Hybrid Piezoelectric–Magnetic Self‐Sensing Actuator using Novel Dual‐Alignment Magnetic/Mechanical Processing for Vibration Control of Whole‐Body Vibrations

Multi‐stimuli‐responsive/‐functional polymeric materials can respond to numerous stimuli and execute multiple tasks, overcoming barriers faced by single‐stimuli materials. Herein, the development of hybrid piezoelectric–magnetic self‐sensing actuator (HPMSA) that can both sense and actuate is proposed. This iron oxide/functionalized carbon nanotube/polyvinylidene fluoride film optimizes both piezoelectric and magnetic properties through dual‐alignment fabrication, utilizing strong element bonds for simultaneous alignment. Magnetic nanoparticles are advantageous over nanorods due to latter's randomized shape anisotropy decreasing magnetization. The dual magnetic and mechanical processing increases polar β‐crystal content to 88%, where magnetic alignment alone increases degree of crystallinity to 66%. As a vibration damper, HPMSA operates within 40–600 Hz frequency, with a sensing sensitivity of 2.5 mV g−1 and 0.72 m s−2 weighted acceleration damping, lowering passenger health risks. Piezoelectric and magnetic relationship shows 0.19 V increase with 125 mT applied. The flexible HPMSA can integrate onto a curved surface and sense/dampen vibrations of an air motor, propeller drone, and simulated tremors. The HPMSA provides tremendous potential and understanding into multi‐stimuli‐responsive/functional materials, simultaneous alignment, and vibration control in the next generation of transportation vehicles for human safety.

requirement for actuation. [33]Despite these disadvantages, magneticstimuli-responsive property has fast response, easy actuation control, [34] and can be easily integrated into many polymer-based matrices, allowing for the fabrication of hybrid and multi-stimuliresponsive systems.This includes magnetic/shape-memory actuators, [35] magnetic hydrogel actuators, [36] and thermos-responsive magnetic nanoparticles, [37] each system utilizing magnetic-responsive property to enhance the performance of its hybrid system.
This paper proposes a novel multi-stimuli-responsive and multifunctional hybrid piezoelectric-magnetic self-sensing actuator (HPMSA) utilizing unique dual-alignment-processing technique.The HPMSA will conduct sensing through piezoelectric effect and actuation through magnetic movement.The optimization of both properties can be achieved through adequate alignment of the polar β crystals of PVDF (sensing) and the magnetic particles (actuation).Therefore, the novel dual-alignment-processing technique utilizes both magnetic and mechanical alignment to align multiple elements simultaneously.This is achieved through the creation of strong intermolecular bonding between the magnetic iron (II, III) oxide (Fe 3 O 4 ) nanoparticles, CNT, and crystals of PVDF through chemical functionalization of CNT (f-CNT).By introducing reactive hydroxyl groups to the sidewalls of CNT, the f-CNT will bond with Fe 3 O 4 and PVDF, linking each element to one another.Therefore, it is hypothesized that any alignment processing experienced by one element (i.e., Fe 3 O 4 through magnetic field) will align the other elements as well (i.e., f-CNT and PVDF crystal), successfully optimizing both piezoelectric and magnetic properties simultaneously.The proposed HPMSA is considered to be used in the application of an active vibration damper to sense and dampen excessive vibrations caused during the operation of next generation of vehicles, from electrical to aero to autonomous.[43] In this new generation of nonstop transportation, with new travel methods arising such as electric vehicles and urban air mobility (UAMs), [44] consequences of WBV are affecting beyond workers to typical passengers.Passive vibration control is not able to adapt to a constant changing environment over time, reducing its effectiveness. [45,46]Traditional mechanical active vibration systems can adapt and readjust to sense and dampen, but they are bulky and require high-power input. [47]Thus, a thin flexible single device that can both sense and dampen these harmful vibrations while easily being integrated into any transportation system is greatly beneficial.To the author's knowledge, this is the first system that utilizes piezoelectric and magnetic properties in a single hybrid film that can function as both a sensor and an actuator by utilizing different stimuli-responsive systems for an active vibration damper system.

Chemical f-CNT
CNTs have been identified as an ideal candidate for functional applications, and able to utilize functionalization treatment to modify its properties. [48][51] Nitration method allows the functionalization of large amount of material, reducing production cost of functionalized carbon nanomaterials and promoting commercialization.The chemical functionalization by nitration of single-walled carbon nanotubes (SWCNT) was processed through the fabrication method outlined in Ugalde et al., [52] employing an acid functionalization with a combination of nitric acid (HNO 3 ) and sulfuric acid (H 2 SO 4 ) under magnetic stirring and heat.The chemical functionalization of SWCNTcreated functional hydroxyl groups on its sidewalls, which allows Fe 3 O 4 nanoparticles and PVDF crystals to attach onto this sidewall through intermolecular bonding.A pristine SWCNT buckypaper and chemically functionalized SWCNT (f-CNT) were compared using Fourier-transform infrared (FTIR) spectroscopy peaks that represent multiple functional groups (Figure 1a).The pristine CNT in black shows peaks at %1520 cm À1 for C═C asymmetric stretching [53][54][55] and %1630 cm À1 for vibration stretching of C═C bond. [56,57]It also shows peaks from 2920 to 2960 cm À1 from the methyl groups located at the defect sites on the CNT. [58,59]The f-CNT shows peaks at 1030/1060 cm À1 , which is typical of C─O, C─O─C stretching, and C─OH bonds, [55,56,60] and at 3360-3400 cm À1 which confirms existence of OH bonds. [58,60]Other peaks were found at 1386 cm À1 (C─O), [54,56] 1700 cm À1 (C═O), [56,59,60] 2329 cm À1 and 3728 cm À1 (COOH), [61,62] and 2850 cm À1 (CH 2 ); [58] all confirming the successful f-CNT to create ÀOH and ÀCOOH groups.With these functional groups, the sidewalls of CNT become reactive, easily creating intermolecular bonds when Fe 3 O 4 nanoparticles and PVDF melt are added.The Fe 3 O 4 on the sidewalls of f-CNT can also be seen by SEM images, showing Fe 3 O 4 physical placement along the CNT sidewalls and a nice nanoparticle distribution.In Figure 1b, the schematic of the attachment of both Fe 3 O 4 and the hydrogen atoms of PVDF through ultrasonication is shown, along with SEM image of the (Fe 3 O 4 /f-CNT)/PVDF nanocomposite with %50 nm diameter Fe 3 O 4 nanoparticles and %29 nm diameter of CNTs.With energydispersive X-ray (EDX), the elemental mapping of (Fe 3 O 4 /f-CNT)/ PVDF can be completed, observing carbon (C) and fluorine (F) due to the CNT and PVDF, oxygen from the f-CNT, and a nice distribution of iron (Fe).

Effect of f-CNT for Alignment Potential
The purpose of f-CNT was to create reactive CNT sidewalls.This will allow the elements of PVDF to attach easily to CNT to form crystals. [30,31] This could be seen by the raw FTIR data in Supporting Information A and Figure S1, observing an increase in the intensity of the absorbance peaks for sample with Fe 3 O 4 / f-CNT in PVDF compared to CNT in PVDF or pure PVDF.Another purpose of f-CNT was to combine Fe 3 O 4 and CNT into one cohesive filler which will allow multiple-alignment-processing methods.This requires Fe 3 O 4 to move with CNT, and vice versa.To confirm this, the counterpart samples using pristine CNT were fabricated, comparing Fe 3 O 4 /f-CNT/PVDF versus Fe 3 O 4 / CNT/PVDF.Initially, the effect of embedding f-CNT was analyzed (after compression molding) before any alignment processing was applied.While the polar β-crystal content was steady around 33%, it was observed that the overall degree of crystallinity increased when using f-CNT, versus pristine CNT (Figure 2a,b).This is due to the reactive CNT sidewalls forming stronger bonds with the atoms of PVDF, along with the high aspect ratio of CNT. [29]This allowed the polymeric chains in the amorphous phase to be arranged with long CNT, resulting in the crystal phase of PVDF.It was seen that the use of f-CNT increased the degree of crystallinity by %7.4%.
The magnetic alignment processing was conducted with the nanocomposite in a melted state due to its low viscosity to minimize fabrication time. [63]A ferrite magnet directly under the 100 μm thick (Fe 3 O 4 /f-CNT)/PVDF film was used to apply a norm magnetic field of 77 mT, which was measured using a gauss meter 1.8 mm away from magnetic source (Model 460 3-Channel Gaussmeter, Lake Shore Cryotronics). [64]To support the reporting of 77 mT of magnetic field, the magnetic gradient with distance away from the ferrite magnet source was analyzed through COMSOL Multiphysics simulation, as seen in Supporting Information B and Figure S2.It was observed that within a distance of <5 mm from the magnet surface, the field of 77 mT is acceptable for reporting.While neodymium magnets can provide a higher magnetic field, due to the high temperature required during the compression molding procedure to turn HPMSA into a melt, a ferrite magnet was chosen with its high operating temperature. [65,66]To support the simultaneous movement of Fe 3 O 4 and f-CNT and PVDF crystals, the effect of magnetic field was seen as the resulting PVDF crystals, where Fe 3 O 4 would move f-CNT, which would subsequently move PVDF crystals.Comparing the crystal before and after the application of magnetic field, it can be clearly seen that there is a minimal increase for samples fabricated with nonfunctionalized CNT (Figure 2c) and a significant increase in crystals for samples fabricated with f-CNT (Figure 2d).The crystals for samples with f-CNT increase from an average of 50.7% for compression molded to 57.3% for compression molded with magnetic field.CNT is not magnetic, and therefore typically requires a high magneticfield application for alignment of 5-25 T. [67][68][69] While some literature has used various methods such as coatings, CNT alignment still requires 0.3-0.4T. [70] By creating strong intermolecular bonds between CNT and Fe 3 O 4 , CNT were able to move at a much lower magnetic field of 0.077 T due to the highly magnetic Fe 3 O 4 , which in turn aligned the chains of PVDF from its amorphous phase to crystalline phase.Samples made with nonfunctionalized CNT, there is barley an increase from 43.6% to 45.8% from compression molded to compression molded with magnetic field, respectively.Additionally, transmission electron microscope (TEM, H7500, Hitachi) was utilized to show high agglomeration with pristine CNT samples, and additional X-ray diffraction (XRD, PW1830, Phillips) analysis to further support the increase in crystallinity in the sample when using f-CNT (Supporting Information C and Figures S3  and S4). [71,72]This shows the advantages of utilizing f-CNT and magnetic fields for the enhancement of PVDF crystals, and highlights use of very low-magnetic field on CNT.
From the overall degree of crystallinity results, the effect of nanofillers were more significantly seen in samples with 40 and 60 wt% Fe 3 O 4 in f-CNT, and 0.5 and 1 wt% (Fe 3 O 4 /f-CNT) nanofiller in PVDF.Additionally, from a previous study, it was seen that samples above 1 wt% CNT saw an increase in conductivity of the material. [29]To effectively sense using piezoelectric effect, a dielectric material is ideal.Therefore, moving forward in the article, samples with 40 and 60 wt% of Fe 3 O 4 in f-CNT, and 0.5 and 1 wt% (Fe 3 O 4 /f-CNT) in PVDF, will be focused.For ease of reading, these sample names were simplified (Table 1).

Effect of Magnetic Alignment and Fe 3 O 4 Nanospheres versus Nanorods
The proposed study relies on Fe 3 O 4 nanoparticles on the sidewalls of f-CNTs to move f-CNTs in a singular direction during magnetic-field application, where this process will require a torque motion.However, theoretically, the spherical shape of the nanoparticles cannot produce a magnetic torque due to the spheres being magnetically isotropic and having no preferred magnetization direction. [73]This creates a force motion, relying on the attractive force toward the magnetic source.Therefore, Fe 3 O 4 magnetic nanorods are utilized and compared due to their shape anisotropy, allowing the nanorods to be aligned along their long axis when experiencing a magnetic field and creating a torque motion. [74]The schematic comparing the movement of magnetic nanospheres versus nanorods can be seen in Supporting Information C and Figure S5.
The effect of magnetic alignment on the HPMSA was measured using a magnet property measurement system (MPMS) at room temperature, observing the nanocomposite's magnetic property using a magnetization versus applied magnetic field (M versus H) graph.Comparing 40 wt% Fe 3 O 4 nanoparticles (spherical shape) and nanorods, the saturation magnetization, M s , of the nanorods is lower than that of nanoparticles at the same loading (Figure 3a).This is due to the magnetic anisotropy of the nanorods from its shape, which greatly affects its magnetic property. [75]As seen in Figure 3c, with the random alignment of the Fe 3 O 4 nanorods during fabrication, its alignment can either have a high-or low-demagnetization factor, N, by having its short   or long axis parallel to the magnetic field, respectively. [73,76]herefore, if the nanorods are not oriented with the long axis parallel to the external magnetic field, the high N value lowers the magnetization value in the direction of the external magnetic field. [77,78]With a nanofiller that vastly changes depending on its own alignment, the nanorods result in a filler that are more difficult to align parallel to a magnetic field compared to nanospheres. [79,80]The similar sizes of nanosphere and nanorods can be seen in Supporting Information C and Figure S6a,b for clearer comparison, showing a deformed shape for nanorods due to the ultrasonication processing.It is common for rod shapes to have a higher magnetic coercivity, H c , than spherical shapes; [81,82] however, the randomness of alignment makes this difference very small with 6 mT of difference (Supporting Information C and Figure S6c).This can also be seen by the Fe 3 O 4 nanorods' effect on the resulting crystal alignment of PVDF, after compression molding with an external magnetic field.Both the overall degree of crystallinity and the total β-crystal content are lower when using Fe 3 O 4 nanorods (Figure 3b), compared to using Fe 3 O 4 nanospheres (Figure 3e).This is more evident for samples with 60 wt% of Fe 3 O 4 , having 3%-6% decrease in total β-crystal content.The symmetry of the sphere allows the nanospheres to have the same N in any direction, and to be able to neglect their placement angle with respect to the applied magnetic field.
Additionally, with the application of magnetic field during compression molding, the nanospheres formed chains where the connected Fe 3 O 4 nanosphere chain created magnetic anisotropy generating a magnetic torque motion (Figure 3d). [83,84]This resulted in the alignment of both crystals from the amorphous phase and the polar β crystals, seeing a high content of 67% and 23%, respectively, for 1-40.This increase in crystallinity was also seen by the larger melting peaks in the raw differential scanning calorimetry (DSC) data, and further quantification utilizing XRD (Supporting Information C and Figure S4). [71,72]There were better results for samples with 40 wt% Fe 3 O 4 than 60 wt%, most likely due to the agglomeration caused by the excess Fe 3 O 4 hindering crystals.Therefore, from both magnetic property and PVDF crystals standpoint, this study continued with using Fe 3 O 4 nanoparticles of spherical shape due to the many downsides of Fe 3 O 4 nanorods from its magnetic anisotropy.This was decreased to 113 and 102 mT for 40 and 60 wt% Fe 3 O 4 , respectively, with compression molding under a magnetic field.
From this onset calculation, even with the slight increase in magnetic particles from 40 to 60 wt%, the samples could reach M sat at 10 mT lower field.By aligning magnetic particles, this was further lowered by an additional 5 mT for 60 wt% Fe 3 O 4 .This shows that there is a degree of magnetic alignment when samples are compression molded with 77 mT of magnetic field.The onset magnetic field can be seen in Supporting Information D and Figure S7.

Effect of Novel Dual-Alignment Processing-Magnetic and Mechanical
The dual-alignment-processing method utilizes magnetic alignment with an external magnetic field and then subsequent mechanical alignment by mechanical stretching method.As seen in Figure 4a-c, each alignment method increases the alignment of magnetic particles and PVDF crystals in one direction.After both processing methods, it is expected for the amorphous phase of PVDF to change into the structured PVDF crystals and see an increase in the polar β crystals (Figure 4c).Mechanical stretching of PVDF is a common method to increase polar β crystals. [85]ocusing on the effect of the alignment processes on the polar β crystals, which are responsible for the piezoelectric-sensing ability, the resulting β-crystal content after processing can be seen in Figure 4d,e.β crystals are shown before any alignment processing (no alignment, grey) to after compression molding with magnetic field applied (magnetic alignment, orange), to compression molding then mechanical stretching (mechanical stretchingmagenta), and to dual-alignment process of magnetic then mechanical alignment (magnetic þ stretching, green).The positive effect of the external magnetic field during compression molding was discussed in the previous sections, and this can be seen with the increase in β-crystal content.There is significant increase when mechanical stretching method is applied, as expected, increasing to %80%.Magnetic alignment still plays a role however, as seen by the %4% increase in β crystals when both magnetic and stretching are applied, compared to just stretching.
It is noted that while this increase was small compared to stretching, the magnetic alignment process was executed through a very low-magnetic field of 77 mT which would normally not be able to align CNT or the crystals of the polymer PVDF.This supports the success of the dual-alignment technique of magnetic and mechanical on the polar β crystals of PVDF, which in turn will enhance the piezoelectric sensing of the HPMSA.
With the current samples, there is very minimal amount of Fe 3 O 4 existing in the entire nanocomposite (0.2-0.6 wt% of nanocomposite).This is due to limiting the amount of conductive CNT in the polymer matrix to sustain the dielectric property of PVDF for enhanced dynamic sensing.Since Fe 3 O 4 is attached to the conductive CNT through functionalization, this also limited the amount of magnetic property in the nanocomposite.This led to minimal magnetic damping ability (Supporting Information E and Figure S8a).Therefore, additional Fe 3 O 4 nanoparticles were added to the (Fe 3 O 4 /f-CNT)/PVDF nanocomposites.To ensure that the newly fabricated samples with additional Fe 3 O 4 could undergo the mechanical stretching processing step and did not hinder polar β-crystal growth, mechanical and crystal testing were executed.After parametric testing with additional Fe 3 O 4 loading from 0 to 30 wt%, addition of 10 wt% Fe 3 O 4 was chosen from its ability to sustain the flexible mechanical property required for mechanical stretching (Supporting Information E and Figure S8b).To ensure that this addition of nanoparticles did not hinder the polar β crystals, before and after addition were compared, as seen in Figure 4f.It seems that the extra Fe 3 O 4 nanoparticles had minimal changes to the β crystals of PVDF, which would maintain its piezoelectric-sensing ability.Thus, sensing and damping of vibrations were using samples with þ10 wt% Fe 3 O 4 .

Sensing and Damping of Harmonic Vibration
For many years, the negative health effects of vibration have been well established in the structural and medical field, where exposure to even small amplitude vibration can lead to long-term health and structural hazards. [38]The WBV occurring in aero vehicles is one of the most severe, causing detrimental health impacts such as back problems, nerve disorders, circulatory system disorder, and even hearing loss. [40]Therefore, the reduction in WBV in aero vehicles by sensing the harmful vibrations and reducing its movement is a great start to vibration damping for other fields.UAMs are next generation of vehicles for ease and inclusive transportation but its adaptation into commercial use has not been finalized.Literature on UAMs emphasizes the importance of aircraft engine and flight modes on the resulting vibrations. [86]Thus, referencing high-frequency motors for drones (40-200 Hz), as well as helicopters due to the similarity of vertical takeoff and landing design, the frequency range for testing was chosen to be 40-600 Hz.The HPMSA film is also to be used as a vibration damper to reduce excessive vibration that is harmful to human bodies.Referring to health guidelines zones in Design Criteria Standard by the Department of Defense, [87] the weighted acceleration was chosen at 7 m s À2 since this was the zone separating "Caution" and "Health risks were likely" criteria.It is noted that both the frequency range and acceleration may change as UAMs become more developed and modified for commercial use.The HPMSA were tested at 40, 100, 200, 400, and 600 Hz at 7 m s À2 acceleration to analyze its vibration-sensing ability.
To analyze the performance of the HPMSA, a shaker system with a cantilever setup was utilized with one end fully clamped (fixed) and the free end with a proof mass (Figure 5a).The HPMSA used silver electrodes and copper tape for wire attachment and used a shaker/accelerometer closed feedback loop to apply harmonic base motion.The measured HPMSA output signal was compared to that from a laser vibrometer, both measured by an oscilloscope.Figure 5b,c compares 1-40 with þ10 wt% Fe 3 O 4 and its exact loading counterpart but using Fe 3 O 4 nanorods (R).In the previous section, the crystals analysis deemed that the Fe 3 O 4 nanorods could not align and support the development of polar β crystals as effectively as Fe 3 O 4 nanoparticles.From the piezoelectric-sensing results, this previous conclusion is additionally supported which is directly correlated to its polar β-crystal content.The voltage output from the vibration is much higher for samples using nanoparticles versus nanorods, supporting our decision to continue with nanoparticles.HPMSA needs a constant output signal over a frequency range to be an effective sensor.Therefore, its sensing sensitivity was measured using a frequency sweep from 40 to 900 Hz (Figure 5d).All samples have approximately 2.5-3 mV g À1 , where 1-60 with þ10 wt% Fe 3 O 4 has the most consistent result through this frequency range, making it the most reliable.The frequency range goes beyond what is required for UAMs (40-600 Hz), which demonstrates its degree of safety factor and potential further uses.The signals of HPMSA with 1 wt% nanofiller are lower than that of those with 0.5 wt% nanofiller, most likely due to the slight increase in conductivity from the additional conductive CNT in the polymer matrix, drifting the nanocomposite further from a dielectric material.The sensing ability of 0.5-40, and 0.5-40, 0.5-60, and 1-60 with 10 wt% can be seen in Supporting Information F and Figure S9.The consistency of 1-60 with þ10 wt% Fe 3 O 4 can also be seen by its steady output of signal of 0.017 V for 100-105 Hz at 1 Hz interval, where it is also seen with the vibrometer's constant output (Supporting Information F and Figure S10).As the modulus of samples effect the sensing performance in such a diverse bending strain environment, its mechanical property was analyzed which also supported both the reduction in crystallinity and disuse of samples with higher filler content (Supporting Information F and Figure S11).
The vibration damping performance using magnetic actuation was analyzed at 0.7 Â g acceleration and frequency range of 40-600 Hz.To induce magnetic damping, a ferrite ceramic magnet was used at a distance 2 mm away to reduce the vibration amplitude and speed, as seen in Figure 5e.The measured signal output of the undamped and damped cantilever vibration was converted into weighted acceleration using equation from ISO 2631, considering frequency and assuming z-axis unidirectional vibration. [88]Damping magnitude increases with frequency, most likely due to the decrease in amplitude (Figure 5f ).The sensing ability of 0.5-60 with 10 wt% Fe 3 O 4 has the greatest damping magnitude across all frequencies from 40 to 600 Hz, achieving maximum of 0.72 m s À2 difference in weighted acceleration with magnetic damping.Using the health guidelines for WBV for passengers as a reference, the undamped (U) and damped (D) weighted acceleration could be plotted (Figure 5g).Depending on the exposure time, the decrease in weighted acceleration does shift from "Health Risks are Likely" zone to "Caution" zone.Additionally, it is noted that the HPMSA is 0.4% mass of the substrate it is damping, highlighting its high damping ability in terms of its small size.This supports the positive damping results of HPMSA film to reduce safety risks of passengers in aircrafts.
It was noted that the damping magnitude measured by the vibrometer and HPMSA were significantly different.It is proposed that with the application of a magnetic field, the magnetic particles move with this field causing nanocomposite deformation.This deformation changes the polarity of the material itself, creating movement of electrons and additional voltage generation.This phenomenon of magnetoelectric effect (Figure 6a) will need to be considered for future studies to apply the vibration damper in an active loop.This is to ensure that after the first feedback signal, the enhancement of piezoelectric effect from the external magnetic field is considered in the calculations.The changes in the piezoelectric signal output from an applied magnetic field can be seen in Figure 6b.It was observed that at 0 mT of magnetic field, HPMSA film has 0.38 V of output.When there is 56, 77, and 125 mT of magnetic field on HPMSA, this output voltage increases to 0.39, 0.51, and 0.57 V, respectively.

Demonstration
The HPMSA thin film proposed can be used in many applications that require sensing and damping of its vibrations.As a demonstration of the dual sensing and actuation abilities, HPMSA was attached to an air compressor and a prototype UAM to sense and dampen their vibrations during operation (Supporting Information G Video S1 and Figure S12, respectively).For the UAM, only one of the propellers was turned on for operation to ensure that the vehicle would not fly away during the measurement process.Figure 7a,b shows the vibrationsensing results from both HPMSA and the vibrometer of the air compressor and UAM.The vibrometer shows more noise compared to the HPMSA, indicating that while the vibrometer is very sensitive, it is detecting too much for a vibration that covers a wide range of frequencies.In contrast, the HPMSA gave clean vibration readings with minimal noise from both the air compressor and the UAM.The HPMSA detected the important frequencies and was able to dampen this vibration to lower its This includes the HPMSA sample on a cantilever with silver electrodes, copper tape for wire attachment, and proof mass, which is clamped onto the cantilever beam setup.The cantilever beam setup comprises a shaker and an accelerometer.The accelerometer provides feedback control of the shaker through the computer interface and controller to ensure the correct frequency and acceleration is applied.The signal from the HPMSA is read through the oscilloscope after going through the operational amplifier.To verify the vibration measured by HPMSA, the vibrometer with a laser sensor is measured using an oscilloscope.b) HPMSA output sensing signal from 1 to 40 with þ10 wt% Fe 3 O 4 nanoparticles, and c) 1-40 with þ10% Fe 3 O 4 using nanorods.d) Sensing sensitivity from 40-900 Hz of HPMSA with various loading.e) Schematic of the effect of magnetic damping of HPMSA cantilever vibration, reduce vibration amplitude.f ) Damped weighted acceleration using 77 mT of magnetic field 2 mm away from HPMSA.The damping performance is measured using a vibrometer.g) The resulting damped weighted acceleration on the health guidelines for whole-body vibration, comparing undamped (U) and damped (D) signal.
amplitude.While the HPMSA is geared toward transportation systems, its use can go beyond this field, such as the medical industry.Essential tremor (ET), for example, is a common movement disorder that hinders the use of hands and forearms due to its involuntary shaking, with a frequency of 6-12 Hz with large variations amplitude. [89,90]To best mimic ET, a prosthetic hand with a spoon to simulate eating was attached to the shaker system.With the HPMSA thin film attached to the handle of the spoon, the entire system was excited at 12 Hz and 3 Â g of acceleration.
The HPMSA could effectively sense this distinct vibration and dampen it (Figure 7c and Supporting Information G Video S2).
There are many advantages to the HPMSA thin film, especially its flexibility and conformability that allows the device to be applied to any surface geometry.Figure 8a showcases a curved cantilever with a curved HPMSA film attached.The shaker was excited at 40 Hz at 0.7 Â g acceleration in the z-axis (up and down).However, with the curved shape, we expect there to be additional movement in the plane parallel to the ground (sideways) and other axis due to the proof mass at the end.The vibrometer was not able to get a clear reading due to its laser reflecting off at non-180°angles.This led to peaks at various frequencies with enhanced noise (Figure 8b).The vibrometer was  also seen as unreliable when the vibrating object moved during operation, resulting in high noise (Supporting Information G Video S3 and Figure S13).The HPMSA film was able to achieve a clearer reading of this randomized vibration and was also able to dampen this vibration at 40, 100, 200, and 600 Hz (Figure 8c-f and Supporting Information G Video S4).

Conclusion
This paper reports a novel multi-stimuli-responsive and multifunctional self-sensing actuator fabricated with a dual-alignment technique to be used for vibration sensing and damping of nextgeneration vehicle vibrations.This HPMSA thin film utilized a piezoelectric polymer matrix embedded with functionalization of CNTs (f-CNT) attached with iron (II, III) oxide magnetic nanoparticles.Due to the intermolecular bonds created between the crystals of PVDF, f-CNT, and Fe 3 O 4 , the dual-alignment technique relied on the alignment of one component to align the rest.It was observed that with the successful oxidation f-CNT using acid treatment, the crystals of PVDF became more aligned from an external magnetic field of 77 mT, increasing overall degree of crystallinity.The nonmagnetic property of both f-CNT and PVDF leads to the conclusion that the Fe 3 O 4 nanoparticles were aligned by the low-magnetic field, which in turn aligned f-CNT, which then aligned the crystals of PVDF.The shape effect of Fe 3 O 4 nanosphere (nanoparticles) versus nanorods was compared, where nanospheres were more effective in alignment of the PVDF crystals and enhancement of magnetic property.This was due to the magnetic anisotropy caused by the shape asymmetry of nanorods which was hindered by the randomized placement of the nanorods in the nanocomposite matrix.This led to high-demagnetization factor parallel to the applied magnetic field, and lowered PVDF-crystal content and magnetization saturation compared to nanoparticles.With both magnetic and mechanical alignment processing, the polar β-crystal content increased to a maximum of 88% and was %5% greater compared to just mechanical stretching processing.This supported both the effect of magnetic alignment and the advantage of having both alignment-processing methods.To increase magnetic damping effect, 10 wt% of Fe 3 O 4 nanoparticles were added to the (Fe 3 O 4 /f-CNT)/PVDF nanocomposite film, taking into account its Young's modulus to allow ease of mechanical stretching fabrication method without hindering PVDF-crystal content.To target the vibration sensing and damping of next generation of vehicles such as UAMS, a frequency range of 40-600 Hz with 0.7 Â g acceleration was chosen, following health guidelines for WBV.It was observed that 1-60 with þ10 wt% Fe 3 O 4 sample had the most consistent voltage output per acceleration for sensing (2.5 mV g À1 ).Additionally, the sensing ability of samples with nanosphere had lower voltage output compared to the same loading counterpart using nanoparticles, further supporting the use of nanoparticles.It was observed that 0.5-60 with 10 wt% Fe 3 O 4 has the greatest damping magnitude across all frequencies from 40 to 600 Hz, achieving maximum of 0.72 m s À2 difference in weighted acceleration with magnetic damping using 77 mT of magnetic field.This lowered the weighted acceleration from "Health Risks are Likely" zone to "Caution" zone, successfully increasing safety of passengers of vibrating vehicles.The relationship between magnetic and piezoelectric property in one hybrid material was a positive one, with the piezoelectric signal output increasing from 0.38 V for 0 mT of magnetic field to 0.57 V with 125 mT of magnetic field.As a demonstration, the HPMSA was used to sense and dampen the vibration during the operation of an air compressor and a prototype UAM, showing clear sensing and damping ability compared to the noisy vibrometer output.The robust application of HPMSA was showcased by testing a prosthetic hand mimicking ETs at 12 Hz, successfully sensing and damping this low frequency.Lastly, the flexibility and conformability which leads to ease of integration of HPMSA was demonstrated by placing this film onto a curved cantilever, and successfully sensing and damping vibrations.This paper reports the clear success of a novel dual-functional thin film that is both a sensor and an actuator in one single flexible device by utilizing low power and dual-alignment fabrication methods.Its demonstrations provide its potential beyond the next generation of vehicles but for any vibration systems, such as health care.The HPMSA thin film will bring further insight into multifunctional hybrid systems, and for the advancement of vibration control for the next generation of vehicles.
Chemical f-CNTs: The chemical functionalization by nitration of SWCNTs were processed through the fabrication method outlined in Ugalde et al., [52] SWCNTs were mixed with a combination of nitric acid (HNO 3 , 60% concentration, Sigma-Aldrich) and sulfuric acid (H 2 SO 4 , 98% concentration, Sigma-Aldrich) at a 1:3 volume ratio, respectively This mixture was magnetically stirred at 70 °C on a hot plate for 4 h.Afterward, the functionalized SWCNTs (f-CNTs) were filtered and washed with deionized water until pH of 7.
Fabrication of Fe 3 O 4 /f-CNT/PVDF with Dual-Alignment-Processing: For the fabrication of the HPMSA, f-CNTs were mixed in DMF solvent, dissolved PVDF, and Fe 3 O 4 nanofillers using ultrasonication (Q500, Qsonica) at 100 W for 10 min.The dispersed Fe 3 O 4 /f-CNT/PVDF mixture was coagulated in distilled water and dried to evaporate the water.The coagulation method allowed for immediate solidification of the polymer matrix with the fillers, minimizing filler aggregation. [91]The dual-alignment process utilized both magnetic and mechanical stretching where both processes would indirectly and directly align the crystals of PVDF and magnetic particles.The alignment of magnetic particles was conducted with the polymer in a melted state.The coagulated Fe 3 O 4 /f-CNT/PVDF were compression molded at 180 °C for 5 min, then applied 5000 lbs-f for 10 min, all under an external magnetic field of 77 mT using a ferrite magnet.The magnetically aligned Fe 3 O 4 /f-CNT/PVDF thin film was cut into rectangular pieces of 8 Â 20 mm and mechanically stretched using a tensile tester (5848 MicroTester, Instron) to 500% stretch ratio at 80 °C.The initial weight percentage (wt%) of Fe 3 O 4 to f-CNT were 20, 40, and 60 wt%.The initial wt% of Fe 3 O 4 /f-CNT nanofiller in PVDF was 0, 0.1, 0.5, 1, 1.5, and 2 wt%.This was considered initial wt% due to the narrowing of scope after analysis of each processing results and various characterization tests.TEM (H7500, Hitachi) was utilized to observe and compare agglomeration for samples containing pristine CNT versus f-CNT.
Piezoelectric Crystals: FTIR (Bruker Alpha) was used to analyze the effect of acid treatment for functionalization of SWCNTs, and the effect of each processing step on the crystals of PVDF matrix.Infrared spectra of PVDF nanocomposites were obtained by averaging signals from 32 scans from 400 to 1500 cm À1 at a resolution of 4 cm À1 .To distinguish the different PVDF-crystals phases, namely the nonpolar α and polar β crystals, the polar β-crystal content was calculated using Equation (1) where F(β) is the β-crystal content; and A β and A α are the absorption bands of β and α crystals, respectively.These absorption bands were located at 840 and 766 cm À1 for β and α crystals, respectively. [29,92]DSC (Q20, TA Instrument) was used to characterize the degree of crystallinity (χ c ) of the semicrystalline PVDF nanocomposite.Thermal analysis was conducted from 40 to 220 °C at a rate of 10 °C min À1 .χ c was calculated using Equation ( 2) where ΔH f is the heat of fusion of the PVDF nanocomposite sample being tested, and ΔH f,c is the heat of fusion of 100% crystalline PVDF sample (104.7 J g À1 ). [93]XRD (PW1830, Phillips) was utilized to further support the analysis of crystallinity in the nanocomposite, analyzing from 2θ = 10°-45°a t a scanning step of 0.02°.To quantify the crystallinity of the nanocomposite, the area under the peaks was used along with Equation ( 3) where S c and S a are the total areas of crystalline and amorphous peaks, respectively. [94]echanical and Magnetic Property: Mechanical properties were analyzed using a tensile tester (5848 MicroTester, Instron) for Young's modulus following ASTM standards.The degree of Fe 3 O 4 alignment was qualified by quantifying the magnetic property of the nanocomposite through using Quantum Design Magnetic Properties Measurement System (MPMS XL, Quantum Design).With the MPMS, the magnetic profile of the material was seen with changes in applied magnetic field at room temperature (300 K).The MPMS measures in one direction, and therefore the direction parallel to the theorized alignment direction was measured.
Sensing and Damping: The cantilever beam setup for HPMSMA vibration performance testing was attached to a vibration setup consisted of a shaker/ accelerometer closed feedback loop.The shaker (The Modal Shop Inc. 2075E) provided harmonic base motion to the entire cantilever beam setup.Input parameters of acceleration and frequency were controlled through a computer interface (DAC-8) with feedback signals from an accelerometer on the cantilever beam setup (Kistler Instrument Corp.) for closed-loop control.Voltage signal generated by the HPMSA went through an operational amplifier (LM358, Dual OpA, Onsemi) and was gauged by an oscilloscope (Tektronix 3014).To verify the vibration parameters induced in the cantilever, a laser vibrometer (OFV-534, Polytec Inc.) was used to measure the velocity of the proof mass.This signal was processed with the oscilloscope.

Figure 1 .
Figure 1.a) Schematic of chemical functionalization of single-walled carbon nanotubes (SWCNT) to create hydroxyl groups on carbon nanotube (CNT) sidewalls.Fourier-transform infrared (FTIR) spectroscopy result of pristine versus functionalized CNT (f-CNT)*.Scanning electron microscope (SEM) image of Fe 3 O 4 attached to the sidewalls of f-CNT at 10 000Â magnification.b) Schematic of f-CNT mixed with magnetic Fe 3 O 4 nanoparticles and piezoelectric polymer polyvinylidene fluoride (PVDF) melt, with the attachment using intermolecular bonding to the sidewalls of f-CNT.SEM images of (Fe 3 O 4 /f-CNT)/PVDF at 200 000Â magnification.Elemental mapping of (Fe 3 O 4 /f-CNT)/PVDF using energy-dispersive X-ray (EDX) showing the presence of oxygen and even distribution of elements and Fe 3 O 4 .*Note that the signal for pristine CNT in (A) has been increased for ease of viewing.

Figure 2 .
Figure 2. PVDF degree of crystallinity with compression molding with Fe 3 O 4 in a) CNT and b) f-CNT; PVDF degree of crystallinity with compression molding under magnetic field of 77 mT with Fe 3 O 4 in c) CNT and d) f-CNT.

Name (Fe 3
O 4 /f-CNT) nanofiller wt% in PVDF Fe 3 O 4 wt% in f Figure 3f shows 0.5 wt% nanofiller/PVDF with either 40 or 60 wt% loading of Fe 3 O 4 nanoparticles.There is a slight increase in the saturation magnetization, M s , from 40 to 60 wt% Fe 3 O 4 .This increase is minimal because these samples contain 0.5% of the 40 or 60 wt% of Fe 3 O 4 , making the magnetic difference negligible.The small difference in M s is most likely due to agglomeration of fillers creating an uneven distribution within the 5 Â 5 mm sample characterized in the MPMS system.After compression molding under a magnetic field (denoted as "M"), both 40 and 60 wt% samples reach M s faster.By calculating its onset point, it was seen that with just compression molding 40 and 60 wt% Fe 3 O 4 reached M s at 118 and 107 mT, respectively.

Figure 3 .
Figure 3. Magnetic property and resulting crystals of HPMSA film.a) M versus H graph of 0.5-40 using Fe 3 O 4 nanoparticles versus nanorods after compression molding under an applied magnetic field of 77 mT (denoted as "M").b) Total polar β and nonpolar α crystals of PVDF using Fe 3 O 4 nanorods after compression molding with magnetic field.c) Schematic of high-and low-demagnetization factor (N) depending on the rod orientation to the external magnetic field.d) Schematic of Fe 3 O 4 nanoparticles forming a chain during fabrication and creating asymmetry and magnetic torque.e) Total polar β and nonpolar α crystals of PVDF using Fe 3 O 4 nanospheres after compression molding with magnetic field.f ) M versus H graph of 0.5-40 and 0.5-60 using nanoparticles (spheres) after compression molding, and compression molding under an applied magnetic field of 77 mT (denoted as "M").

Figure 5 .
Figure5.a) Vibration-sensing and damping test setup.This includes the HPMSA sample on a cantilever with silver electrodes, copper tape for wire attachment, and proof mass, which is clamped onto the cantilever beam setup.The cantilever beam setup comprises a shaker and an accelerometer.The accelerometer provides feedback control of the shaker through the computer interface and controller to ensure the correct frequency and acceleration is applied.The signal from the HPMSA is read through the oscilloscope after going through the operational amplifier.To verify the vibration measured by HPMSA, the vibrometer with a laser sensor is measured using an oscilloscope.b) HPMSA output sensing signal from 1 to 40 with þ10 wt% Fe 3 O 4 nanoparticles, and c) 1-40 with þ10% Fe 3 O 4 using nanorods.d) Sensing sensitivity from 40-900 Hz of HPMSA with various loading.e) Schematic of the effect of magnetic damping of HPMSA cantilever vibration, reduce vibration amplitude.f ) Damped weighted acceleration using 77 mT of magnetic field 2 mm away from HPMSA.The damping performance is measured using a vibrometer.g) The resulting damped weighted acceleration on the health guidelines for whole-body vibration, comparing undamped (U) and damped (D) signal.

Figure 6 .
Figure 6.a) Voltage generation from an external magnetic field in a material that contain both piezoelectric and magnetic properties due to the mechanical deformation.b) Piezoelectric signal output in voltage when the sample is at rest, and exposed to an external magnetic field of 56, 77, and 125 mT.

Figure 7 .
Figure 7. Application of HPMSA for vibration sensing and damping.a) Air compressor vibration sensing and damping (with 77 mT) showing output signal from HPMSA and vibrometer.b) Urban air mobility (UAM) propeller vibration sensing and damping (with 77 mT) showing output signal from HPMSA and vibrometer.c) Simulating essential tremors with 12 Hz frequency and 3 Â g acceleration, and the resulting damping (with 77 mT) with HPMSA thin film.

Figure 8 .
Figure 8. Vibration-sensing and damping results of curved cantilever.a) Curved cantilever setup with curved HPMSA film attached, and the laser from vibrometer reflecting into the cantilever holder.b) The sensing results of HPMSA and vibrometer when exciting the curved cantilever at 40 Hz and 0.7 Â g acceleration in the z-axis (up and down).The sensing and damping output signal of HPMSA at 0.7 Â g acceleration and frequencies of c) 40 Hz, d) 100 Hz, e) 200 Hz, and f ) 600 Hz.

Table 1 .
Simplification of sample loading.