Flexible, Bending Stable, and Biocompatible Silk Fibroin/NiFe Films for Bio‐Integrated Microwave Applications

Integration of magnetic materials into bio‐integrated flexible electronics will bring multi‐functionality, such as magnetoception and wireless communication. In practical application, the performance of magnetic materials and devices should remain stable during cyclic bending–unbending and they can be degradable within a designated lifetime. However, direct integration of magnetic materials with flexible and biocompatible materials and maintaining mechanically stable microwave performance have not been achieved yet. Here, ultrathin (5–50 nm) permalloy (NiFe) films deposited on biocompatible silk fibroin (SF) films are shown to be bending stable, and dissolvable. The ferromagnetic properties of NiFe films deposited on SF, Kapton, and silicon are studied and compared by ferromagnetic resonance. Benefiting from the low surface roughness of fibroin, which is only ≈0.8 nm, SF/NiFe films exhibit comparable ferromagnetic properties to Si/NiFe, better than that of Kapton/NiFe films. Moreover, the ferromagnetic resonance field of SF/NiFe films remains highly stable during cyclic bending that is exposed to both compressive and tensile strain/stress, and its temperature stability is very close to that of Si/NiFe. Finally, the whole SF/NiFe films are demonstrated to be dissolvable and biocompatible. Therefore, SF/NiFe films can be a useful and stable platform for flexible and biocompatible microwave applications.


Introduction
Bio-integrated flexible electronics are taking the place of traditional solid-state electronics in unconventional environments at biotic/abiotic interfaces.3][4][5] Unlike traditional solid-state electronics that are based on rigid substrates and packaged up to isolate from the external environment, bio-integrated flexible electronics often work in unconventional and complicated environments inside the body, and they are usually implanted and attached to active tissues or organs with curved surface. [6]To maintain stable performance and reduce the chance of suffering from surgery, lightweight, mechanically stable, and biocompatible electronic devices are always required.

X. Zhang
State Key Laboratory for Manufacturing Systems Engineering, The Key Laboratory of Biomedical Information Engineering of Ministry of Education, Center for Mitochondrial Biology and Medicine, School of Life Science and Technology, International Joint Laboratory for Micro/Nano Manufacturing and Measurement Technology, Xi'an Key Laboratory for Biomedical Testing and High-end Equipment Xi'an Jiaotong University Xi'an, Shannxi 710049, China E-mail: xiaohuizhang@mail.xjtu.edu.cnIntegration of magnetic materials and introduction of magnetism into flexible electronics will bring multi-functionality, such as sensing external stimuli, [7] magnetoception, [8] and RF/microwave applications. [9]Various magnetic devices were fabricated on flexible substrates, such as giant magnetoimpedance sensors, [7] giant magneto-resistance sensors, [8,[10][11][12][13][14] and anisotropic magneto-resistance sensors, [15] antennas, [4,9] as well as wireless sensors based on ferromagnetic resonance. [16,17]][20] However, those reported flexible magnetic devices are either not biocompatible or very sensitive to external strain that are unable to maintain stable performance during practical application.][12] Although a few polymers mentioned above are biocompatible, their low degradability in biological systems and natural environment still restricts their application of implantable and transient electronics. [4,22]lexible, biocompatible substrates are necessary to supporting and encapsulating the magnetic devices.Silk fibroin (SF), an abundant natural biomaterial, provides an appealing platform for developing biocompatible and implantable devices in bio-integrated electronics. [6,23]It is lightweight, flexible, and mechanical robust, enabling conformal contact between flexible electronics and tissues/organs. [6][26] Hence, SF films could serve as both passive supporting and packaging/sealing material.
Magnetic devices on flexible substrates should be bendingstable and mechanically robust, which means the devices are insensitive to external strain/stress.This is mainly determined by the magnetostriction of magnetic materials and they are limited to very few oxides and metals that have small magnetostriction, such as yttrium iron garnet, [27] lithium ferrite, [28] and permalloy. [29]Ferromagnetic oxide thin films are usually deposited at high temperature >500 °C, but SF films cannot withstand such a high temperature.Meanwhile, ferromagnetic oxides are usually chemically stable and very difficult to degrade in the natural environment.Hence, they are unsuitable for transient bio-integrated electronics.Permalloy, on the other hand, is a widely used soft metallic ferromagnet with ultralow magnetostriction very close to zero. [30]Flexible magnetic sensors based on permalloy exhibit stable sensing performance. [8,15]However, the relatively large surface roughness of commercial available polymer substrates weakens the sensing performance, and a buffer layer was needed to reduce the surface roughness and enhance the performance. [15][34] However, there is still a lack of research on direct large-area deposition of magnetic materials on SF substrate to realize biocompatible, controlled degradation magnetic devices.
In this study, typical soft magnetic materials NiFe ultrathin films were directly deposited on SF films by magnetron sputtering.Both the static and dynamic magnetic properties were examined and they are comparable with those that grow on rigid silicon substrates and better than that on flexible polyimide substrates.The microwave magnetism of SF/NiFe films is highly stable during bending with a smallest radius of curvature ≈2.5 mm.They are demonstrated to be biocompatible, making them a useful and stable platform for flexible, biocompatible microwave applications.

Results and Discussion
Figure 1a shows the process of preparing SF/NiFe films for flexible electronics.In the beginning, the SF solution was prepared by a well-established procedure. [4]A small amount of SF solution was then dropped and cast to spread out on a flat poly(methyylmethacrylate) (PMMA) substrate.After well crystallization in air, uniform SF films with thickness ≈16-80 μm were obtained, and the thickness could be controlled by the amount of SF solution per area.Figure 1b shows one flexible, transparent, and stretchable sample with a large area over 14 cm 2 and it was ready for direct deposition of ultrathin NiFe (5-50 nm) films by magnetron sputtering.Patterned magnetic thin films were prepared by shadow mask.Later, the SF/NiFe films could be easily peeled off from PMMA substrates.Figure 1c shows optical images of SF/NiFe films in bent states.They were highly flexible and a small radius of curvature (ROC) ≈300 μm was demonstrated, corresponding to a bending strain ≈6.3 ± 0.4% for a 37 μm SF films.
Kapton is a commonly used flexible substrate in many studies and has shown great potential in biocompatible applications. [35]n this study, we used Kapton as the reference material for comparison.We assessed the surface roughness of both Kapton and the SF film before and after NiFe deposition using atomic force microscopy.Figure 1d,e shows the surface morphology of SF films and Kapton , respectively.When examining a 5 × 5 μm area, we found that the SF film exhibited a similar roughness (rootmean-square roughness Rq = 0.802 nm) compared to Kapton (Rq = 1.02 nm).The surface roughness of the SF film and Kapton after NiFe deposition with varying thicknesses is presented in Figures S1 and S2 (Supporting Information), respectively.These results indicate that the roughness of the samples remains comparable, suggesting similar magnetic properties in the NiFe layer.Figure 1f is the X-ray diffraction (XRD) curve of the SF/NiFe film, which reveals a (111) texture in the NiFe layer, indicating a face-centered cubic (BCC) structure.This structural characteristic contributes to the ferromagnetic metallic properties of the NiFe layer on the SF film.
The dynamic ferromagnetic properties of SF/NiFe films were examined by broadband ferromagnetic resonance (FMR), [36] sweeping magnetic field at a microwave frequency (f) ranging between 3 and 12 GHz.Figure 2a shows a schematic illustration of broadband FMR measurement based on a coplanar waveguide.Typical FMR spectra of SF/NiFe(20 nm) films are presented in Figure 2b, from which the resonance field (H r ) and the linewidth (ΔH) were extracted.Figure 2c,d shows, respectively, the H r -f and ΔH-f curves of SF/NiFe films with different thickness of NiFe layer ranging between 5 and 50 nm.The effective magnetization (4M eff ) and Gilbert damping constant () were then analyzed by the following Kittel equations: where  is the gyromagnetic ratio and ΔH 0 is the intrinsic linewidth.The FMR performance of NiFe thin films deposited on silicon and Kapton was also measured.The obtained 4M eff and  as a function of NiFe thickness were plotted in Figure 2e,f, respectively.The effective magnetization 4M eff increased and the damping constant decreased with increasing thickness of the NiFe layer on all three kinds of substrates.This is consistent with previous observations due to the surface effect in ultrathin ferromagnetic films. [37]For NiFe films deposited on silicon substrate, when the thickness increased from 5 to 50 nm, 4M eff increased from 8442 to 10 550 Oe, and it increased from 8104 to 10 368 Oe on SF/NiFe films.For the same thickness of NiFe layer, the SF/NiFe films exhibited larger 4M eff than Kapton/NiFe films, which is mainly due to the smoother surface of SF films as observed in Figure 1d.The damping constant decreased from 8.7 × 10 −3 to 6.1 × 10 −3 for Si/NiFe films and from 9.8 × 10 −3 to 6.8 × 10 −3 for SF/NiFe films.Those values are considerable to previous reports with similar thick NiFe layers. [38]Figure 2 confirms that SF films are useful substrates for direct deposition of ultrathin ferromagnetic films and it is even better than traditionally widely used polyimide substrates.
The flexible SF/NiFe films exhibit very stable microwave magnetism during the bending test.Figure 3 shows both narrowband and broadband FMR measurement results of SF/NiFe(20 nm) films exposed to compressive or tensile bending strain/stress.Figure 3a shows the in-plane field-sweep FMR spectra with decreasing radius of curvature (ROC), and no obvious change in their shape or shift of the spectra could be observed.Such a narrowband FMR measurement was conducted in a microwave resonance cavity with high-quality value and the SF/NiFe films were stuck onto round PTFE molds with different ROC from 25 to 2.5 mm (the inset of Figure 3b) and the NiFe layer were exposed to compressive strain/stress.Figure 3b compares both FMR field and linewidth with respect to the radius of curvature, and confirms that they remain highly stable during bending.The FMR field decreased very slightly from 1016 to 1013 Oe (<0.3%) while the linewidth remains 46.8 Oe.We also confirm its stable performance during bending by broadband FMR under different microwave frequencies and a bended coplanar waveguide was used (the inset of Figure 3c).Figure 3c shows FMR fields with respect to the microwave frequency for SF/NiFe(20) films and Figure 3d shows corresponding frequency-dependent linewidth.Again, no obvious change was observed in both curves over a wide frequency range from 3 to 12 GHz which means both 4M eff and Gilbert damping constant  remains stable during the bending test.Figure 3e compares the in-plane magnetic hysteresis loops of the SF/NiFe films at the flat/unbent state and the bended state with ROC = 2.5 mm.Both the coercive field (H c ) and the remnant magnetization (M r ) seem to be nearly unchanged during bending when SF/NiFe films are exposed to tensile strain/stress.For practical applications, mechanical retention is also important for the flexible RF/microwave electronics.Here, we examined this property in SF/NiFe(20 nm) films by recording the FMR spectra over a bending time up to 24 h and the results are shown in Figure 3f.Both the FMR field and the linewidth are nearly invariable after 1 day, demonstrating excellent mechanical retention characteristics of the SF/NiFe films.The durability of the SF/NiFe film (50 nm) under bending cycles was assessed.The samples were subjected to automatic bending on a translation stage at a frequency of 0.5 Hz, with a minimum bending radius of ≈2.5 to 3 mm.Figure 3g shows the FMR curve of the SF/NiFe film under various bending cycles, while Figure 3h illustrates the shift in the FMR field corresponding to these bending cycles.Notably, the alteration in the FMR field remained <1% even after 15000 bending cycles.The result indicates a good durability of the SF/NiFe film.
The highly bending-stable and good microwave magnetism of SF/NiFe films benefits mainly from the low roughness of SF films (Figure 1d) and the ultralow magnetostriction of permalloy thin films which is only 1 ppm. [37]It seems that the adhesion between NiFe and SF is good and avoids exfoliation during cyclic bending.In this study, the bending strain-induced shift of FMR field could be estimated by: [39] where  s is the magnetostriction of permalloy films,  is the bending stress-strain/stress, and M s is the magnetization of permalloy films.The maximum bending strain in NiFe layer depends on many factors, such as the thickness of NiFe, the thickness of SF film, and its Young's modulus.Here, we have measured the elastic properties of SF film and the strain-stress curve  is shown in Figure S4 (Supporting Information), revealing its Young's modulus ≈1.28 GPa, close to the previous report. [37]The bending strain with respect to the radius of curvature is shown in Figure S5 (Supporting Information), and the maximum bending strain in the NiFe layer is far <1%.Even if we consider a magnetostriction up to −1 ppm, the maximum shift of FMR field was only ≈3 Oe, consistent with the observation of this study.
We further demonstrate that the temperature stability of flexible SF/NiFe films is very close to that of rigid Si/NiFe. Figure 4 shows temperature-dependent narrowband FMR measurement results.Figure 4a,b compares FMR spectra in the temperature range −50 to 50 °C for SF/NiFe and Si/NiFe films, respectively.For both samples, their FMR field shifts upward by rising temperature which is mainly due to the decrease of 4M eff .Figure 4c,d shows temperature-dependent FMR field shift and linewidth, respectively.For Si/NiFe films, H r increased nearly linearly from 939.8 (−50 °C) to 966.4 Oe (50 °C) with a slope of 0.264 Oe K −1 and corresponding ΔH increased from 34.47 to 37.53 Oe.For SF/NiFe, H r increased also nearly linearly from 990.7 (−50 °C) to 1021.6 Oe (50 °C) with a slope of 0.302 Oe K −1 and corresponding ΔH increased from 44.02 to 46.05 Oe.The temperature coefficient of SF/NiFe is very close to that of Si/NiFe, demonstrating its good temperature stability.SF/NiFe films also have good dissolvability and biocompatibility.Apparently, SF/NiFe films could degrade rapidly in water within 24 h (Figure 5a).To extend the working life of the devices prepared by SF/NiFe, they could be encapsulated in chemically modified SF (see Experimental Section).As a result, the dis-solution became slow in chemically modified SF/NiFe/SF films (Figure 5b).For them, there was no obvious dissolution within 24 h as compared to rare SF/NiFe films.The biocompatibility of SF/NiFe films was demonstrated by testing the cell viability that directly culture cells on the surface of ferromagnetic thin films.Figure 5c,d shows living and dead cells after 72 h of culture on the surface of glass/NiFe and SF/NiFe, respectively.The majority are living cells (green ones) and very few dead cells (red ones) could be observed.The cell viability was revealed to be 93.5 ± 0.7% and 99%, respectively.They are at a level comparable to the survival rates observed in the bare SF film groups (shown in Figure S7, Supporting Information).The flexible, bending stable, and biocompatible SF/NiFe films offer exciting possibilities for bio-integrated microwave applications.Their unique combination of properties opens up new avenues for the development of innovative devices that seamlessly integrate with biological systems, enabling advanced healthcare, communication, and environmental monitoring.

Summary
In summary, ultrathin NiFe (5-50 nm) films were directly deposited on SF films with low roughness (0.802 nm).SF/NiFe films exhibited considerable ferromagnetic properties (4M eff and damping) close to Si/NiFe films, better than that of Kapton/NiFe films.The microwave magnetism of SF/NiFe films was highly bending-stable when NiFe layer was exposed to both compressive and tensile strain/stress, together with stable mechanical retention performance.In addition, their temperature stability was very close to that of Si/NiFe over a wide temperature range from −50 to 50 °C.Moreover, the whole SF/NiFe films are demonstrated to be biocompatible, and the degradation could be modulated by encapsulation of chemically modified SF.Hence, SF/NiFe films could be a useful and stable platform for flexible and biocompatible microwave applications.

Experimental Section
Preparation of SF Films: First, Bombyx mori silkworm cocoons were cut into ≈10 cm in length and boiled in 0.02 m Na 2 CO 3 solution for 20 min to wipe off sericin proteins.Then rinsed the SF in deionized water, and squeezed out the excess water.After natural drying overnight, the SF was sufficiently dissolved in 9.3 m LiBr solution at 60 °C for 4 h to make the SF rehydration and then dialyzed in deionized water for 4 days with replacing deionized water every 12 h.The post-dialysis SF solution was then centrifugal with 10 000 rpm twice to remove insoluble impurities.Finally, postdialysis SF solution (5 to 7 wt.%) was slowly dripped onto polyethylene substrates and dried one day in the ultra-clean room to produce ≈50 μm thick SF film.
Deposition of Metal Thin Films: NiFe(5-50 nm) thin films were deposited onto SF films by DC magnetron sputtering (Phase-IIJ, AJA International Inc., USA) with a base pressure <8 × 10 −8 Torr, a working Ar pressure of 3 mTorr and DC power of 30 W. The film thickness is a nominal thickness that was calibrated by a quartz crystal microbalance (SQM-160, Inficon Inc., USA).The deposition rates were 0.017 Å s −1 for NiFe.The as-deposited films were cut into a size of 5 × 5 mm for FMR measurement.
Magnetic Properties Characterization: The broadband FMR measurements were carried out on a home-made measurement system, including a rf/microwave signal generator (N5173B, Keysight Technologies, USA), a lock-in amplifier (SR-830, Stanford Research Systems, USA), a coplanar waveguide, a Helmholtz coil, and an electromagnet.For conventional measurement, a flat coplanar waveguide was used.For the bending test, a bended coplanar waveguide was used.In both kinds of measurement, the SF/NiFe films were attached on the waveguide by a small piece of Kapton tape to ensure full contact between the soft SF/NiFe films and the coplanar waveguide.The narrowband FMR spectra were recorded using a high sensitivity X-band (≈9.3 GHz) electron paramagnetic resonance system (JEOL, JES-FA200) by swept static magnetic field and conventional modulation and phase-sensitive detection techniques.Several PTFE rods with different diameters were used for bending test.
Degradation Test: The SF/NiFe films were placed between two fresh SF films to form a sandwich-like stack.The edge of the stack was sealed by SF solution as an adhesion layer and the stack was then cross-linked at 120 °C for 60 s under pressure to achieve complete adhesion of the SF layers and fully encapsulation of the SF/NiFe films.Subsequently, the packaged film was subjected to immersion in methanol for a duration of 30 min, followed by air drying at room temperature.They were cut into small sizes for the following degradation test.The samples were placed into 50 mL of 1.0 m phosphate-buffered saline (PBS, pH 7.4, Sigma-Aldrich, USA) at room temperature and the degradation states were recorded by a camera at different time.
Cell Viability Test: Glass/NiFe(20 nm), bare SF, and SF/NiFe samples were prepared for this test.NIH-3T3 fibroblast were seeded on the specimen with a cell density of 5000 cells cm −2 and left undisturbed for 2 h to allow for cell attachment.Subsequently, 500 μL Dulbecco's modified eagle medium (Gibco, USA) supplemented with 10% fetal bovine serum (Gibco, USA) was added.After 3 days of culture in an incubator, a live/dead assay was performed by calcein AM/ethidium homodimer-1 (Life Technologies, USA) according to the protocol suggested by the manufacturer.The numbers of live and dead cells were counted via ImagePro Plus software through the obtained images taken using Olympus IX 81 fluorescence microscope (Olympus, Japan).Cell viability (%) was calculated by dividing the number of live cells by the number of total cells.Three samples of each film were tested to obtain an average value of cell viability.

Figure 1 .
Figure 1.Fabrication of flexible SF/NiFe films.a) Schematics of the processing of SF and SF/NiFe thin films, b) optical images of a transparent SF film with the area up to 14 cm 2 , c) optical images of SF/NiFe films in bent states, surface AFM morphology of d) SF films and e) Kapton films (5 × 5 μm), and f) XRD curve of the SF/NiFe films.

Figure 2 .
Figure 2. Ferromagnetic resonance of NiFe thin films deposited on SF, Kapton, and Si.a) Schematic illustration of broadband ferromagnetic resonance measurement, b) typical ferromagnetic resonance spectra of SF/NiFe films from 3 to 12 GHz.c) FMR field with respect to microwave frequency for SF/NiFe films from which 4M eff was calculated, d) FMR line-width with respect to microwave frequency for SF/NiFe films from which Gilbert damping constant was calculated, thickness dependence of e) 4M eff , and f) Gilbert damping constant .

Figure 3 .
Figure 3. Narrowband and broadband FMR spectra of SF/NiFe (20 nm) films during bending test.a) In-plane field-sweep FMR spectra with various ROC under tensile strain/stress.b) FMR fields (H r ) and linewidth (ΔH) as a function of ROC, the inset shows the bending configuration for narrowband FMR test.Microwave frequency dependent c) FMR fields and d) FMR linewidth for SF/NiFe films in unbent state and bent state with ROC = 5 mm under compressive strain/stress, the inset in (c) shows the bending configuration for broadband FMR test.e) In-plane magnetic hysteresis loops in unbent state and bent state with ROC = 2.5 mm.f) In-plane FMR spectra of SF/NiFe films in unbent state, bent at ROC = 2.5 mm after 24 h and then returned to unbent state.g) The FMR spectra of SF/NiFe (50 nm) were measured in a flat state after undergoing various bending cycles.h) The corresponding FMR field (bottom set) and the displacement of Kapton/NiFe samples (upper set).

Figure 4 .
Figure 4. Temperature stability of SF/NiFe(20 nm) films.Temperature-dependent FMR spectra of a) SF/NiFe films and b) Si/NiFe from −50 to 50 °C, and corresponding temperature-dependent c) FMR field shift and d) linewidth change.

Figure 5 .
Figure 5. Dissolvability and biocompatibility of SF/NiFe films.a) Images of time sequence for SF/NiFe films during degradation in water.b) Images of time sequence for SF/NiFe/SF films encapsulated in chemically modified SF during degradation in water.Fluorescence image of live and dead cells after 72 h culture on the surface of c) NiFe or d) SF/NiFe films deposited on glass.e) Percentages of live cells after 72 h culture on the surface of NiFe and SF/NiFe films.