Nanomagnetic Elastomers for Realizing Highly Responsive Micro- and Nanosystems

Evolution has produced natural systems that generate motion and sense external stimuli at the micro- and nanoscales. At extremely small scales, the intricate motions and large deformations shown by these biosystems are due to a tipping balance between their structural compliance and the actuating force generated in them. Artificially mimicking such ingenious systems for scientific and engineering applications has been approached through the development and use of different smart materials mostly limited to microscale dimensions. To push the application range down to the nanoscale, we developed a material preparation process that yields a library of nanomagnetic elastomers with high magnetic particle concentrations. Through this process, we have realized a material with the highest magnetic-to-elastic force ratio, as is shown by an extensive mechanical and magnetic characterization of the materials. Furthermore, we have fabricated and actuated micro- and nanostructures mimicking cilia, demonstrating the extreme compliance and responsiveness of the developed materials.

The synthesis of nanomagnetic elastomers consists of the steps shown in Fig. S1.First, is the preparation of magnetite nano particles from iron salt in an aqueous solution [1][2][3] and second, coating the particles by a polymer followed by their separation from the solution and the uncoated particles and finally tuning of their concentration to yield a nanomagnetic material.The entire process is carried out in a custom-built setup inside a fumehood.The step-by-step procedure is detailed below: ❏ Ferric and ferrous chloride solutions are prepared by adding 3.64 gm of former and 2.23 gm of the latter salt to 60 ml of DI water separately in two glass beaker.The salts are taken from the glovebox in glass-vials and then directly added to the water beaker placed on a weighing scale.
❏ The salts are stirred using a glass rod for around 5 − 10 minutes and mixed together in a 200 ml capacity glass bottle/ vessel (from VWR).The mixed solution has two salts in a molar concentration ratio of 2:1 (ferric to ferrous chloride).
❏ The reaction vessel containing the solution is closed with a GL − 45 cap (from VWR) modified to hold four transfer tubes (from Tygon) and a DC motor (DCX 19 S from Maxon) at its center as shown in Fig. S2.Three of the four tubes have an external diameter of 3 mm and the fourth one has a diameter of 5 mm.The three smaller tubes extend into the vessel till around 1 cm above the final solution level while as the fourth tube stays very close to the inner surface of the cap, extending only few millimeters inside.To ensure leak-tight tubing in the cap, the holes are made 100 µm smaller than the tube diameters.The motor at the center is secured to the cap using screws with a sealing tape placed between the motor and the cap.The top side of the motor is closed off with a PMMA cap and sealed using glue to prevent leakage through the motor.
The sealing tape is prepared using a laser cutter with the right size and location of the holes for the motor shaft and the screws to pass through.The PMMA cap is made using a micro-milling machine.The motor shaft is connected to a glass stirrer through a push-fit assembly using a teflon-tape layer in between.The tube opposite the biggest diameter tube is connected to a nitrogen supply and one of the other two tubes is connected to a burette.All the tubes are fitted with a luer-lock or valve at the end.
❏ After the setup, the vessel is flushed with nitrogen for 30 minutes to reduce the oxygen concentration to a minimum level.During nitrogen flushing, only the biggest diameter tube is kept open as it removes the air by collecting it at the topmost level, being close to the inner side of the GL − 45 cap.The other two tubes are opened and closed for few seconds after every 10 minutes to ensure no oxygen is trapped in the tubes.
❏ After 30 minutes of flushing, the burette is filled with ammonium hydroxide solution and the stirrer is turned on.The stirrer speed is set between 300 − 400 rpm.
❏ Stirring for a few minutes, the solution is titrated with the ammonium hydroxide solution to precipitate the nanomagnetic particles.The amount of N H 4 OH added to the solution depends on the type of polymer to be used for coating the particles, see Fig. S1.The quantity of nanomagnetic particles produced is known by balancing the particle synthesis reaction. 4The titration is followed by stirring the solution for around 10 minutes and then a polymer is added using a glass syringe.The amount of polymer added in case of AMS-152, AMS-162 and AMS-191 is 2 ml, 2 − 3 ml and 6 ml respectively.
❏ All the tubes are closed and the nitrogen supply is stopped, isolating the environment inside of the reaction vessel from the outside oxygen-rich environment.The solution is left to stir for the next 14 − 16 hrs to allow particle-polymer bonding.
❏ With sufficient time given for bonding, the stirring is stopped and the cap is removed.
The magnetic nanoparticles along with the polymer, forming a thick black phase, are settled using a permanent magnet and the clear solution left above is decanted.
Particles that tend to run away in the end with the decanting solution are held back by letting a small volume of the solution to stay back.
❏ To separate the bonded/ unbonded particles from the extra polymer and the small amounts of the by-products, around 200 ml methanol is added to the particle-polymer complex and the content is stirred using a glass rod for 5−10minutes.The particles are again settled on a permanent magnet and the clear content left above is decanted.This step is followed two more times but with a smaller quantity of methanol ∼ 150 ml.In the third step of methanol addition, the stirring rod is rinsed/ washed with methanol and the solution is stirred without the glass rod.
❏ Continuing without the glass rod, the methanol is decanting and around 150 ml of DI water is added.Stirring turns the particle-polymer complex into lumps/ flakes floating in the added water.Use of stirring rod with water makes the lumps to stick to the rod which is difficult to rinse off later on.Stirring for a few minutes, the particles are settled again and the content left above is poured out.This step is also repeated three times.
❏ The water decanting is again followed by methanol addition and stirring for three more times, each time adding around 100 ml of methanol.Methanol addition breaks down the lumps formed during water addition into finer fragments.The extensive process of washing the particles with methanol and water removes all the extra polymer and other byproducts leaving behind all the coated as well as uncoated particles.
❏ To remove the uncoated particles, 30 ml of chloroform is added and the vessel is closed with a new cap and the content is sonicated for half an hour.Sonication enables finer dispersion of the particles in the chloroform.The solution is now poured from the 200 ml vessel to smaller vials.The coated particles make a stable suspension with the chloroform whereas the uncoated particles are settled on a permanent magnet by letting them to settle for around 1 hr.
❏ The chloroform containing the coated particles are poured into another vial and the settled particles in the first vial are dried by letting the chloroform to evaporate in a fumehood.
❏ A small quantity of the particle-containing chloroform solution (∼ 1 ml) is removed into a small glass vial (using a glass pipette) and its weight is measured.
❏ The chloroform is allowed to evaporate in a fumehood and the final quantity of coated particles left in the vial is compared with the initial quantity to calculate the weight of the coated particle content in the bulk of the chloroform solution.
❏ Knowing the quantity of particles produced from balancing the synthesis chemical reaction 4 and excluding the uncoated sediments, the quantity/ percentage of the particles and polymer in the chloroform solution is calculated.Typically, a maximum particle percentage of 40 − 50 wt% is obtained.
❏ To prepare a nanomagnetic elastomer from the ferrofluid, a required amount of ferrofluid is transferred to a vial and a measured amount of polymer is added, same as the coated polymer, depending on the concentration of the nanomagnetic elastomer to be prepared.
❏ Curing agent dicumyl peroxide equal to 10wt% of the polymer present in the chloroform solution, including the coated quantity, is added and the solution is first stirred for a few minutes with a glass rod and then sonicated for half an hour.
❏ The solution is left for evaporation until all the chloroform evaporates leaving behind a uniform mixture of particles in the polymer − a ferrofluid.
❏ The ferrofluid is either molded using polycarbonate track etched membranes (PCTE) to make cilia structures 5 or cured directly in the glass vial using a vacuum oven at 130 0 C for 8 − 9 hrs.The cured elastomer is removed from the vials and cut into rectangular sections to prepare samples of 3.5 × 2.5 × 2.5 mm for the compression test.

Text S2: Compression test setup accuracy
❏ The compression test setup consists of a motorized stage from Physik Instrumente model number M-405.DG, which is controlled by a DC motor controller C-863 from Mercury, and a force sensor from VWR with a model number LPW-303i.The minimum step size of the compressing stage is 0.001 mm, and the smallest force that can be measured is around 9.8 µN or 0.001g.These least count values of the components used are much smaller than the range of the applied strain and of the measured stress in this study, which gives the setup a high accuracy.

Figure S1 :
Figure S1: Schematic of the material synthesis process steps.Step 1: Ferric and ferrous chloride salts dissolved in deionized water.Step 2: both salt solutions are put into the reaction vessel.Step 3: precipitation of magnetite nanoparticles by titrating the salts with ammonium hydroxide in a sealed vessel.The blue arrows represent different amounts of ammonium hydroxide corresponding to different polymers used for particle coating.Step 4: coating of particles while they are being stirred continuously with the blue arrows showing the possibility to add varieties of AMS polymers (aminopropylmethylsiloxane with dimethylsiloxane), which vary in molecular weights.Step 5: removal of reaction byproducts and mainly the removal of excess polymer not used for coating through the use of solvents methanol and water.Step 6: separation of uncoated magnetic particles from coated particles suspended in chloroform.Step 7: tuning the concentration of the magnetic particles by weight with the same type of base polymer used to coat the particles.

FigureFigure S4 :FigureFigure S6 :
Figure S3: (a) A SEM (scanning electron microscopy) image of FF-AMS-191 40 wt% shows the uniform distribution of the coated magnetic particles in the polymer matrix.(b) An AFM (atomic force microscopy) image of the same magnetic polymer as in (a) obtained using phase contrast dynamic mode of the AFM.The particle size is around 20 nm.

Figure S7 :
Figure S7: Similar to the magnetization curves obtained for the polymers FF-AMS-191 and FF-AMS-162 the magnetization curves obtained for the nanomagnetic elastomer FF-AMS-152 show an increasing saturation value for higher magnetic content.

Figure S8 :
Figure S8: Similar to the linearly increasing mass magnetization values at 100 mT the saturation magnetization values also show an increasing trend for all the three nanomagnetic elastomer types with increasing magnetic concentration.