5 shows that the birefringence induced interference colors of the film/devices under cross-polarized reflected light could be tuned from blue to yellow by changing the film thickness from 2 to 4 µm. The images were taken at a device alignment direction of 45˚ relative to the polarizer direction so that CNC aligned parallel to the long axis of the device would have maximum intensity (images at different rotations are provided in Figure S5). These tunable optical properties may provide new opportunities for all pass optical filters, wavelength selective adaptive optics, and phase modulators in liquid crystal MEMS technology.37-39


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Figure 4. Reflected light microscopy images showing stress gradient effects in 300 µm long CNC cantilevers and doubly clamped beams (DCBs). Arrows indicate the shear alignment direction. (a) The parallel CNC alignment on 2 µm thick beam showing stress induced deflection. (b) Perpendicular CNC alignment on a 2 µm thick beam, the image distortion is due to the downward beam curvature and curling of the end. (c) Parallel CNC alignment on 4 µm thick beam. (d) Perpendicular CNC alignment on a 4 µm thick beam. (e) and (f) Doubly clamped 4 µm thick, 300 µm long beams with parallel CNC alignment resulted in intact beams, but perpendicular CNC alignment resulted in cracking of 300 µm long beams. Scale bars are 100 µm.

Figure 5. Reflected cross-polarized reflected light microscopy images showing tunable optical properties in CNC cantilever arrays resulting from birefringence induced interference. Arrows indicate the shear casting direction. (a) A 2 µm thick device array and (b) A 4 µm thick device array of cantilevers. The uniform interference colors; blue in (a) and yellow in (b) were observed


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at the 45° rotation of alignment director in between two polarizers crossed at 90°. Dark spots are attributed to local orientation of domains in a slightly different direction. Scale bars are 50 µm.

Based on the linear elastic beam bending theory, the elastic modulus of CNC-MEMS was obtained using point load beam bending by nanoindentation40,41 of 100 µm long DCBs. Longer beams were not studied, as they were more prone to buckling or breakage which resulted in nonlinear beam bending (Figure S6). The initial linear regions of the load versus deflection curves (Figure 6a) were used to calculate the slope (µN/µm) and corresponding elastic moduli using the following equations40,41: ‫=ܧ‬ ‫=ܫ‬

௠௅య

(1)

ଵଽଶூ ௪௧ య

(2)

ଵଶ

where E is the elastic moduli, m is the slope (µN/µm), L is the beam length, I is the area moment inertia, w is the beam width, and t is the beam thickness. DCBs 100 µm long, 50 µm wide with 2 and 4 µm thicknesses were tested from eight different wafers to obtain elastic moduli in both the parallel and perpendicular alignment directions. Figure 6b shows the calculated elastic moduli and the anisotropic mechanical properties of the CNC-DCBs. DCBs with parallel CNC alignment had elastic moduli of 65 GPa (σ = 25 GPa, n = 42) for 2 µm thick devices and 51 GPa (σ = 14 GPa, n = 31) for 4 µm thick devices. These values are within one third of typical moduli for polysilicon. For devices with perpendicular alignment, a number of the devices had cracking or breakage which prevented testing. For devices where mechanical properties could be obtained, the moduli were much lower: 17 GPa (σ = 1.2 GPa, n = 11) and 25 (σ = 3.1 GPa, n = 11). These measured elastic moduli are even


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higher than those achieved by Reising et al. for 41 µm thick aligned CNC films; the maximum values they obtained were 32 GPa for parallel alignment and 7.0 GPa for perpendicular alignment based on tensile testing.42 Interestingly, the ratio of the average parallel to perpendicular moduli, or anisotropy ratios are comparable to those reported by both Reising et al. and Passantino et al.42,43 for 40 – 50 µm thick films.

The relatively high standard deviations in the devices’ elastic moduli are attributed to variations resulting from manually producing the wafers and microstructural variations within a given wafer. The higher standard deviations in the parallel versus perpendicular devices are believed to be an artifact of not all perpendicular devices not being testable. It is expected that more consistent mechanical properties could be achieved by improving the consistency of the CNC film and ensuring all films had equivalent moisture content and were tested at the same humidity. Improving the uniformity of aligned CNC films is an area of ongoing research; over areas as large as a 4” wafer microstructural variations occur to a combination of imperfect packing due to the CNCs’ polydispersity and radially dependent drying induced microstructural changes. Ensuring devices are maintained and tested at a consistent humidity would reduce variations in mechanical properties due to moisture content. Wu et al.44 and Benítez et al.45 reported increasing humidity and moisture uptake resulted in physical property changes in CNC films such as decreased elastic modulus, hardness, and tensile strength in CNC films. These property changes were attributed to disruption of CNC films’ hydrogen bonding network. It is envisioned that devices from pristine CNC films could be packaged in foil pouches for single use, similar to qualitative paper based sensors. Also, the stability of CNC films and MEMS devices can be significantly


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improved through simple covalent functionalization schemes and this is the subject of ongoing work.23

Figure 6. Micromechanical properties of CNC-MEMS devices. (a) Load versus deflection curve of a 100 µm long, 30 µm wide, and 2 µm thick DCB showing deflection distance until it hits the substrate; the slope of the selected linear region used in the calculation of elastic modulus calculation based on linear-elastic beam theory. (b) Elastic moduli of 2 µm and 4 µm thick CNC DCBs showing different elastic moduli when the CNC were aligned parallel and perpendicular to the beams’ long axes. Error bars are the standard deviation of at least 11 tests for perpendicular alignment, 31 tests for 4 µm thick devices with parallel alignment, and 42 devices for 2 µm thick devices with parallel alignment (c) 4 µm thick MST shuttle x and y position changes


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during mechanical actuation upon applied voltage on an open loop piezo assisted micromanipulator. The suspended shuttle was pushed in the x direction (see Figure S10) until the 60 µm long fracture beams on both sides fractured giving the breaking length for fracture strength calculation. (d) Deflection profiles of a gold sputter coated CNC cantilever (500 µm long, 30 µm wide, and 4 µm thick) due to electrostatic actuation by applying DC voltage using 2D phase shifting interferometry.

PSI was used to determine the freestanding and actuated CBA height profiles (see Figure S7) and confirm the compressive stress gradients (downward arc) indicated by the optical images in Figure 4. The microscopically measured angles of rotation for released RST devices (Figure S8) were input into the following equation to obtain the residual stress33: ߪ௙ =

ாఏௗ

(3)

ଶ

where E is the elastic modulus, θ is the angle of rotation in radian, and d is the distance between two suspended beams holding the RST I-beam. The compressive residual stresses in the 2 µm and 4 µm thick RSTs were 280 kPa (σ = 4.8 kPa) and 41 kPa (σ = 4.8 kPa), respectively. These values are lower than the 295 to 420 MPa46 obtained for standard low pressure chemical vapor deposited (LPCVD) polysilicon films. It should be noted that sputter coating the cantilevers with gold plasma (for SEM imaging) changed the nature of the height profile to an upward arc indicating the transition to a tensile stress gradient (Figure S9); this was attributed to stress relaxation.47 Figure 6c shows the MST actuation graph for 4 µm thick MST. The shuttle was pushed against the stopping posts using an incremental DC voltage to a piezo-assisted micromanipulator; the fracture distance was 4.8 µm for 60 µm long fracture beams on the suspended shuttle (see Figure


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S9). The fracture strength of the actuated MSTs’ was calculated based on the distance the shuttle travelled in the x-direction before suspended beam fractured using the following equation.48 ߪ௙ =

ଷா௪ఋ೑

(4)

ଶ௅೎ మ

where E is the elastic modulus, w is the width of the fracture beam, ߜ௙ is the fracture distance (Figure 6c), and ‫ܮ‬௖ is the distance between fracture post and suspended shuttle (see Figure S9). The calculated fracture strength of 4 µm thick CNC MSTs with parallel alignment was 1.1 ± 0.2 GPa which is more than one third of the typical 2.6 ± 0.4 GPa for polysilicon.49 The MSTs made from 2 µm thick films were more flexible. Even at the maximum shuttle displacement possible for this MST design the suspended beams did not fracture. To further demonstrate the device functionality, CBAs were sputter coated with gold, and the actuation pads were subjected to a DC voltage source while the silicon substrate was electrically grounded. As shown in Figure 6d, the setup works as a parallel plate capacitor when the DC voltage is applied. Due to the buildup of capacitive charge between the cantilever surface and the silicon substrate, the freestanding ends of the cantilevers were deflected towards the substrate as the voltage was increased (Figures 6d, S11). The charge induced electrostatic force was directly proportional to the square of the applied voltage. The buildup of this electrostatic force pulled the free end of the CBA downward while the elastic force restored the initial position. At least 10 V DC was required to electrostatically actuate the freestanding cantilever (Figure 6d); this is attributed to resistive losses in the CNC film. The curvature of the beam even before actuation was due to stress gradient throughout the structural film. This result shows that the nearly dielectric CNC microstructures can be repeatedly electrostatically actuated after depositing a thin conductive layer of sputtered metal. In addition, recent studies on piezoelectric sensing50 and electromechanical response51 of nanocellulose may also provide a new avenue for


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research on CNC MEMS as biosensors. CNCs’ organic chemistry can enable biomarker immobilization using standard protocols providing an inexpensive disposable material platform. Therefore, the piezoelectric properties of CNC films50,51 and the ability to co-assemble conductive or piezoelectric nanomaterials with CNC are promising for CNC MEMS actuation and sensing.

CONCLUSIONS A completely new material platform for MEMS fabrication based on CNC extracted from woody biomass has been demonstrated. The flow alignment of aqueous CNC to produce the device layer enables anisotropic mechanical and optical properties. Standard MEMS devices with feature sizes as low as 6 µm were fabricated via a low temperature fabrication process. The resulting devices were found to have remarkable mechanical properties; for CNC aligned parallel to the long axes of the devices, both the moduli and fracture strengths were roughly one third those of silicon. Moreover, the fracture strength, stress gradient, and optical properties of the CNC film can be modulated using the CNC film processing conditions; this tunability is unlike any conventionally used MEMS materials. Therefore, CNC MEMS provide new opportunities to tailor device mechanical and optical properties to suit chosen applications. These findings may lead to a new MEMS fabrication paradigm. CNCs’ organic chemistry can enable biomarker immobilization using standard protocols. In addition, the piezoelectric properties of CNC films,50,51 as well the ability to co-assemble conductive or piezoelectric nanomaterials into the films, is promising for CNC MEMS actuation and sensing. Therefore, CNC MEMS are expected to be particularly relevant for the development of low cost and disposable point of care BioMEMS devices for real time multi-analyte sensing for disease biomarkers.8,52


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.

ASSOCIATED CONTENT Supporting Information Figure S1-S11 contain the following: masks used for anchor and device layer patterning during CNC MEMS microfabrication, cholesteric microstructure of a CNC dispersion, AFM height scan images on CNC and Ti/TiO2 coated CNC films, smallest feature size of MST fracture beams, cross-polarized reflected light images showing birefringence intesnity at different angles of alignment directions, load versus deflection profiles for DCBs, PSI height profiles of freestanding and mechanically actuated cantilevers, freestanding RSTs showing counterclockwise rotation after release, SEM image of CBA after relaxation showing upward arc profile, actuation of MSTs’ suspended shuttle, PSI 2D height profiles of electrostatically actuated cantilever, and m.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] Author Contributions V.A.D, W.R.A, and C.L.K originated the idea and conceived the research. N.A. conducted initial experiments on etching CNC films and wrote the MEMScripts used for actuation and phase shifting interferometry studies. P.S. developed the method that resulted in actuatable devices,


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then designed and fabricated the reported CNC MEMS followed by testing of device properties. All authors contributed to the writing and editing of the manuscript. Notes The authors declare no competing financial interests.

ACKNOWLEDGMENTS The authors acknowledge NSF CMMI-1131633 and CMMI-1130825 for funding. Alexander Haywood is acknowledged for preliminary experiments that provided a foundation for this manuscript; these experiments are described in Reference 22. Michael Hamilton, George Hernandez, and Charles Ellis are acknowledged for assistance with fabrication and Bart Prorok and Marianne Sullivan are acknowledged for assistance with nanoindentation protocols and measurements.

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Table of Contents (TOC) Image.


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Microelectromechanical Systems from Aligned Cellulose Nanocrystal

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