Smart Fluids: Survey of Principles for Current, Imminent, and Future Technological Applications

Honors Thesis Spring 2010

By: Katrin Passlack

Faculty Mentor: Dr. David Miller

University of Oklahoma Department of Aerospace and Mechanical Engineering Joe C. and Carole Kerr McClendon Honors College

TABLE OF CONTENTS

DEDICATION ...... ii LIST OF TABLES...... iii LIST OF FIGURES ...... v NOMENCLATURE...... vi

PART 1: THE SCIENCE OF SMART FLUIDS CHAPTER 1: THE CONCEPT OF A SMART FLUID ...... 1 CHAPTER 2: MR FLUIDS ...... 3 CHAPTER 3: ER FLUIDS...... 9 CHAPTER 4: ST FLUIDS ...... 12

PART 2: CURRENT, IMMINENT, AND FUTURE APPLCATIONS CHAPTER 5: CURRENT APPLICATIONS ...... 17 CHAPTER 6: IMMINENT APPLICATIONS ...... 21 Section 1: Rowing Ergometers ...... 21 Section 2: Alpine Skis ...... 22 Section 3: Active Knee Rehabilitation Devices...... 23 Section 4: Footwear ...... 24 Section 5: Prosthetics and Exoskeletons...... 27 Section 6: Programmable Joint Braces ...... 30 Section 7: Machining Processes...... 31 Section 8 Automobile Headrests ...... 33 CHAPTER 7: TECHNOLOGICAL DEVELOPMENTS NECESSARY FOR INTEGRATION ...... 35 CHAPTER 8: FUTURE APPLICATIONS ...... 36

REFERENCES...... 37

i DEDICATION

For Coach Andy, for fighting for me every step of the way this year.

For Jessica, my little sis, because watching you step up and deal with everything when everything was just too much, inspired me to do the same.

ii LIST OF FIGURES

FIGURE 1: ER PARTICLE NEWTONIAN AND BINGHAM PLASTIC STATES [1]...... 1 FIGURE 2: SHEAR RATE VS. YIELD STRESS FOR NEWTONIAN FLUIDS AND BINGHAM PLASTICS [1] ...... 1 FIGURE 3: MR SANDWICH BEAM EXPERIMENTAL SET UP [3]...... 4 FIGURE 4: MR SANDWICH BEAM COMPOSITION [3]...... 4 FIGURE 5: FREE VIBRATION RESPONSE OF THE PET MR SANDWICH BEAM [3]...... 5 FIGURE 6: FREE VIBRATION RESPONSE OF THE AL MR SANDWICH BEAM [3]...... 5 FIGURE 7: FIRST NATURAL FREQUENCIES FOR A PET MR BEAM [3] ...... 5 FIGURE 8: FIRST NATURAL FREQUENCIES FOR AN AL MR BEAM [3]...... 5 FIGURE 9: MR BEAM VIBRATION TESTING SET‐UP [7] ...... 6 FIGURE 10: MR BEAM DEFORMED AND UNDEFORMED CROSS SECTIONS [7]...... 6 FIGURE 11: MR SIMPLY SUPPORTED BEAM RESPONSE [7]...... 7 FIGURE 12: MR CLAMPED‐FREE BEAM RESPONSE [7]...... 7 FIGURE 13: MR THICKNESS RATIO EFFECT ON NATURAL FREQUENCIES [7]...... 8 FIGURE 14: MR THICKNESS RATIO EFFECT ON LOSS FACTOR FOR SIMPLY SUPPORTED BEAM [7]...... 8 FIGURE 15: ER COLUMN FORMATION FOR SILICON IN SILICON OIL [8]...... 9 FIGURE 16: ER COLUMN FORMATION FOR CORNSTARCH IN SILICON OIL [8]...... 9 FIGURE 17: TOP VIEW ER COLUMN PHENOMENON 1%WT SILICON [8]...... 10 FIGURE 18:TOP VIEW ER COLUMN PHENOMENON 0.2%WT SILICON [8]...... 10 FIGURE 19: THREE LAYER PVC BEAM WITH INTEGRATED ST FLUID [14]...... 12 FIGURE 20: VIBRATING BEAM TESTING SCHEMATIC [14]...... 12 FIGURE 21: STF VISCOSITY DEPENDENCE ON STRAIN [14]...... 13 FIGURE 22: RELATIONSHIP OF INITIAL TIP DISPLACEMENT TO RELATIVE THICKENING FOR STF [14]...... 13 FIGURE 23: 10% W/W FUMED SILICA FREQUENCY VS. COMPLEX VISCOSITY [13] ...... 14 FIGURE 24: 12.5% W/W FUMED SILICA FREQUENCY VS. COMPLEX VISCOSITY [13] ...... 14 FIGURE 25: 15% W/W FUMED SILICA FREQUENCY VS. COMPLEX VISCOSITY [13] ...... 14 FIGURE 26: 17.5% W/W FUMED SILICA FREQUENCY VS. COMPLEX VISCOSITY [13] ...... 14 FIGURE 27: FIBER (A) AND CYLINDRICAL (B) ST COMPOSITE STRUCTURES WITH APPLIED SHEAR [12]...... 15 FIGURE 28: ST FIBER COMPOSITE SCHEMATIC [12] ...... 15 FIGURE 29: ST FIBER COMPOSITE TESTING SET UP [12]...... 15 FIGURE 30: ST FIBER COMPOSITE FREQUENCY SWEEPS [12] ...... 16 FIGURE 31: CRITICAL STRAIN RATE FOR THICKENING OF ST COMPOSITE [12]...... 16 FIGURE 32: SEISMIC MR DAMPING SYSTEM IN TOKYO, JAPAN [4]...... 18 FIGURE 33: MR VIRTUAL ENDOVASCULAR TELESURGURY [4]...... 18 FIGURE 34: ER CLUTCH CONTROLLED ROBOT ARM [1] ...... 19 FIGURE 35: BIEDERMANN MOTECH MR PROSTHETIC KNEE [16] ...... 19 FIGURE 36: THE UNSTABLE ROWING SIMULATOR [19]...... 21 FIGURE 37: THE STF SKI DESIGN [15]...... 22 FIGURE 38: STF SKI TESTING RESULTS [15]...... 22 FIGURE 39: ACTIVE KNEE REHABILITATION DEVICE WITH ER BREAK [20]...... 23 FIGURE 40: VERTICAL GRFS IN DIFFERENT RUNNING STYLES [22]...... 24 FIGURE 41: GRFS IN RUNNING ON DIFFERENT SURFACES [22]...... 24 FIGURE 42: THE ENERGY SAVING WEDGE CONCEPT [24] ...... 25 FIGURE 43: ADIDAS SHOE PATENT DESIGN SKETCH [26]...... 26 FIGURE 44: EVO SHOES [28]...... 26 FIGURE 45:PHOTO OF MR PROSTHETIC KNEE [29]...... 27 FIGURE 46: MR PROSTHETIC KNEE SCHEMATIC [29]...... 27 FIGURE 47: THE FIVE PHASES OF HUMAN GAIT [29] ...... 28

iii FIGURE 48: MR PROSTHETIC ACTIVATION AND CONTROL SCHEMA [29] ...... 28 FIGURE 49: EXOSKELETON OVERVIEW WITH MR KNEE BREAK [30] ...... 29 FIGURE 50: MR KNEE BREAK FOR EXOSKELETON [30]...... 29 FIGURE 51: MR BREAK EXOSKELETON ENERGY CONSUMPTION [30]...... 29 FIGURE 52: PNEUMATIC ENERGY STORING ORTHOSIS [31]...... 29 FIGURE 53: CNC MACHINE WITH PIEZOELECTRIC ACTUATOR [6] ...... 32 FIGURE 54: SMART BORING TOOL WITH A PIEZOELECTRIC ACTUATOR [6] ...... 32 FIGURE 55: TUNABLE BORING BAR WITH ER FLUID [6] ...... 32 FIGURE 56: POLYURETHANE RETICULATED FOAM W/ AND W/O MR FLUID [34]...... 33 FIGURE 57: RETICULATED FOAM WITH MR FLUID SCHEMATIC [34] ...... 33 FIGURE 58: MR FOAM ENERGY ABSORPTION FOR VARYING MAGNETIC FIELD STRENGTHS [34] ...... 34 FIGURE 59: MR FOAM ENERGY ABSORPTION FOR DIFFERENT MR FLUID VOLUME FRACTIONS [34]...... 34 FIGURE 60: RETICULATED MR FOAM HEADREST SCHEMATIC [34]...... 34 FIGURE 61: MR FOAM AND DRY FOAM HEADREST SIMULATION HEIGHT RESULTS [34]...... 34

iv LIST OF TABLES

TABLE 1: COMPARISON OF PROPERTIES OF ER AND MR SMART FLUIDS [1, 2]...... 2 TABLE 2: NATURAL FREQUENCY RESPONSE OF MR BEAM [6]...... 7

v NOMENCLATURE

%wt = percent by weight dB = decibels mm = millimeter w/w = weight/weight

AFO = ankle foot orhtosis Al = aluminum AKROD=active knee rehabilitation orthotic device CCB = clamped clamped beam CFB = clamped free beam COT = metabolic cost of transport EPFL = École Polytechnique Fédérale de Lausanne (Swiss Federal Institutes of Technology) ER = electro-rheological ESO = energy storing orthosis F2MC =Fluidic Flexible Matrix Composite FST = fast tool servo G = Gauss GRF = ground reaction force IOC = international Olympic committee MR = magneto-rheological ODOE = On Demand Operational Exoskeleton OTD = order to disorder PET= polyethylene terephthalate PPG = polypropylene glycol PZT = piezoelectric ceramics SSB = simply supprted beam ST = Shear thickening T = Tesla V = Volt VBT = Vibrating beam testing

vi PART 1: THE SCIENCE OF SMART FLUIDS

CHAPTER 1: THE CONCEPT OF A SMART FLUID

The term smart fluid is broadly defined as a fluid that acts Newtonian until a specific stimulus is applied. When a stimulus of the proper type and sufficient strength is applied, micrometer-sized particles suspended in a dielectric carrier fluid will align such that the resistance to flow of the smart fluid, the viscosity, significantly increases [1, 2] and thus the fluid becomes quasi solid [3]. The two predominant types of smart fluids are electro-rheological (ER) and magneto-rheological (MR) fluids, which are primarily complimentary and not competitive technologies, each with its own advantages [4]. Both fluids consist of a dielectric carrier liquid, but differ in the type of suspended particle and the applied filed necessary to trigger thickening. ER and MR fluids were independently discovered in the 1940s. Some sporadic work on ER fluids persisted throughout the following decades, but it was not until MR fluids were rediscovered in the 1990s that commercial applications of smart fluids entered the marketplace [1]. The first commercialization of MR fluids occurred in 1995 as rotary brakes for aerobic exercise equipment [2, 4]. Some everyday items already incorporate smart materials such as coffeepots, cars, and glasses, as well as brakes, dampers, clutches, and shock absorbing systems [2]. To date, the most marketed application of MR fluids are the damping struts in the 2002 Cadillac Seville STS [1].

The alignment phenomenon is presented for an ER fluid in Figure 1, where state (a) shows the Newtonian behavior of the fluid and state (b) shows the fluid in the presence of an applied field when it behaves similarly to a Bingham plastic. A Bingham plastic is a non-Newtonian fluid whose yield stress must be exceeded before flow can occur [2]. As the electric stimulus is applied, the particles acquire a dipole moment and form chains aligned along the applied field, which causes the suspension to become less viscous and solidify [2]. The thickening phenomenon for MR fluids is similar is appearance and effect as that experienced by the ER fluid in Figure 1.

The difference in shear rate versus shear stress for states (a) and (b) are shown in Figure 2.

Figure 1: ER particle Newtonian and Bingham plastic Figure 2: Shear rate vs. yield stress for Newtonian fluids states [1] and Bingham plastics [1]

1 There exist several classes of intrinsically adaptive materials other than ER and MR fluids. These include: 1) Shape Memory Polymers/Alloys: These materials undergo microscopic structural molecular changes in response to external stimuli (mainly heat) [2, 5] 2) Piezoelectric Ceramics (PZT): The stiffness of PZTs can be modulated via electromechanical coupling [5]. 3) Fluidic Flexible Matrix Composite (F2MC): This material consists of high performance fibers containing a high bulk modulus fluid. Opening or closing the inlet valves can modulate the stiffness of an F2MC tube. 4) Shear Thickening Fluids (ST): These materials approximate the response of MR fluids, but require only a shear stress stimulus and thus require no power to actuate.

Although the thickening effect is comparable between MR and ER fluids, there are fundamental differences in both the composition and behavior of these two fluids. The main differences between MR and ER Fluids are presented in Table 1.

Table 1: Comparison of properties of ER and MR Smart Fluids [1, 2, 6]

Property ER MR Type of Particle Semi-conducting Magnetically soft Carrier liquid kinematic viscosity 10-50 cSt 10-50 cSt Density - 3-4 g/cm3 Particle Diameter 5-50 µm 0.1-10 µm Volume fraction of particles Up to 50% Up to 50%

4 kW/ 1 T Minimum stimulus strength mm up to 10 kV 2-25 V

Maximum yield strength 3-5 kPa 50-100 kPa Reaction time Virtually instantaneous A few milliseconds Operating Temperature Range 15 to 90 oC -40 to 150 oC

2 CHAPTER 2: MR FLUIDS

One of the key components of an MR fluid is choosing a particle with a large saturation magnetism of relatively pure, soft iron particles (e.g. carbonyl iron) suspended in mineral oil, synthetic oil, water, and/or glycol [1]. It has been suggested that the most suitable particles are alloys of iron and cobalt (saturation magnetizations of 2.4 T), which cost less and create a stronger MR fluid than pure iron particles [1]. Although MR fluid particle size can range from 0.1-10 µm, most MR fluids utilize particles 3-5 microns in size [2]. Functional smart fluid solutions can be obtained using larger or smaller particles, but particles larger in size are more difficult to suspend in solution, and particles smaller in size, although easier to suspend in solution, are much more difficult to manufacture [2].

The total yield strength developed in an MR fluid is a linear function of both the magnetic field strength and the shear rate, as presented in Equation 1 [2]:

(1)

The wall roughness also factors into the magnitude of the yield strength developed by the fluid, especially at low applied magnetic field strengths [2]. Response times of 6.5 ms have been recorded for MR fluids, and thus the thickening effect is almost immediately reversible [2, 3].

MR fluids are relatively insensitive to temperature variations and contamination, and can produce a yield stress up to 100 kPa powered via a small voltage source [1]. Other additives, stabilizers and surfactants, may also be included in the MR composition which may inhibit gravitational settling, enhance lubricity, modify the initial carrier fluid viscosity, and/or inhibit wear [1, 2]. Stabilizers serve to keep the particles suspended, while surfactants serve to coat the suspended particles to enhance the polarization effects form the MR field is applied [2].

MR fluids tend to be heavy, but this is typically compensated for by the small volume of MR fluid needed to achieve the desired effects [1]. Additionally, MR fluids require magnets, which can oftentimes be bulky [4]. MR fluids are typically more expensive than ER fluids [1], but can develop yield strengths 20-50 times greater than those developed by ER fluids and are driven by a relatively small power supply compared to ER fluids, around 50W [2]. Additional benefits of MR over ER fluids include a higher stiffness development potential, and thus higher controllability [3].

MR Fluids have been integrated into cantilever sandwich beams to experimentally determine the damping response characteristics for different magnetic field intensities and configurations [3]. It has been shown that the MR beam damping capabilities are nearly twice those of an ER cantilever beam [7]. There are however, several difficulties that must be overcome to successfully test an MR sandwich beam. Firstly, the magnetic poles used to generate the stimulus

3 field cannot be directly embedded in the beam layers, as the magnetic field must be externally generated, and the material of the beam encompassing the MR fluid must be non-magnetic, such that the effect of the MR fluid on damping can be isolated [3]. Another difficulty arises in that albeit that the field lines are perpendicular to the beam in an equilibrium position, this angle will continuously change as the beam vibrates. Since the magnetic poles are located outside of the beam, a rather strong, homogeneous magnetic field must be generated in order to fully activate the MR fluid [3]. Lastly, since the particles suspended in MR fluids are magnetic, the beam may bend when subjected to a strong magnetic field, depending on both the type and thickness of material that the MR fluid is encased in [3]. The response of the MR cantilever beam is strongly influenced by many fluid and structural parameters, including, the field intensity, layer thickness, beam geometry, complex shear modulus of the MR fluid, and boundary conditions [7].

On one study, the MR cantilever beam was composed of three layers, as shown in Figure 4. Two types of beams were examined, each with a PET interstitial layer to retain an even distribution of MR fluid. One type of beam was also encased in PET and the other beam was encased in aluminum [3]. Aluminum is well suited for encasing MR fluids, as its relative magnetic permeability is equal to unity, whereas the clear color or PET can be beneficial to endure that no air bubbles are trapped in the MR compartment [3]. The beams were machined and glued together via superglue, after which the MR fluid was syringe injected, and the holes were sealed and allowed to dry [3]. The MR fluid for this particular sandwich structure was MRF-132Dg (Lord Corporation) and is hydrocarbon based with a base viscosity of 0.092 Pa⋅s and approximately 81% solid content (fluid has both high magnetic permeability and low coercivity) [3]. Permanent magnets were used to stimulate the MR fluid, and the magnetic field strength was altered by changing the distance between the magnets [3]. The beam was excited via free vibration, an impact hammer, and a shaker, and the beam was shown to vibrate at the same natural frequency for each displacement method [3]. The vibration response was measured via a laser vibrometer as shown in the experimental set up in Figure 3.

Figure 3: MR sandwich beam experimental set up [3] Figure 4: MR sandwich beam composition [3]

The damping properties of the MR beam were shown to increase considerably for both the PET and the Al beam when the magnetic field was applied, as shown in Figures 5 and 6, respectively [3]. For the Pet beam the damping ratio increased from 0.46% to 0.65% for free-vibration [3]. Similarly, although the Al beam required a strong magnetic field of 0.23 T, only up to 0.14 T for the PET beam, the damping ratio of the Al beam increased from 1.8% to 3.7% [3]. For each beam, the damping properties decreased in magnitude as the if the MR fluid was only activated

4 in select regions, and higher controllability was observed if the beam was activated closer to the free end [3].

Figure 5: Free vibration response of the PET MR Figure 6: Free vibration response of the Al MR sandwich beam [3] sandwich beam [3] (a) without magnetic field (a) without magnetic field (b) with magnetic field (b) with magnetic field

The controllability of MR sandwich beam response can be demonstrated by applying a variety of magnetic field strengths at different locations. For the PET beam, shown in Figure 7, the natural frequency of the beam was controllable by 26.9% and the vibration levels were decreased by as much as 2.33 dB [3]. The PET beam was only tested up to a field strength of 0.14 T, because at higher field strengths, the beam was naturally attracted to the magnets. For the Aluminum beam, shown in Figure 8, the natural frequency was shown to be controllable by 17.5% and as much as 15.7 dB, especially when the magnetic field was applied closer to the free end of the beam [3].

Figure 7: First natural frequencies for a PET MR beam Figure 8: First natural frequencies for an Al MR beam [3] [3]

Interestingly it has been shown that the stiffening effect of MR cantilever beams is less if the beam is placed vertically instead of horizontally [3]. The natural frequency for the vertical configuration only increased by half as much as that for the horizontal configuration with the same applied magnetic field strength and magnetic configuration. This effect has not yet been attributed to any single factor, but may be influenced by gravity, attractive forces between the beam and the magnets, the angle of the shaker stinger, or the weight distribution of the structures [3].

5

Albeit, published in a peer-reviewed journal, Figures 7 and 8 present a logical dilemma. From Figures 5 and 6, the time for the MR beam to damp clearly decreases when the magnetic field is applied. However, if the time required for the MR beam to damp decreases, then the first natural frequency of the beam should increase, for the higher the natural frequency becomes the faster the beam will damp. In 1989, is was first experimentally shown that the natural frequencies of an ER cantilever beam increase with increasing field strength, and it is reasonable to expect a similar phenomenon for MR fluids, whereas the opposite phenomenon was presented as a peer reviewed result in Figures 7 and 8.

One recent study, published extensive data that negates the findings of Figures 7 and 8, while using a very similar MR cantilever beam experimental set-up. The MR beam for this experiment, as shown in Figure 10, consisted of two thin aluminum strips, with zero magnetic permeability, a 1.5mm gap filled with the MR fluid, and a 1.5mm layer of Buna-N rubber to maintain the uniform gap width [7]. The MR beam was excited via a shaker and tested for three end conditions: (i) simply supported (SSB), (ii) clamped-free (CFB), and (iii) clamped-clamped (CCB) via an experimental set-up as shown in Figure 9.

Figure 9: MR beam vibration testing set-up [7] Figure 10: MR beam deformed and undeformed cross sections [7]

Indubitably, the findings for the 1989 ER fluid study were validated for MR fluids along each node of the beam and for field intensities of 0-500 G (0-0.05T) [7]. It is noted that these field strengths are significantly lower than the field strengths utilized in the disputed article above. Thus, the damping effect here may in fact be due more to the actual damping effect of the MR fluid than the stabilizing effect that a magnetic field may have on the individual magnetic particles in the fluid contained in the beam, as the particles are less likely to vibrate when under the influence of a magnetic field, which may also contribute to a damping effect. Table 2, shows that even for all three boundary conditions, the natural frequency increased at each node (mode) as the magnetic field strength increased [7]. To reiterate, “The results consistently show that the natural frequencies corresponding to all the modes increase with increases in the magnetic field, irrespective of the boundary condition [7].” Thus, it appears as though Figure 7 and 8 are not an accurate interpretation of the MR cantilever beam study.

6 Table 2: Natural frequency response of MR beam [7]

Additionally, it was shown that at higher excitation frequencies, the MR beam displacement decreases as shown in Figures 11 for SSBs and in Figure 12 for CBFs [7]. Additionally, these figures show that peak displacement further decreases with increasing applied magnetic field strength, thus directly linking the decrease in displacement to the thickening phenomenon. However, there appears to be a saturation tendency that occurs with increasing magnetic field strength. The increase in the magnetic field amplitude from 0 to 250 G leads to a 9.5% maximum reduction in the first displacement peak of an SSB (Figure 11), and a 5.9% maximum reduction in the second and fourth displacement peaks for a CFB (Figure 12) [7]. However, further increases in field intensity do not elicit effects of similar magnitude, and thus the displacement reduction effect appears to be easily saturated, even at low magnetic field strengths [7].

Figure 11: MR simply supported beam response [7] Figure 12: MR clamped-free beam response [7]

The thickness ratio of the MR fluid layer also affects both the natural frequency and the loss factor of the MR beam. Thickness ratio is give by h2/h1 as shown in Figure 10. The loss factor is

7 the ratio of the energy dissipated per radian to the total strain energy [7]. The loss factor generally increases with an increase in magnetic field strength for all beam end conditions [7]. For increasing MR fluid thickness ratios, the natural frequency response of the beam tends to decrease, possibly because the relative variation in the mass of the structure is higher than that of the stiffness of the MR fluid, as shown in Figure 13 [7]. On the other hand, the loss factor generally increases with only for the first two modes, as shown for an SSB under a 500T magnetic field in Figure 14 [7]. As the thickness of the MR fluid increases, both the dissipated energy and the strain energy necessarily increase; however, the relative change in dissipated energy is considerably higher [7].

Figure 13: MR thickness ratio effect on natural Figure 14: MR thickness ratio effect on loss factor for frequencies [7] simply supported beam [7]

8 Chapter 3: ER Fluids

ER fluids consist of polarized particles in colloidal suspension with an oil of a lower dielectric constant than the polarized particles [8]. As the electric field strength increases, the dipole force between neighboring particles becomes attractive along the direction of the electric field, but remain repulsive for dipole moments away from the direction of the electric field at intersection angles greater than 55o [8]. The particles will always increase the effective viscosity of the ER fluid via chain formation along the direction of the applied electric field as the attraction between the dipoles favors this direction, while the thermal diffusion of particles tends to randomize particle position [8]. Within seconds of the application of an electric field, neighboring particles form columns, and at a certain critical electric field intensity, the osmotic pressure becomes negative [9] and the ER fluid sticks to not only itself, but also to its container, and thus effectively acts like a solid [8]. Figure 15 shows this chain formation phenomenon for silicon powder in silicon oil (0.5 wt%) and an electric field intensity of 300 V/mm and Figure 16 shows the chain formation phenomenon for cornstarch and silicon oil (0.5 wt%) and an electric field intensity of 900 V/mm [8].

Figure 15: ER column formation for silicon in Figure 16: ER column formation for cornstarch in

silicon oil [8] silicon oil [8]

ER fluids must be contained between a suitable arrangement of electrodes. The application of the electric field causes the particles to polarize and as the field is gradually increased, the particles will form chainlike structures, as shown in Figure 15 and 16, and thus create an effective yield stress that must be overcome before flow can occur [1]. Although ER fluids require large power sources to generate significant increases in viscosity, the size of the actuating elements is usually relatively small [4].

Column formation in ER fluids is very well defined, as shown in Figures 17 and 18. Figure 17 shows the column formation phenomenon for silicon powder (1%wt) and en electric field strength of 1000 V/mm, and Figure 18 shows silicon powder (0.2%wt) and en electric field strength of 1000 V/mm. The most stable arrangement of the ER fluid is the body centered tetragonal configuration, as this structure yields the lowest energy level while maintaining a stable structure [8]. However, shear stress can also be introduced into the ER fluid, by moving the two electrode plates in parallel but opposite directions. This shear stress will cause the chains to tilt and stretch, and eventually fail if the tilt becomes too great, as the tilt diminishes the attractive forces acting between neighboring molecules [8].

9

Figure 17: Top view ER column phenomenon Figure 18:Top view ER column phenomenon 0.2%wt 1%wt silicon [8] silicon [8]

One advantage of ER fluids is that ER fluids do not exhibit the same residual yield stress effects as MR fluids. In an MR fluid, residual particle interactions may occur due to the ferromagnetic nature of the particles which may induce very small vibrations that will not trigger an MR fluid thickening effect as these vibrations are below the magnetic stimulus threshold for a viscosity increase to occur [4]. This phenomenon does not occur in ER fluids [4].

Much like MR fluids, the process to create ER fluids is tedious, and must be precisely executed [8]. Similarly to MR fluids, the shear stress developed in the ER fluid is given by equation 2 [8]:

(2) where is the shear stress of the ER fluid for electric field E and shear rate γ (γ is defined as where u is the fluid velocity and y is the displacement perpendicular to u), is the viscosity of the fluid, and is the yield shear stress [8].

Is has been found that the particles in the ER fluid continue to flow after the initial application of the electric field, but will stop after a few seconds when chain formation begins to occur [8]. However, chain formation may not be statically reversible, as is has been shown that upon the removal of the electric field, the ER particles do not return to their random locations in a static fluid. The ER fluid must also be exposed to dynamic shear stress (i.e. the shear rate cannot be zero), in order for the random to regular arrangement transition to be completely reversible [8].

ER fluids have been seen as a purely academic exercise since around 1990, when the focus of applications shifted to the damping properties of MR fluids [1]. ER fluids exhibit similar responses to cantilever beam testing as MR fluids, and while most early research was conducted on ER fluids, the focus began to shift towards MR fluids partially because MR fluids exhibit a higher level of bandwidth control [7]. It was first experimentally concluded that the damping ratio along with the natural frequency of an ER fluid increases as the applied electric field increaseds in 1989 [10]. ER cantilever beam testing will not be covered in detail here, as it is very similar to MR and ST beam testing and the results are similar as well, except lower in magnitude. It has however, been shown that the damping factor in a five layer ER beam could be larger than the damping factor in a three layer ER beam [11].

10 Although ER sandwich cantilever beams have a lower stiffness development potential, they do harbor several advantages over MR sandwich beams. ER structures can be fully activated by homogeneous electric fields and the controllability of the stiffness and damping characteristics of ER beams has been demonstrated via activating different regions of the ER beam [3]. Additionally, an ER stimulus generation electrodes, can be embedded directly into the external layers of the beam [3]. Although ER beams may prove more practical for experimentation, their activated yield stress only reaches 2-5 kPa [3], approximately 5% of the range achievable via MR fluids.

11 CHAPTER 4: SHEAR THICKENING FLUIDS

ST fluids are different from MR and ER Fluids, as they do not require any electrical or magnetic stimulus, and thus do not require an external power source. However, ST fluids display similar increases in viscosity under shear and vibration frequency stimuli as MR and ER fluids[12]. Initially ST fluids were investigated when the thickening (changes in suspension microstructure via particle aggregation) phenomenon become problematic for processing equipment [13]. Initially, ST fluids were thought to thicken due to an order-to-disorder phenomenon (ODT), but more recent experiments have shown that thickening is due to hydrodynamic clustering, where the hydrodynamic clusters are compact groups of particles that stick together due to short-range lubrication forces [13]. At low shear rates, ST particles exist in an equilibrium pervaded by Brownian repulsive forces; with a sufficient increase in shear rate, ST particles become highly clustered [12].

ST fluids consist of sub-micron sized particles, concentrated and suspended in a carrier fluid [12]. At low shear rates, ST fluids undergo thinning (even at increasing low shear rates [12]) until a critical shear rate or frequency of oscillation is reached, at which the viscosity increases sharply (as seen in steady state experiments) [12-14]. The thickening phenomenon is usually complete by one decade of shear rate after the onset of thickening [13] and thickening can lead to simultaneous changes in both stiffness and damping [12]. The thickening phenomenon is immediately reversible upon the removal of the shear stress [12].

It has been shown that ST fluids in sandwich structure beams will display an increase in viscosity which begets an increase in both structural rigidity and damping [14]. This was shown by encasing 15% w/w fumed silica (12nm particle size) in polypropylene glycol (PPG). The experimental set up is shown in Figure 19 and consists of two layers of fumed silica (suspended in PPG after vigorous drying) sandwiched between three plates of PVC [14]. Oscillatory stresses were exerted on the tip of the cantilever beam (dynamic sinusoidal flexural deformation), and the shear rate of the beam was investigated as a function of frequency via vibrating beam testing (VBT) [14]. The vibration exciter system was coupled to the beam via a force transducer, as shown in Figure 20 [14].

Figure 19: Three layer PVC beam with integrated ST fluid [14] Figure 20: Vibrating beam testing schematic [14]

12 Figure 21 shows the how complex viscosity increases as stain increases for different frequencies [14]. For all frequency values, the complex viscosity does not increase until a certain minimum strain stimulus is reached. Interestingly, the behavior of the ST fluid after thickening has occurred varies depending on the frequency. For lower frequencies, the viscosity begins to decrease after thickening has occurred and frequency continue to increase, where as for higher frequency values, the strain may not have been sufficiently increased for this phenomenon to occur. Additionally the thickening transition is initiated at relatively low frequencies, but the critical frequency increased monotonically with increasing initial set strain [14]. The length of the relative thickening along the cantilever beam was also determined to be a function of the initial displacement of the tip, as shown in Figure 22.

Figure 22: Relationship of initial tip displacement to Figure 21: STF viscosity dependence on strain [14] relative thickening for STF [14]

The extent and sharpness of the thin to thick transition depends on several factors. These include the particle size and shape, particle content, particle-particle interactions, and continuous phase viscosity. The effect of the particle content on the thickening response is illustrated Figures 23- 26, wherein the w/w percentage of fumed silica (12nm particle size) in polypropylene glycol (PPG) varies from 10% to 17.5%.

13

Figure 23: 10% w/w fumed silica frequency vs. complex Figure 24: 12.5% w/w fumed silica frequency vs. viscosity [13] complex viscosity [13]

Figure 25: 15% w/w fumed silica frequency vs. complex Figure 26: 17.5% w/w fumed silica frequency vs. complex viscosity [13] viscosity [13]

The behavior of ST fluids at high frequency oscillations is still disputed; it has been suggested that a minimum strain amplitude must be present for thickening to occur even at the highest frequencies, while other authors maintain that high frequency behavior has a similar frequency dependence as low frequency behavior [12, 13]. After the end of the thickening transition, three possibilities for the further increased frequency domain exist [13]: 1) The specimen may fracture due to the high viscosity. 2) The viscosity may become independent of frequency. 3) The viscosity may decrease as frequency continues to increase. It has been suggested that at sufficiently high frequencies, the shear thickening effect may disappear completely, as it has been shown that at frequencies above the transition range, the viscosity of the ST fluid decreases approximating unique power law [13].

14 Shear fluids have also been integrated into composite fiber structures, which have the potential to exhibit large increases in damping properties, as the ST fluid stored in the inter-fiber spaces has a different elastic stiffness than the fibers and thus large shear strains result [12]. Two potential fiber reinforced ST composites are depicted in Figure 27 [12].

Figure 27: Fiber (a) and cylindrical (b) ST composite structures with applied shear [12]

One such a fiber ST composite was designed utilizing KE-P50 particles (500 nm in size) and polyethylene glycol (PEG) along with 10x7x4 mm composite silicone matrixes and glass fiber- epoxy rods [12]. The construction process of such a matrix is rather elaborate. First, the rods must be inserted into PEET tubes (poly(tetraflouroethene)) which were then cast into silicone to create a silicone sample with holes. The holes were then filled with the ST fluid and the glass tubes were gently inserted while one side of the holes was sealed off [12]. The sample is depicted in Figure 28. The volume fraction of the fibers and the thickness of the ST fluµid layer were 0.25 and 50µm [12]. The ST fiber composite was tested in a clamp set-up similar to the one used in Figure 20 and is depicted in Figure 29.

Figure 28: ST fiber composite schematic [12] Figure 29: ST Fiber composite testing set up [12]

The ST composite fiber was tested in a range of frequencies from 0.1 to 100 Hz and force amplitudes from 0.1 to 1.5 N. The ST composite specimens showed the expected pre and post transition behavior independent of the applied strain amplitude for each frequency sweep consisting of a power law decrease before and after the thickening transition [12], as shown in Figure 30. However, the ST composite specimens were prone to fracture after the transition

15 (brittle behavior) [12]. Furthermore, the critical strain amplitude as a function of frequency for the ST composite is shown in Figure 31. The shear modulus and the shear damping ratio were found to not change significantly with carrying frequency or applied force, but were, however, found to vary as much as 20% to 40%, which may be due to the different friction conditions [12]. Although more research is needed to fully determine and mathematically model ST composite materials, these fiber composites can be tailored to exhibit specific, load-controlled, dynamic properties [12]. Interestingly, the research on ST fiber composites was funded both the Sports and Rehabilitation Engineering program at EPFL [12].

Figure 31: Critical strain rate for thickening of ST Figure 30: ST fiber composite frequency sweeps [12] composite [12]

16 Part 2: Current, Imminent, and Future Smart Fluid Applications

CHAPTER 5: CURRENT APPLICATIONS

The suspension struts on the Cadillac STS 2002 have been the most prominent application of MR fluids [1]. Lord Corp. (Delphi Corp.) commercially markets this strut technology as “MagnaRide” which has also been utilized in the 2003 and 2004 Chevrolet Corvette. The main advantages of MR MagnaRide technology include a 40% reduction in mechanical components in the strut assembly (mostly valves), the elimination of the traditional shock absorber fluid, as well as an adaptation capability at a frequency of 500 Hz [4]. However, the cost to benefit ratio for MR strut technology is not yet established enough for widespread use in the auto-industry [4]. However, Ford has channeled its MR research and development efforts towards the integration of MR fluids into seatbelts, steering columns, and clutch systems [4]. Significant reduction in the vehicle to driver force transmission has been achieved via MR fluids via a driver seat suspension mechanics [1]. Future applications of MR fluids in the auto industry will aim to reduce noise and vibration from the engine before it enters the cabin and to reduce the number of mechanical connections between the steering wheel and the drive wheels.

Other applications of MR fluids include the use of MR brakes to control the antenna system on a NASA rocket and in the form of a rotary break to deliver feedback sensations in forklift trucks [1]. MR dampers are used for real-time vibrational control in heavy-duty trucks, as well as for low cost linear and rotary positional and velocity control systems for pneumatic actuators [2]. MR dampers are also used to isolate buildings from seismic shocks and have been designed into a cable stayed Ding Ting Lake [4] bridge in China to control wind induced vibrations [1, 2]. Although initially costly to implement, MR seismic damage control systems appear to be quite effective, especially when close to the epicenter of the eqrthquake. [4] A schematic example of a seismic damping system at the Museum of Emerging Science and Engineering in Tokyo, Japan, is presented in Figure 32. Washing machine dampers, car shock absorbers, and tactile force feedback in steer-by-wire systems, also utilize MR fluids. [2] MR fluids have also shown to be promising for microscopic polishing, which is possible due to the abrasive properties of the MR fluid and may be enhanced by future developments towards solid phase MR fluids [1, 2]. MR fluids are also being investigated for incorporation into gun barrels to control the recoil, as well as in dampers for helicopter blades [4].

MR and ER fluids have shown great promise for integration into hepatic devices. By controlling the stiffness of a hepatic surface, the surface can mimic tactile pressure, and eventually even mimic the sense of touch. This technology has been integrated into a tablet that can both read and output Braille letters, allowing the user to both read and write [4]. Smart Technology is currently testing smaller prototypes of this tablet that utilizes three rows and three columns of plastic pins activated my ER valves, and the full version will feature 128 by 64 individual activators [4]. ER fluids have strikingly similar rheological properties of biological tissues, as they can only resist forces and not generate them, which is ideal for hepatic systems [4]. Thus, surgical simulations can also be created via hepatic MR devices and may eventually dominate the surgical training technology as MR devices create a realistic hepatic interface [4]. An example of such a hepatic surgical MR interface is shown in Figure 33.

17

Figure 32: Seismic MR damping system in Tokyo, Japan Figure 33: MR virtual endovascular telesurgury [4] [4] (a) is the top view and (b) is the side view

Current applications of MR smart fluids operate on three distinct MR systems: 1) MR break: An MR brake operates via direct shear. The MR fluid is placed between two surfaces, each moving with respect to the other. The break allows for the continuous control of torque by varying the applied magnetic field [2]. 2) MR clutch: The clutch also operates in direct shear mode and serves to transfer power from one shaft to another. As the magnetic field and thus the viscosity surrounding the first shaft is increased, the torque is transmitted to the second shaft. Useful torque transmission has been shown to occur after 1-2 milliseconds after the initial increase of magnetic field strength [2]. 3) MR dampers: These dampers are infinitely variable and can change from liquid to solid phase in a few milliseconds. MR dampers are semi-active devices, that are considered fail-safe and are useful for energy absorption in mechanical systems [2].

Although smart fluid applications have focused on MR fluids, some applications of ER fluids have also been developed. To parallel the advances of MR automotive technology, Smart Technology Ltd. has developed a ER suspension damping system and aims for this system to permeate the European auto industry [4]. NASA has also begun developing ER fluid based hepatic system for remote access system control [4]. A squeeze-flow mode of ER fluids has shown promise for use in structural vibration control, and an ER clutch has also been developed [1]. An ER clutch mechanism has been incorporated into a force display system, which may have potential for application in virtual reality systems [1]. ER fluids have been used for tension control in a wire cutting discharge mechanism for machining operations as well as in rotating machinery components as controllable seals [1]. ER fluids have also been incorporated into a dual clutch system to control a robot arm prototype [1], as shown in Figure 34.

18

Figure 35: Biedermann Motech MR Prosthetic Knee Figure 34: ER clutch controlled robot arm [1] [16]

The properties of smart fluids may make them ideal candidates for integration into prosthetics, orthotic devices, and sports equipment. MR fluids have already been successfully integrated into advanced prosthetic devices for real-time gait control [2]. Prosthetics provide an ideal opportunity for application as the fit of the prosthetic is dynamic during wear [5]. The interface pressures in a prosthetic socket have been shown to range from 0-342 kPa and the shear stresses have been shown to range from 0-57 kPa. Thus, a smart fluid could provide a dynamic fit that varies according to the current socket fit conditions and thus reduce the load that is transferred to the residual limb, thus increasing comfort and extending potential wear time [5]. Ankle-foot orthotic (AFO) devices have mainly focused on preventing the toe-drop phenomenon, by regulating joint stiffness and system damping [5].

An MR prosthetic knee, the Prolite™ Smart Magnetix™ Above-the-Knee (AK) Prosthetic [16], has been developed and is commercially marketed in Germany by Biedermann Motech GmbH in conjunction with LORD Corp [1, 4]. This MR prosthetic aims to increase gait balance, stability, and energy efficiency [16]. The Smart Magetix Technology was adapted from LORD corporations use of MR technology in their Motion Master™ Ride Management System for truck seat dampers [16]. The Smart Magnetix knee, as shown in Figure 35, consists of a hydraulic piston that is controlled to match the environmental requirements placed on the prosthetic in real time in order to account for walking speed, stairs, terrain slope, and temperature, such that user comfort is maximized without conscious input from the user [4]. The electrical current through the MR fluid is controlled by microprocessors that determined the environmental constrains and then feed this information to the controller [4]. Beiderman Motech claims that the response time of their prosthetic knee is only 10ms and that the MR force response can be varied almost infinitely [16]. The smart magnetic knee has superior damping capabilities, as the damping component is passive and thus does not introduce any additional

19 potentially destabilizing forces to the system, as with active damping systems, and, additionally, costs less than an active damping system [16].

One of the most promising applications of ST fluids to date, it the integration of ST fluids into Kevlar® mats, as the ST fluid can absorb large amounts of shock resulting from the impact of a high velocity projectile [14]. Some advances in this area have also incorporated electro textiles, and thus ER fluids may also play a role in the development of smart textiles [17]. Other potential applications include smart seat belt design [18] and athletic equipment, as ST fluids do not require a power source while still displaying shock absorbing properties.

20 Chapter 6: Imminent Applications

Section 1: Rowing Ergometers

It has been suggested that the resistive force in an rowing ergometer (erg) can been established via both MR and smart fluids [19], yet no concrete examples can be located in existing literature. An interesting concept however, could be to apply smart fluids to the base of an erg in order to create an unstable environment, thereby forcing the rower to maintain proper balance on the erg, better simulating a boat. In the erg design presented in Figure 36 the erg can swing about three axes, including ±20mm vertically, ±2o transversely, and ±10o longitudinally [19]. This rowing simulator was determined to include a swing of 2o-3o (similar to rowing on a boat) at lower rates of rowing, and showed an increased 3rd vertebral displacement at higher rates, showing that the athlete had to work harder to maintain his/her balance [19]. The muscle activity of the rowing simulator should also be different for stable and unstable conditions [19]. An application of smart fluids may result in a more realistic unstable environment, as the smart fluid could (1) prevent the erg from tipping over yet provide superior instability at low rates, and (2) mimic the hydrodynamic stabilizing effect of the water on the boat hull, as well as the linear inertia of the athlete, [19] that occur at higher rates.

Figure 36: The unstable rowing simulator [19]

21 Section 2: Alpine Skis

The macro-scale integration of ST fluid into an alpine ski has been shown to cause damping to occur much more quickly than in a normal ski for both large and small experimental deformations with a hammer. The STF damping in the ski is driven by transforming the flexural deformations of the ski tip onto gliding plate via a carbon fiber reinforced polymer (CFRT), as shown in figure 37 [15]. The ST fluid was placed between the gliding plate and the ski surface in a small cavity created by removing the upper layer of ski material [15]. The ski was tested under normal, damping device, and damping device with the ST fluid comditions; the results of the ski testing are shown in Figure 38 [15].

Figure 38: STF Ski Testing Results [15] Figure 37: The STF ski design [15] (a, c, and e are small initial deformation, b, d, and f are large initial deformations, a and b are a normal ski, b and c are a ski with just the damping device, and e and f are the damping device with the STF)

22 Section 3: Active Knee Rehabilitation Device

Some advances in applying smart fluids to control torque in knee rehabilitation devices have also been published, yet no commercial smart fluid device has proven robust enough to market commercially [20, 21] . Marvoidis and Bar-Cohen have begun developing this technology as it shows promise for individually tailored, doctor prescribed, at-home rehabilitation programs that can also be monitored for patient compliance [4].

Two distinct approaches to knee rehabilitation have incorporated ER and/or MR fluids. The first device mimics an isokinetic dynamometer. The core of the device is a damper, driven by an MR fluid, which provides a passive force for muscle strengthening [21]. The damper is incorporated into a chair and is controlled by a laptop, in order to keep the cost low. The MR fluid requires only a low power supply and an angular sensor records the joint position. The device could potentially be programmed to increase the force progressively either as the patient improves, or on a schedule set by a physical therapist [21].

The second knee rehabilitation device is intended for extended wear in between therapy sessions in order to maintain proper knee alignment and during normal activities [20]. This device (the AKROD) is a knee brace with an ER break built into the joint in order to control for disturbances such as external torques, such that these disturbances do not interfere with proper knee function an alignment. The device is depicted in Figure 39. The ER break was able to maintain the desired torque almost instantly [20].

Figure 39: Active knee rehabilitation device with ER break [20]

23 Section 4: Footwear

The footwear debate is still an unsolved mystery. The root cause of running injuries remains an unresolved issue, although most footwear manufacturers focus on designing shoes to reduce GRFs at impact, as visible in a barrage of air pockets, fluid pockets, and spring like attachments to the heel portion of many shoe designs. However, the initial GRF may not actually be the most potent factor contributing to running injuries.

According to Bartlett, the biomechanical requirements of a running shoe are as follows [22]: 1) Attenuation of repetitive GRFs 2) Maintenance of rear-foot control for stability – this includes subtalar joint stabilization to avoid excessive pronation of supination 3) Traction/friction of the outsole 4) Allowance for difference foot-strike styles and pressure distributions 5) No exacerbations of arch irregularities 6) Heat dissipation 7) Wearer comfort The use of smart fluids in footwear could address points 1, 2, and 4.

Interestingly, it has been reported that the running surface impacts the movement pattern of the runner. Heal strike is 31% more likely on a compliant surface than on a non-compliant surface [22], and although this data explicitly compared grass to asphalt, it can reasonably be extrapolated to the material of a running shoe. Thus making running shoes more compliant may actually increase heal strike and thus increase the exposure to higher GRF, as shown by Figure 40. A caveat to this assertion is that that overall GRF in running on a compliant surface is actually greater for the longer duration of mid-foot contact than the GRF on a non-compliant surface, as shown in Figure 41. The initial GRF spike in running on non-compliant surfaces is still greater than the highest GRF for running on compliant surfaces, but only lasts for a short duration of time, and thus may not be the critical factor in muscle, tendon, and bone loading injuries [22].

Figure 40: Vertical GRFs in different running Figure 41: GRFs in running on different surfaces styles [22] [22]

Additional complications to footwear design arise from studies showing that bone-on-bone, and tendon contact forces only responded very minimally to changes in midsole hardness [22]. For ground stiffness below 350 kN/m shoe sole stiffness has been shown to have almost no influence

24 on the initial GRF; additionally, there is a good correlation between decreasing shoe sole stiffness and the attenuation of the first GRF [23]. Furthermore, internal impact forces have been shown to be greater during the propulsion phase than during the impact phase [22]. Lastly, several footwear companies have claimed that their shoes return more energy to the wearer, yet studies have shown that soles do not have the spring properties necessary to return energy at the correct times, location, or frequency, leading to only a 1% of return for the total energy needed for each stride.

If energy return is considered, then smart fluids may actually produce an undesirable effect in footwear. Shoes generally have a low energy return ratio, as the deformation in the forefoot shoe sole is generally small and cannot be increased, as this would lead instability. It has been estimated that a marathon runner expends about 500 J of energy for each ground contact, and thus even a relatively large deformation of 10mm, the maximal returned energy is only 2% [24]. Additionally, it has been shown that running shoe soles are only 60-70% efficient, and thus in reality only 1.4% of the energy would be returned with a 10mm deformation (yet, this may still be substantial) [24]. Lastly, the energy must be returned at the right location and at the right frequency. Most shoe manufacturers have focused on energy return at the heel, although energy must be returned at the forefoot in order to be useful. The midsole also has energy storing capabilities that have not been exploited. The optimal frequency of energy return will vary with the type of stress imposed on the shoes; however, the optimal loaded natural frequency of sprinting surfaces is 5 Hz [24].

If a viscous material is introduced into the shoe midsole, this, in theory, may decrease the work requirements of a stride via the damping of lower extremity vibrations [24]. Experimental conclusions tend to support these results but are still inconclusive [24]. An additional approach to preventing energy loss, albeit potentially uncomfortable, suggests inserting a wedge into the midsole of the shoe. Since the heel strike only serves to dissipate energy during sprinting, and the athlete has to immediately perform more work to lift the heel off the ground again, it may be best of the heel strike were completely eliminated from the stride [24]. An illustration of this concept, which has never been experimentally validated, is presented in Figure 42.

Figure 42: The energy saving wedge concept [24]

Assuming that peak impact forces are actually one of the leading causes of lower back pain and running injuries, an MR fluid (50g Ford Oil BO 75W-90, 10g Lithium Grease, and 150g of Carbonyl iron FMR-ford 50±10) has shown potential for use to modify the real time damping coefficient of a shoe. This approach is only valid if the conclusion that spring stiffness of the shoe does not significantly influence GRF forces and that the damping factor predominates the musculoskeletal system holds true. Thus, the premise of incorporating an MR fluid into footwear

25 is that the damping coefficient could be modified in real time according to the athlete’s needs [25].

There is plenty of room in most current shoe designs for the integration of MR, ER, or ST Fluids. Most running shoes on the market still follow the original idea pattern for running shoes developed in the 1970s and exemplified by the Adidas® patented running shoe, shown Figure 43. However, most contemporary shoe designs incorporate other elements such as air bubbles, spring like mechanisms, or gel pouches into shoes, especially in the heal region. Thus, incorporating a smart fluid into a running shoe, although only questionably beneficial, appears to be very technically feasible.

Figure 43: Adidas shoe patent design sketch [26]

One of the main difficulties of this approach, is that the IOC bans all “powered devices” which would inherently be a part of an ER or MR fluid system [15]. An alternative to the ER/MR approach lies in directly incorporating shear-thickening fluids into footwear.

One company, Terra Plana, has taken a different approach through their “barefoot” design line. Theses Evo shoes (shown in Figure 44) have an ultra-thin thermoplastic urethane sole (TPU), a “sucked in arch area” that grabs the foot, and a TPU and mesh upper for breathability and support. Evos weigh only 17oz a pair. The shoe capitalizes on recent studies [27] that have found that barefoot runners land on the middle to the front part of the foot as opposed to the heel, thus reducing the heel strike GRF that most contemporary shoe designs try to minimize [28].

Figure 44: Evo Shoes [28]

26 Section 5: Prosthetics and Exoskeletons

Although one commercially MR integrated knee prosthetic is currently marketed, there still exists much potential for improvement and further application of MR fluids to prosthetics as well as exoskeletons. Prosthetics serve to mimic the function of a limb that no longer physically exists, whereas an exoskeleton intends to increase the functionality of limbs that still physically exist but are functioning sub par due to tissue damage or increased loading.

An MR knee prosthetic has been developed, that using only local sensing of force, torque, and position, controls for early stance damping, thus allowing for early stance knee flexion during prosthetic use [29]. Early stance knee bending is an important component of heel and leg shock absorption during normal gait and can only be achieved via mechanically active prosthetics, which can detect stairs, sitting, stumbling, and other non-standard gait behaviors [29]. Several adaptive hydraulic prosthetic knees have been developed, but these cannot dynamically adapt to gait behavior, as the torque and damping levels are set by a prosthetist and cannot be altered by the patient [29]. The MR prosthetic knee consist of an MR break actuator (1), a potentiometer angle sensor (2), strain gage sensors (3), and an electronics board containing a battery (4) as shown in Figure 45 [29]. The MR component, as shown in Figure 46, consists of iron particles utilized in shear mode and powered by an electro magnet and a magnetic circuit [29]. The MR effect was created via a four-disk system, an inner and outer spline each connected to an inner and outer disk respectively. When the knee rotates the inner spline rotated with respect to the outer spline and thus the inner disk rotated with respect to the outer disk. Between the inner and outer disk, a 20 micron gap was filled with MR fluid which could be stimulated via an electromagnet [29]. Thus, when the MR fluid is activated, the torque required to rotate the knee increases [29].

Figure 46: MR prosthetic knee schematic [29] Figure 45:Photo of MR prosthetic knee [29]

27 The degree of MR fluid activation is coupled to the phase of gait of the subject. There are five phases of the human gait cycle, as depicted in Figure 47 [29]: 1. Stance flexion: The knee flexes slightly after the heel strike to produce a mechanism for shock absorption. A relatively high level of damping is applied via the MR knee brake in this phase to inhibit the knee from buckling. 2. Stance extension: The knee joint extends after maximum flexion has occurred. A high damping level from the MR break is also necessary during this gait phase. 3. Pre-swing/Late stance: the knee begins to flex again in preparation for the swing phase and the adjacent foot strikes the ground. The MR activation during this phase was set to zero. 4. Swing flexion: Hip and knee are flexed, the foot leaves the ground, and the leg swings forward. The damping level was dynamically adjusted during this phase according to cadence speed. 5. Swing extension: The knee begins to extend while still swinging forward until the heel again strikes the ground. The MR resistance was also dynamically adjusted during swing extension. Imaginably, the feedback control mechanism for the activation of the MR fluid is governed by a complex algorithm via the force, torque, and displacement feedback of the integrated sensors in the active knee prosthetic. The control algorithm is presented in Figure 48 and has been shown to be effective in inducing early stance knee flexion [29].

Figure 47: The five phases of human gait [29] Figure 48: MR prosthetic activation and control schema [29]

A similar MR break has also been utilized in a load augmentation exoskeleton. Goal of the exoskeleton is to, despite its increased weight of the exoskeleton itself that the subject must carry, reduce the oxygen consumption cost of walking with a heavy pack as shown in Figure 49 [30]. The MR break for the exoskeleton can attain a maximum torque or 60 Nm while only consuming an average of 1W of electrical power [30]. The knee damper was comprised of 76 MR fluid gaps and 77 blades as shown in Figure 50. As shown in the cross section of Figure 50A the variable knee damper creates a shear stress in the MR fluid as the adjacent upper and lower

28 blades move relative to each other as the knee extends, yet can also be modulated via the electromagnet system [30].

Figure 49: Exoskeleton overview with MR knee break Figure 50: MR knee break for exoskeleton [30] [30]

The above MR break quasi-passive exoskeleton was successful in decreasing the oxygen cost of walking via the activation of the MR fluid, but did not lower the oxygen cost of walking below the oxygen cost of walking incurred from carrying the backpack without the exoskeleton, as shown in Figure 51. The quasi-passive exoskeleton increased the metabolic cost of transportation (COT) by 10% compared to the standard loaded backpack, yet the zero-impendence exoskeleton increased the COT by 23%, suggesting that the controlled MR break contributed to the reduction in COT. This further suggests that MR break technology could be implemented into systems that aim to restore paraplegic walking, such as the energy storing orthosis (ESO), of which an early model is shown in Figure 52 [31]. The arrow in Figure 52 indicates the pneumatic break that is currently used to control knee flexion, which could be replaced with an MR break for potential improved torque control.

Figure 51: MR break exoskeleton energy consumption [30] Figure 52: Pneumatic energy storing orthosis [31]

Apart from torque control, smart fluids also have potential for integration into the actual prosthetic socket in order to provide a customized, dynamically adjusting fit for the prosthetic limb, thus increasing potential wear time, user comfort, and even functionality.

29 Section 6: Programmable Joint Braces

Joint braces are commonly used in sports, yet often times do not directly protect the joint from the malignant ranges of motion, but rather constrict joint motion in general, which may hinder sports performance while not effectively protecting the joint. If smart fluids could be integrated into the material of the brace and activated such that the brace was only stiff in the malignant range of motion, but fully fluid and thus flexible in all other ranges of motion, the function of the joint brace would be much more efficient.

The integration of smart fluids into hip protectors may be able to provide an improved solution for preventing hip fractures in the elderly. Hip fractures in the elderly are very serious, as one third of patients are wheelchair or bed bound, and one study found that around 26% of hip fracture patients die within a year [32]. Hip protectors have been shown to be effective in preventing the occurrence of hip fractures as a consequence of falls [33], but also may make walking considerably more difficult due to their inherent motion restrictive properties. Via attaching an accelerometer to the hip joint, an electrical or magnetic signal could activate the smart fluid in the time between the initial hip acceleration and the impact. There exists sufficient lead time between the initial acceleration and the impact for this sequence of events as the lead- time for sideways falls has been reported around 200ms and 98ms for backward falls [32]. Additionally, as long as the weight can be kept as minimal as possible, the smart fluid could allow for closer to normal range of motion than a standard hip brace.

30 Section 7: Machining Processes

Smart (active) materials allow for the direct integration of the sensing and actuation into a machine, tool, or structure in an unobtrusive manner. This improves on the bulky instrumentation that is often necessary for the machinery in question to operate efficiently. Perhaps even more importantly, smart materials provide a continuous spatial range of spacing feedback and thus eliminate the need for sensing at several distinct locations [6].

Smart fluids are able to transform energy into motion and mechanical motion into energy, thus rising above the functionality of a simple transducer. Additionally, smart fluids have the ability to provide a fast dynamic response, a relatively high force with a fine resolution, as well as high stiffness and frequency bandwidths as required for efficient and accurate machining processes [6]. Some of the advantages of utilizing smart fluids for machining processes include: light weight, compactness, low power consumption, ease of activation, high operating frequency, and low cost [6]. The main setback that results from integrating smart fluids into machining processes is the settling that occurs in the fluid after a period of rest, which degrades smart fluid performance [6]. Another difficulty lies in modeling the expected behavior of smart fluids. Although several mathematical models have been proposed and shown to have reasonable accuracy within certain operating conditions, the behavior of a smart fluid depends on a great number of variables and thus is difficult to model accurately and completely.

Most of the current research in the use of active materials in machining processes has focused on the integration of piezoelectric actuators. Piezoelectric materials generate an electric charge when mechanically stressed and a strain response when an electric field is applied [6]. Piezoelectric materials have been integrated into [6]: 1) Fast tool servos (FTS) for diamond turning in order to improve accuracy vie the elimination of unnecessary vibrations. Up to a 75% reduction in error in the surface finish has been recorded with the use of piezoelectric materials. 2) Piezoelectric actuators have been incorporated into conventional CNC machines to provide ultra precision cutting, as shown in Figure 53. 3) Ultrasonic cutting, wherein the tip of the lathe-tool work piece is set to vibrate at a certain amplitude and frequency in order to improve the dynamic stiffness of the lathe. Piezoelectric materials work well for this application due to their immense capacity to transform electrical energy into mechanical vibration. 4) To reduce chatter in milling and turning operations. One such application of piezoelectric material resulted in a six-fold increase in cutting stability. 5) Boring applications, as the compactness of the piezoelectric material does not interfere with the bar’s clearance requirement. One such proposed smart tool in presented in Figure 54. 6) Piezoelectric-driven precision position tables 7) A smart disk composed of a stack of piezoelectric actuators to provide robust and active damping (up to 86%) on a microlithography machine. 8) A thin film sensor has been developed by Los Alamos National Laboratories that serves to monitor tool vibration characteristics.

31

Figure 53: CNC machine with piezoelectric actuator [6]

Figure 54: Smart boring tool with a piezoelectric actuator [6]

Some applications of ER and MR fluids to machining processes have also been investigated. ER fluids have been used to achieve chatter suppression via tunable boring bars for high precision polishing [6]. Additionally, ER fluids have been presented in theoretical concepts wherein they are integrated directly into the cutting tool such that the stiffness and thus the damping effect can be controlled dynamically [6]. The schematic for such a tunable boring bar is presented in Figure 55.

Figure 55: Tunable boring bar with ER fluid [6]

High precision polishing and grinding via both MR fluids have also been investigated for use on lenses, mirrors, and optics [6]. A small section of the material is polished via direct contact with the MR fluid which is directed to a narrow portion of the material via a rotating wheel [6]. A few proposals for the use of ER fluids for polishing applications have also been suggested and remains an active research topic [6].

32 Section 8: Automobile Headrests

Every year over 800,000 whiplash injuries occur from auto accidents, costing the US over $5 million annually [34]. These injuries are often associated with the conflict of stiffness, as during normal operating conditions passengers prefer that the headrest be soft and comfortable, yet under impact conditions, a softer headrest does not provide the supportive and absorptive properties necessary to prevent serious whiplash injuries [34]. Additionally, automotive headrests must conform to a wide range of operating conditions, as ideally, a headrest should prevent whiplash injuries for both adults and children alike, each of which presents a much different set of anatomical criteria.

Smart fluids potentially provide an ideal solution to this conflict of stiffness problem, as they exhibit a wide range of instantaneously adjustable stiffness. One approach to redesigning a car headrest lies in impregnating polyurethane reticulated foam with an MR fluid (and eventually ST fluids as well), as shown in Figure 56 [34]. The reticulated foam is an interconnected network of solid material that forms edges and faces of cells, thus a cellular network [34]. The MR fluid that attaches directly to the cell edges in most influential in changing the stiffness properties of the MR saturated foam, and is shown in Figure 57 [34]. The MR fluid is integrated into the cellular solid via suction. The cellular solid is first compressed and then submerged into a predetermined quantity of MR fluid, and absorbs the MR fluid as it is allowed to expand [34]. The cellular sponge is then compressed and allowed to relax several more times while submerged in the fluid, as to homogenize the MR fluid inside of the cellular pores, resulting in an MR distribution as shown in Figure 56 [34]. The MR fluid covering the struts (edges) of the cellular solid maintains its position due to the finite off state yield stress of the MR fluid [34]. The thickness of the layer of MR fluid covering the struts of the cellular solid is given by:

0 h ≈ τ y /ρg (3)

0 where ρ is the density of the MR fluid, g is the acceleration due to gravity, and τ y is the non- activates state finite velocity of the MR fluid [34]. €

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Figure 56: Polyurethane reticulated foam w/ and w/o MR fluid Figure 57: Reticulated foam with MR fluid [34] schematic [34]

The stiffness properties of MR impregnated foam show a great range of applicability not only to automobile headrests, but also to sports equipment applciations. For example, the landing mats in gymnastics could be modulated to absorb a greater percentage of the energy of landing, thus relieving the gymnasts’ bones of some of the strain encountered during landing. Because the MR

33 fluid clings to the edges of the cellular solid, the reticular foam application bypasses the settling effects that are usually present in MR fluid based damper applications [34]. The energy absorbed by the foam with the MR fluid, can be modulated via both the applied magnetic field strength, as shown in Figure 58, as well as via the volume fraction of MR fluid in the cellular solid, as shown in Figure 59.

Figure 58: MR foam energy absorption for varying Figure 59: MR foam energy absorption for different MR magnetic field strengths [34] fluid volume fractions [34]

The proposed headrest utilizing the MR foam, schematically depicted in Figure 60, will be activated by a magnetic coil in the posterior portion of the headrest, that will be activated according to the mass properties of the passenger and the speed at impact [34]. To simulate the head impact, a clear cylinder “ball drop” device was utilized with a 1.27mm aluminum ball dropped from an average height of 150 cm and attaining an average impact of 5.3 m/s (which is the maximum velocity used in rear-end automobile crash testing) [34]. When the ball was dropped onto an MR fluid filled sample the sample sowed only 30% compression and a rebound velocity of 0.5 m/s, whereas the ball dropped onto dry foam sample bottomed out with 100% compression and a rebound velocity in 1.72 m/s [34]. These results are presented graphically in Figure 61. The coefficient of restitution for the MR fluid filled foam was 0.1 whereas the coefficient of restitution for the dry foam was 0.3, showing that the energy absorption capacity of the MR fluid filled foam was dramatically improved over the dry foam [34].

Figure 60: Reticulated MR foam headrest schematic [34] Figure 61: MR foam and dry foam headrest simulation height results [34]

34 CHAPTER 7: TECHNOLOGICAL DEVELOPMENTS NECESSARY FOR INTEGRATION

MR Fluids MR Fluids exhibit both clumping and settling phenomena with time that are currently only remedially addressed via the use of additives. This reduces the longevity of MR fluids, creating the need for regular replacement of the MR fluid in high-cycle and/or high-load applications [4]. The clumping phenomenon may not be solely caused by gravity and NASA, in conjunction with MIT, is currently conducting the InSPACE experiment, which subjects MR fluids to magnetic pulses in space in order to gain insight into the causes of the settling phenomenon [4].

An additional challenge to commercially incorporating MR fluids into technology lies therein that mass production of MR fluids is much more difficult than the production of MR fluids for lab purposes [4]. The challenge lies in mixing and handling MR fluids in large quantities. Additionally, a barrel of MR fluid weighs slightly over half a ton [4].

ER Fluids ER fluids currently lack the lifetime and temperature range operating conditions to be commercially marketed [4]. Although the operating temperature requirement is only critical for select applications, heavy-duty applications of ER fluids last only a few months, making them ineffective for mass marketing. Other challenges to ER fluid technology include the limited resistance level that can be obtained as well as settling problems, similar in scope to the settling problems in MR fluids [4]. ER fluids will also absorb humidity, which in turn degrades their performance [4].

ST Fluids Several issues must be resolved before smart fluids can effectively be integrated into sports equipment. The micro-integration of ST fluids is a major issue, as it will allow the equipment to retain its current shape while only altering the performance of the material. Micro-integration must fulfill two major conditions for the ST fluid to retain optimal function [15]. 1) The STF must be exposed to high shear strain rates 2) The STF damping effect should remain immediately reversible The permeability, robustness, durability, and aesthetics of smart fluids must also be addressed [15]. ST fluids show great potential for non-energy driven applications, but will need to be directly integrated into micro-scale polymer composites, to create a new class of materials with distinct functional properties [12].

MR Foam Before MR foam can be commercially integrated into headrests, two main issues need to be resolved. The heat transfer of the energy generated during the compression of the MR foam as well as the providing the necessary energy to generate the required magnetic field strength are important concerns [34]. Additionally, the generation of a strong, rapidly modifiable, distributed magnetic field needs to be further developed before MR foam can be commercially integrated into automobile headrests. A capacitor may be able to be utilized to provide the strong rapid magnetic field necessary for energy absorption upon impact with minimal power consumption [34].

35 CHAPTER 8: THE FUTURE OF SMART FLUID APPLICATIONS

Smart fluids have a wide applicability range and thus entertain some interesting potential future applications ranging. Yet there exist several technological challenges (see chapter 9) that place these applications well into the future.

One project is a space-exercise suit to prevent muscle atrophy during prolonged zero-gravity exposure, driven by MR fluid valves incorporated into the suit itself. The concept has merited some R&D efforts at Bar-Cohen-Marvoidis as the On Demand Operational Exoskeleton (ODOE) [4]. The ODOE would be fully adjustable to provide adequate controllability of resistance for different tasks, including using a robotic arm extending from the ODOE used for external of vehicular activities and controlled via the astronaut in the ODOE suit [4]. The current state of the ODOE project involves an ER fluid in a large piston with slots, such that when the piston is moved the ER fluid can travel from one side of the piston to the other via the integrated slots. An electric field can then be applied to selected regions of the piston to regulate viscosity, and thus resistance [4].

Other approaches to ER and MR fluid applications may consist of: 1) Injecting biologically compatible ER and MR fluids directly into the bloodstream to control blood flow to cancerous tumors [4]. 2) Using MR “blood” to flow through robot veins [4]. 3) Magneto-liquid-mirror telescopes which will bend and deform to shield from the twinkling of starlight and thus allow astronomers to make more accurate observations [4]. 4) Spacecraft shock absorbers [4]

Lastly, an interesting future application of smart fluids may be their integration into artificial muscles. Currently, the scope of research focuses on the functionality of prosthetic to joint attachment integration, and mostly aims to integrate polymers into this junction, as natural muscle tissue is basically a polymer [35]. Preliminary investigations into the field have focused on gel actuators, electrochemical polymers, electrostrictive polymers, dielectric elastomeres, and shape memory alloys [35]. However, as smart fluid technology advances, ER fluids and ST fluids potentially have a distinct advantage for artificial muscle applications. Muscle contraction is driven by an electrical signal, and if the threshold of ER fluids were to become low enough, than ER fluids could be seamlessly integrated to reinforce tendon, ligament, and muscle connections. Alternatively, ST fluids could be used to mimic the stretch reflex arcs that exist in the human neuromuscular system, which cause contraction as a protective function. ST fluids could provide a similar protective mechanism when exposed to shear stress via muscular contraction.

36 REFERENCES

[1] R. Stanway, "Smart fluids: current and future developments," Materials Science and Technology, vol. 20, pp. 931-939, 2004. [2] M. Kciuk and R. Turczyn, "Properties and application of magnetorheological fluids," Journal of Achievements in Materials and Manufacturing Engineering, vol. 18, pp. 127- 130, 2006. [3] V. Lara-Prieto, et al., "Vibration characteristics of MR cantilever sandwich beams: experimental study," Smart Materials & Structures, vol. 19, 2010. [4] J. Ouellette, "Smart Fluids Move into the Marketplace," The Industrial Physicist, pp. 14- 17, 2003/2004. [5] M. Philen, "On the applicability of fluidic flexible matrix composite variable impedance materials for prosthetic and orthotic devices," Smart Materials and Structures, vol. 18, p. 104023, 2009. [6] G. Park, et al., "The use of active materials for machining processes: A review," International Journal of Machine Tools and Manufacture, vol. 47, pp. 2189-2206, 2007. [7] V. Rajamohan, et al., "Vibration analysis of a multi-layer beam containing magnetorheological fluid," Smart Materials & Structures, vol. 19, Jan 2010. [8] J. Lue and C. Mao, "Viscosity measurement of electrorheological fluids: corn starch and silicon microparticles mixed with silicone oil," Measurement Science and Technology, vol. 8, pp. 1323-1327, 1997. [9] R. Tao and J. Sun, "Three-dimensional structure of induced electrorheological solid," Physical review letters, vol. 67, pp. 398-401, 1991. [10] M. Gandhi, et al., "A new generation of innovative ultra-advanced intelligent composite materials featuring electro-rheological fluids: an experimental investigation," Journal of Composite Materials, vol. 23, p. 1232, 1989. [11] J. Qiu, et al., "Damping effect of multi-layer beams with embedded electro-rheological fluid," Journal of Intelligent Material Systems and Structures, vol. 10, p. 521, 1999. [12] R. Neagu, et al., "Micromechanics and damping properties of composites integrating shear thickening fluids," Composites Science and Technology, vol. 69, pp. 515-522, 2009. [13] C. Fischer, et al., "Pre-and post-transition behavior of shear-thickening fluids in oscillating shear," Rheologica Acta, vol. 46, pp. 1099-1108, 2007. [14] C. Fischer, et al., "Dynamic properties of sandwich structures with integrated shear- thickening fluids," Smart Materials & Structures, vol. 15, pp. 1467-1475, 2006. [15] C. FISCHER, "Adaptive composite structures for tailored humanñmaterials interaction," ed: Theses, 2007. [16] Other MR Solutions: Medical: Prosthetic Joint. Available: http://www.lord.com/Home/MagnetoRheologicalMRFluid/Applications/OtherMRApplic ationSolutions/Medical/tabid/3791/Default.aspx [17] K. Lee, et al., "Wearable Electro-Textiles for Battlefield Awareness," ed: Storming Media, 2004. [18] R. L. Singh, Dimitris. Design Concept for Smart, Adaptive Seat Belits. [19] A. Domeika, et al., "Unstable simulator of academic rowing," Mechanika, pp. 48-51, 2009. [20] B. Weinberg, et al., "Design, Control and human testing of an active knee rehabilitation orthotic device," 2007, pp. 4126-4133.

37 [21] S. F. Dong, et al., "Adaptive force regulation of muscle strengthening rehabilitation device with magnetorheological fluids," Ieee Transactions on Neural Systems and Rehabilitation Engineering, vol. 14, pp. 55-63, 2006. [22] R. Bartlett, Sports biomechanics: reducing injury and improving performance: Taylor & Francis, 1999. [23] Q. Ly, et al., "Towards a footwear design tool: Influence of shoe midsole properties and ground stiffness on the impact force during running," Journal of Biomechanics, 2009. [24] D. Stefanyshyn, et al., "Work and Energy Influenced by Athletic Equipment," Biomechanics and biology of movement. Champaign, IL, USA: Human Kinetics, p. 49ñ65, 2000. [25] F. Mastrandrea, et al., "Development of MR Fluid for Running Shoes with an Active Damping System," 2009, p. 153. [26] F. Denu, "Sole for footwear, especially sports footwear," ed: Google Patents, 1978. [27] D. Bramble and D. Lieberman, "Endurance running and the evolution of Homo," Nature, vol. 432, pp. 345-352, 2004. [28] C. Carr, "The Cobbler's Child. The scion of the shoemaking family takes a step in a different direction with Terra Plana," TIME, vol. 175, p. Global 10, March 8, 2010 2010. [29] H. Herr and A. Wilkenfeld, "User-adaptive control of a magnetorheological prosthetic knee," Industrial Robot: An International Journal, vol. 30, pp. 42-55, 2003. [30] C. Walsh and K. Endo, "A quasi-passive leg exoskeleton for load-carrying augmentation." [31] M. Goldfarb and W. Durfee, "Design of a controlled-brake orthosis for FES-aided gait," IEEE Transactions on Rehabilitation Engineering, vol. 4, pp. 13-24, 1996. [32] M. N. Nyan, et al., "Distinguishing fall activities from normal activities by angular rate characteristics and high-speed camera characterization," Medical Engineering & Physics, vol. 28, pp. 842-849, 2006. [33] J. Lauritzen, et al., "Effect of external hip protectors on hip fractures," Lancet(British edition), vol. 341, pp. 11-13, 1993. [34] S. Deshmukh and G. McKinley, "Adaptive energy-absorbing materials using field- responsive fluid-impregnated cellular solids," Smart Materials and Structures, vol. 16, pp. 106-113, 2007. [35] H. Herr and R. Kornbluh, "New horizons for orthotic and prosthetic technology: artificial muscle for ambulation," Smart Structures and Materials: Electroactive Polymer Actuators and Devices, 2004.

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