Biodegradable PBAT@CoFe2O4 Foils as Magnetically Active Photothermal Materials for Smart Surface Heating

1 Biodegradable PBAT@CoFe2O4 Foils as Magnetically Active Photothermal Materials for Smart Surface Heating Emilia Zachanowicz1*, Anna Tomaszewska2, Magdalena Kulpa-Greszta2, Dominika Zákutná3, Maria Romerowicz-Misielak2, and Robert Pazik2* 1Polymer Engineering and Technology Division, Wroclaw University of Science Technology, Wyspianskiego 27, Wroclaw, 50-370, Poland 2Faculty of Biotechnology, Collegium Medicum, University of Rzeszow, Pigonia 1, 35-310 Rzeszow, Poland 3Department of Inorganic Chemistry, Charles University, Hlavova 2030, Prague 2, 128 40, Czech Republic *Corresponding authors: Emilia Zachanowicz (WUT), [email protected], Robert Pązik (UR), [email protected] Abstract Our study is devoted to the development, physicochemical characterization, and NIR energy conversion to the heat of biodegradable PBAT@CoFe2O4 composite foil fabricated via the solvent evaporation casting technique. Superparamagnetic CoFe2O4 nanoparticles (6.3 nm) were synthesized through thermal decomposition and incorporated into the PBAT matrix, as confirmed by vibration modes shift, suggesting strong interfacial interactions. The TGA/DTA and DSC analysis showed a significant change in the composite foil degradation and thermal properties induced by the presence of cobalt nanoferrite due to its catalytic activity. Magnetic characterization confirmed superparamagnetic behavior of stock CoFe2O4 and ferrite-doped PBAT composite foil. We observed that under NIR808 laser irradiation, the composite exhibited rapid heating, whereas foil heating under AMF was ineffective due to the immobilization of particles within the polymer matrix. The highest recorded temperature was 115 °C with a specific absorption rate (SAR) of 97.5 W/g (maximum allowed laser power due to the risk of polymer matrix melting). Heating ability was further evaluated using a pork skin ex-vivo model to simulate soft tissue interaction, revealing skin discoloration and surface changes caused by protein coagulation under heat generation. Cytotoxicity tests of reference and composite foil were carried out using NIH/3T3 fibroblast cell line according to ISO standards, showing that foils can be considered safe and non-toxic. The results confirm the potential of NIR-responsive PBAT@CoFe2O4 composites that can be used as smart energyconverting materials for various practical applications. Even though the AMF cannot be applied for heating foils integrated with magnetic particles can be used for the fast separation of polymeric composites from a waste mixture using magnets upon their segregation in the recycling facilities. This feature can be of particular interest upon product end-life. Keywords: PBAT, biodegradable polymers, composite foil, magnetic nanoparticles, self-cleaning surfaces, energy conversion, recycling and separation https://doi.org/10.26434/chemrxiv-2025-46k18 ORCID: https://orcid.org/0000-0001-9417-6514 Content not peer-reviewed by ChemRxiv. License: CC BY 4.0 2 1. Introduction The interest of scientists in the search and development of new and biodegradable polymeric materials constantly grows due to the urgent need to address several issues, such as the efficiency and cost of the recycling process, as well as a serious environmental burden1,2. It is predicted that the global use of plastics will increase to 884 Mt with up to 4725 Mt accumulated in stock up to 20503. Therefore, immediate solutions are mandatory to improve the overall situation. Replacement of traditional polymers for packaging purposes and other industries with high exploitation of macromolecular compounds is in critical demand4. One of the possibilities is to switch toward biodegradable polymers that can be decomposed, disintegrated, depolymerized, or repurposed through environmentally friendly processes5. Among many existing bioplastic materials, poly(butylene adipate-coterephthalate) (PBAT) is very promising due to its 100% biodegradability and unique properties that resemble low-density polyethylene (LDPE)6. PBAT is synthesized via polycondensation reaction of the 1,4-butanediol (BDO), adipic acid (AA), and terephthalic acid (PTA). The process requires elevated temperature (≈ 190 °C), high vacuum, and long reaction time, affecting production costs7. However, PBAT’s key advantage lies in its compostability, while decomposition products can be effectively metabolized by microorganisms8. In recent years, the integration of inorganic nanoparticles into biodegradable polymer matrices has led to the emergence of hybrid polymer-inorganic composites with multifunctional properties, enabling their application in diverse fields such as electronics9, biomedicine10, energy storage11 etc. Among various inorganic fillers, magnetic nanoparticles, particularly spinel ferrites (MFe2O4, where M can be Co2+, Fe2+, Zn2+, Mn2+, etc.) have attracted considerable attention due to their exceptional magnetic properties and pronounced absorption capabilities in the low-energy near-infrared (NIR) spectral region12–14. Very interesting applications of ferrite compounds have been recently shown by Gao and coworkers15–17 through the use of photothermal stimulation to promote the formation of dynamic active-sites and enhance oxygen evolution for modern electrocatalysis. The addition of the magnetic component into the PBAT polymer matrix can result in several interesting features that can be controlled i.e. shape memory (PLA/PBAT/Fe3O4)18, magnetism for electronic devices (PBAT/Fe3O4)19, flexible materials (PBAT/ferrites)20, pollutants photodegradation (PBAT/Fe3O4)21, electromagnetic shielding materials (PBAT/Fe3O4/CNTs)22, https://doi.org/10.26434/chemrxiv-2025-46k18 ORCID: https://orcid.org/0000-0001-9417-6514 Content not peer-reviewed by ChemRxiv. License: CC BY 4.0 3 immobilization of enzymes and magnetic separation (Mg2+-Fe3O4@PBAT/B-CD/PVA)23. However, the use of magnetic PBAT-based composites as NIR-light responsive heat-inductive materials remains largely unexplored, particularly for contactless energy conversion applications as smart materials for general purposes as well as medicine. Composite foils are particularly relevant in the food packaging industry and related sectors, where biodegradability and thermal responsiveness are crucial. PBAT is considered a promising material with the potential to replace traditional polymers. Consequently, there is a high demand for the development of hybrid materials that provide additional functionalities. However, a significant challenge lies in achieving a homogeneous distribution of magnetic fillers within the polymeric matrix while preserving or enhancing the desired functional performance. Furthermore, the interfacial interaction between polymer chains and nanoparticle surfaces is critical in defining the overall properties of the composite, including mechanical stability, magnetic behavior, and long-term durability. In this study, we report the development and comprehensive characterization of biodegradable PBAT@CoFe2O4 composite foils designed for efficient, contactless heating under alternating magnetic field (AMF) and near-infrared (NIR) light. The idea of ferrite utilization as photothermal agents, at least from the two standpoints: (1) they are relatively cheap in comparison to the fabrication of Ag, Au metallic nanoparticles or lanthanide-based light-responsive materials (2) offer multifunctionality through combining several physicochemical properties described above. By embedding spinel CoFe2O4 nanoparticles into the PBAT matrix, we have shown a material that combines magnetic properties with lightdriven thermal conversion, enabling smart applications, including active food packaging, biomedical interfaces, on-demand surface sterilization, and photothermal-responsive coatings. Practical relevance was demonstrated through a proof-of-concept experiment using pork skin as a biological tissue model, highlighting the material’s capacity for localized heating and its potential to induce thermal effects in soft tissue environments. Foil potential in magnetic separation from the waste mixture was shown. Even though AMF heating was not effective, and can be a basis for the end-of-life product separation in recycling facilities. 2. Experimental 2.1. Synthesis of magnetic nanoparticles and composite foil https://doi.org/10.26434/chemrxiv-2025-46k18 ORCID: https://orcid.org/0000-0001-9417-6514 Content not peer-reviewed by ChemRxiv. License: CC BY 4.0 4 The CoFe2O4 nanoparticles were prepared by using the thermal decomposition technique carried out in benzyl alcohol as a solvent by adopting the well-established Niederberger and Bilecka approach24 without the use of a microwave reactor. Briefly, 0.5143 g (2 mmol) of Co(acac)2 (cobalt acetylacetonate, Sigma Aldrich, Poland, 97%) and 1.4128 g (4 mmol) of Fe(acac)3 (iron acetylacetonate, Sigma Aldrich, Poland, 97%) were transferred to the two-neck glass flask together with 70 mL of benzyl alcohol (Sigma Aldrich, Poland, 99%) and protected with a rubber sleeve stopper. All operations were done in a glovebox (P10R250T2, Systemtechnik GMBH, Germany) under the inert gas (N2, Linde, Poland, 99.99%) to prevent substrates from contact with air and humidity. Afterwards, the glass flask with the mixture was mounted to the laboratory set-up consisting of a reflux column, heating mantle, and automatic temperature controller (LTR 2500, Juchheim, Germany) with a Pt-100 sensor. After that, the temperature of the reaction mixture was raised to 205 °C (boiling point of the solvent) and kept for an additional 4 h. The resulting dark-colored solution with visible black particles was cooled to room temperature. CoFe2O4 particles were separated by centrifugation and washing steps (three times) with a mixture of ethanol/acetone (1:1 v/v, both from Stanlab, Poland, pure for analysis). The synthetic procedure was repeated 5 times. Multiple batches of CoFe2O4 nanoparticles were combined to obtain a sufficient quantity of filler material for composite fabrication. The stock sample was divided 50/50, where one part was suspended in ethanol solution (TEM, foil casting) while the other was dried to get a powder necessary for physicochemical characterization (XRD, FTIR-ATR, magnetic characterization). In the case of the PBAT@CoFe2O4 fabrication, a solvent evaporation casting approach25 was used. For this purpose, reference PBAT was prepared by dissolving poly(butylene adipateco-terephthalate (PBAT, Ecoflex® F Blend C1200, BASF, Poland) in tetrahydrofuran (THF, Sigma Aldrich, Poland, 99%) to get a 5% (w/v) solution. Namely, 0.9 g of PBAT was taken, and mixed with 20 ml of THF under constant stirring for 2h at 60 °C. In the next step, the obtained PBAT/THF solution was poured (13 ml) into the mold (on-site made Teflon made – 80x80x3 mm) and left for THF evaporation for 48 h at room temperature. After that, a transparent reference PBAT foil was obtained. The procedure for PBAT foil preparation with magnetic component was carried out in the same way. However, the CoFe2O4 ethanol suspension was first dried, and 0.14 g (around 15% with respect to the PBAT content) of magnetic particles were added to PBAT/THF at the beginning of the solution preparation under sonication and https://doi.org/10.26434/chemrxiv-2025-46k18 ORCID: https://orcid.org/0000-0001-9417-6514 Content not peer-reviewed by ChemRxiv. License: CC BY 4.0 5 further mixing to get a homogeneous particle distribution. After solvent evaporation, the black-colored PBAT@CoFe2O4 foil was obtained. Composite thickness was measured using an electronic thickness meter of 0 -10 mm with an accuracy of 0.001 mm (Merazet, Poland). 2.3. Characterization of physicochemical properties of nanomaterials and composite foil Structural properties of the CoFe2O4 stock nanoparticles were evaluated by means of the X-ray powder diffraction technique (XRD). The pattern was recorded using a Bruker D8 Advanced diffractometer by covering the 2Θ range of 15 - 70° with a 0.02° step and an integration time of 0.8 s. As an X-ray source, a Cu lamp emitting Kα1 1.54060 Å was used, while Kα2 was cut with a Ni filter. Sample preparation involved the evaporation of part of the stock magnetic nanoparticles. Before measurement, the powder was ground using an agate mortar. Recorded data was further curated in Diffrac.Eva software (Bruker, Germany) for background correction, while the final presentation was done in Origin Pro 9.0 (Origin Lab, USA). Fourier transform infrared spectroscopy (FTIR) with an attenuated total reflection (ATR) was performed to study the surface properties of the stock CoFe2O4 nanoparticles, PBAT reference foil, and PBAT@CoFe2O4 composite. No special sample preparation was necessary, except evaporation of stock CoFe2O4 material and gentle cleaning of both foils with ethanol. The FTIR-ATR spectra were recorded using a Thermo Scientific Nicolet iZ10 spectrometer equipped with an ATR accessory (diamond crystal) covering the 3500 - 500 cm-1 spectral range. An instrument calibration procedure was implemented to correct for artefacts and air humidity before measurements. In the case of the CoFe2O4, the HRTEM technique (High resolution transmission electron microscopy) was involved to characterize primary particle size, distribution, and particle morphology using a Tecnai Osiris X-FEG HRTEM microscope (FEI Company, USA) operating at 200 kV. Before imaging, a droplet of the ethanol dispersion of CoFe2O4 particles (≈ 0.25 mg/ml) was deposited on a carbon-covered 200 mesh copper grid (EM Resolutions, United Kingdom) and slowly dried overnight at room temperature. Particle size and crystallographic fringe distances were estimated using ImageJ software (v. 1.54g). Scanning electron microscopy with energy dispersive X-ray spectroscopy was used to image the surface of the PBAT and PBAT@CoFe2O4 samples and quantify the elemental content. Element distribution maps were recorded utilizing an FEI Quanta 3D 200i microscope https://doi.org/10.26434/chemrxiv-2025-46k18 ORCID: https://orcid.org/0000-0001-9417-6514 Content not peer-reviewed by ChemRxiv. License: CC BY 4.0 6 (FEI Company, USA). Before measurements, samples of polymeric foil and composite were placed on an alumina stage and mounted with Cu tape. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were carried out using a TGA/DSC1 Mettler Toledo system (Mettler Toledo, Poland) under N2 atmosphere. For each measurement, around 5 mg of material was taken and heated from 50 to 550 °C with a heating rate of 10 °C/min. In the case of DSC characterization, a similar mass of sample was taken and placed in an aluminum pan. The DSC apparatus was coupled with a TC 100 intercooler while the measurement was performed for the temperature range of 0 – 200 °C with a 10 °C/min heating rate. The resulting data were processed with STARe software and presented in Origin Pro 9. The zero-field-cooled (ZFC) and field-cooled (FC) magnetizations (applied field of 10 mT) and magnetization isotherms at selected temperatures (4 K and 298 K) were measured on the Quantum Design MPMS7XL device (SQUID, USA). The samples were put into the gelatin capsule, fixed with glue to prevent physical rotation of the grains. The measurements of temperature effect induced by the alternating magnetic field (AMF) were conducted using a specially designed set-up provided by NanoScale Biomagnetics, Spain, consisting of a magnetic field generator G2 D5 Series Multimode 1500 W driver equipped with S32 coil. The coil was thermally isolated by a thick-walled polystyrene box filled with additional polymer-based foam insulating material to limit heat exchange with the external environment. The G2 driver in this particular configuration generates AMF with frequencies between 145 – 771 kHz and field intensity is within the range of 3.8 – 34.5 kA/m, depending on the presets (controlled by Maniac software provided by the manufacturer). Assessment of the energy conversion of the NIR laser light (808 nm) into heat on PBAT@CoFe2O4 foils was performed by using a self-constructed set-up that consisted of a continuous wavelength laser module emitting 808 nm line (delivered through 400 µm optical fiber (all from CNI, China), thermally isolated polystyrene box with a place for sample mounting, fast thermovision camera FLIR T660 (FLIR, USA), as well as a laptop computer for recording using ResearchIR software (FLIR, USA). Laser power meter Ophir StarLite with a beam track thermal sensor 10 A-PPS (Ophir, Israel) with a measurable laser power range of 20 mW – 10 W was used for laser module calibration. https://doi.org/10.26434/chemrxiv-2025-46k18 ORCID: https://orcid.org/0000-0001-9417-6514 Content not peer-reviewed by ChemRxiv. License: CC BY 4.0 7 2.5. Characterization of foil cytotoxicity The NIH/3T3 cell line derived from mouse fibroblasts (ECACC 93061524) was obtained from the European Collection of Authenticated Cell Cultures (England) (ECACC). Cells were maintained in DMEM (Dulbecco's Modified Eagle Medium, Sigma Aldrich, Poland) supplemented with 10% newborn calf serum (NCS, Sigma Aldrich, Poland), 1% antibiotic and antimycotic (100 U/ml penicillin, 100 µg/ml streptomycin, and 0.25 µg/ml amphotericin, Sigma Aldrich, Poland). The culture was maintained under standard conditions of 37 °C, 5% CO2, and 100% relative humidity. Cells were passaged regularly using 0.25% Trypsin-EDTA solution, which was stopped by adding NCS medium without subsequent centrifugation. The tests were prepared according to ISO 10993-526 with some modifications. Samples were extracted according to ISO 10993-1227. 2.5.1. Exposure of cells to extracts and evaluation of metabolic activity The preparation of the extracts involved sterilization with 75% ethanol and washing (three times) with sterile PBS of PBAT@CoFe2O4 foil together with the PBAT reference sample. The extracts were prepared by incubating the test samples in a sealed vessel in DMEM culture medium containing 10% of NCS at 37 °C with shaking for 24 h under sterile conditions. The ratio of sample area to medium volume was 125 mm2/ml. The incubation time of the extract was chosen according to ISO recommendations for cytotoxic assays, while extract temperature was set by taking into account the stability of the culture medium as well as the properties of the sample under test. To verify the protective effect of the serum, an extract was prepared in DMEM medium with 5% NCS as well. The extracts were used for in vitro cytotoxic assays. The parent extracts (100%) were diluted in culture medium to obtain dilutions of 50 and 25%. NIH/3T3 cells were seeded in 96-well plates at a density of 7x103 cells/well in DMEM medium with 10% NCS. After 24 h of incubation, the medium was replaced with the prepared extracts (see preparation of extracts). Cells cultured in the presence of a DMEM with the appropriate concentration of NCS (5 or 10%) were used as a reference (hereafter referred to as negative control). All assays were prepared in 6-fold replicates. After a 24 h incubation with the extracts, 20 µl MTT (5mg/mL of PBS) was added to each well and incubated for 3.5 h under standard culture conditions. Then, the medium was gently removed, and 150 µl of DMSO was added to dissolve the formazan crystals. The absorbance was measured at 570 nm using a https://doi.org/10.26434/chemrxiv-2025-46k18 ORCID: https://orcid.org/0000-0001-9417-6514 Content not peer-reviewed by ChemRxiv. License: CC BY 4.0 8 Victor plate reader (Perkin Elmer, Poland). The results are presented as a percentage of metabolically active cells relative to the negative control, which was taken as 100%. 2.5.2. Direct contact assay Before the cytotoxicity test, the PBAT@CoFe2O4 samples and the reference samples used in direct contact were disinfected by immersion in 70% ethanol and then washed three times in PBS. The direct contact cytotoxicity test was performed according to ISO 10993-5 with some modifications. NIH/3T3 cells were seeded in 6-well plates at a density of 0.8x106 cells per well in DMEM medium with 10% NCS and incubated until subconfluent under standard culture conditions. Culture medium was removed and fresh medium was added. The sterile test material was placed directly on the cell monolayer without disrupting its integrity. The culture was incubated under standard culture conditions for 48 h. After 24 and 48 h, the cells around the test samples were examined with a microscope using a Zeiss Axiover inverted microscope with phase contrast objective (Carl Zeiss, Germany) to determine the presence of morphological changes, reduction in cell density or cell lysis, indicating cytotoxicity. An analogous procedure was carried out for the reference sample. At the end of the 48 h incubation, cells were stained with neutral red to assess the grade of reactivity in the direct contact assay. The neutral red solution (40 µg/ml of culture medium) was incubated overnight at 37 °C. The red medium was centrifuged 10 min, 1800 rpm. to remove precipitated dye crystals. The cells were washed with PBS. Then 2 ml of neutral red medium was added to each well and incubated for 2 h under standard culture conditions. The neutral red medium was removed, cells were rinsed with PBS, and images were taken. 3. Results and discussion The PBAT and PBAT@CoFe2O4 foils were prepared by using the solvent evaporation casting technique. Photographs of reference and magnetic foils are shown in Fig. 1. For the preparation of the composite, stock CoFe2O4 nanoparticles were taken as a functional polymer filler that were fabricated through thermal decomposition of organic cobalt and iron complexes. Prior incorporation of cobalt nanoferrite into the organic matrix sample was characterized by XRD, TEM, and FTIR-ATR techniques to evaluate structural properties, particle size, distribution, and surface properties. https://doi.org/10.26434/chemrxiv-2025-46k18 ORCID: https://orcid.org/0000-0001-9417-6514 Content not peer-reviewed by ChemRxiv. License: CC BY 4.0 9 Figure 1. Photographs of PBAT, PBAT@CoFe2O4 foil with magnetic nanoparticles. The analysis of the diffraction patterns (Fig. 2a) exhibited that the CoFe2O4 diffraction corresponds with the reference ICDD card no. 22-1086 attributed to the ferrite cubic structure. As one can note, recorded diffraction peaks are broad with a relatively high signalto-noise ratio associated with the small crystallite size, causing peak broadening and enhanced absorption of the X-ray radiation, which is typical for materials containing iron and cobalt cations. The average crystallite diameter (D) was calculated using a well-known Scherrer’s formula28: 𝐷=    , (1) where k is a constant (0.89),  X-ray wavelength emitted by the Cu lamp (1.54060 Å), β0 is ascribed to apparatus broadening (0.05°); β is a full width at half maximum (FWHM), and  is the maximum of the diffraction peak taken into consideration. The average crystallite size of the CoFe2O4 was 9 nm ± 2.3 nm , and is slightly larger than the particle size estimated from the TEM image analysis 6.3 ± 1 nm, due to the nature of the size estimation and error. Figure 3 presents the results of the TEM characterization of the CoFe2O4 sample. It is worth noting that the prepared particles' morphology resembles spherical shapes with a relatively narrow size distribution. TEM allowed for the imaging of the lattice fringes of nanoparticles, while fast Fourier transformation (FFT) enabled estimation of the interplanar distance (d) 0.49 nm characteristic of (111) crystallographic plane of the CoFe2O4. Visible fringes suggest that the fabricated cobalt ferrite nanoparticles are well crystallized. https://doi.org/10.26434/chemrxiv-2025-46k18 ORCID: https://orcid.org/0000-0001-9417-6514 Content not peer-reviewed by ChemRxiv. License: CC BY 4.0 16 by using the numerical inversion reported (thick dark lines). Resulting moment distributions (gray curves: 200 discrete moment distributions cDpVΔμ for (b) CoFe2O4 NPs and (c) CoFe2O4 NPs embedded in the PBAT polymer; colored tick curves: moment distribution obtained for the highest evidence). From the obtained distribution with the highest evidence, the magnetic size (taking into account the spherical shape) of d = 8.86 nm CoFe2O4 and d = 7.1 nm for the PBAT@CoFe2O4 sample. Nevertheless, the CoFe2O4 sample shows a slightly larger apparent magnetic size due to stronger interparticle interactions. A small magnetic moment distribution around 3.34·1022 Am2 also arises from the surface spin canting or disorder. Those contributions are negligible once magnetic nanoparticles are within the PBAT polymer network. Light-to-heat energy conversion on PBAT@CoFe2O4 foil (Fig. 9) was measured by using 808 nm laser wavelength as a function of the laser power (85 – 485 mW) and distance (50 – 120 mm). The potential for the application of the composite foil in modern, smart photothermal active films capable of non-contact light conversion into heat in sterilization and self-cleaning was evaluated by using a piece of pork meat (commercially available) as an example of skin-like substrate within a laser power range of 135 – 1100 mW (Fig. 10) and at the laser distance of 50 mm above sample. The effect of NIR laser light foil stimulation as a function of laser power is presented in Fig. 9a. As shown, the PBAT@CoFe2O4 composite effectively heats to 115 °C at the highest laser power (485 mW). Since the melting temperature of the PBAT foil is around 130 °C and the addition of CoFe2O4 lowers this temperature, higher laser power settings were not tested due to the risk of polymer matrix melting. It is important to note that the heating speed, defined as the dT/dt value obtained from the heating curves, ranged from 1.9 to 11.9 °C/s depending on the laser power, demonstrating that the foil's response to laser action is very rapid and effective. Following the optimization of laser power, the influence of the light source distance was also studied (Fig. 9b), and typical behavior was observed. Namely, the further the light source was, the greater the laser spot size, leading to lower laser power densities, and a subsequent decrease in Tmax at constant laser power (485 mW). This observation is significant from the perspective of future foil exposure to NIR, indicating that the operator can maintain a greater distance from the irradiated material at the expense of Tmax, but can be compensated by increasing the light source power. The specific absorption rate (SAR), being a measure of the heating efficacy was calculated in accordance with the following equation: 𝑆𝐴𝑅=    , (3) https://doi.org/10.26434/chemrxiv-2025-46k18 ORCID: https://orcid.org/0000-0001-9417-6514 Content not peer-reviewed by ChemRxiv. License: CC BY 4.0 17 where Ctotal is the mass averaged heat capacity of the foil sample (CPBAT = 1.33 J/gK, CCoFe ferrite = 0.67 J/gK, Ctotal = 1.22 J/gK), mtotal – describes the mass of the foil sample (13 mg), mactive – is the mass of the CoFe2O4 in the composite material (1.95 mg)41. All results were gathered in Table 2, while Fig. 9c shows a graphical presentation of Tmax, dT/dt, and SAR dependence on the laser power for clarity. Figure 9. Heating curves of the PBAT@CoFe2O4 foil under NIR exposure at 808 nm laser stimulation (485 mW) as a function of laser distance (a), and laser output power at a distance of 50 mm (b). Laser power dependence on heating speed (dT/dt), Tmax, and SAR (c). Table 2. Typical parameters Tmax, heating speed dT/dt and SAR associated with the heat induction during contactless NIR stimulation of the PBAT@CoFe2O4 foil as a function of the laser power. Laser power (mW) dT/dt (°C/s) Tmax (°C) SAR (W/g) 85 1.93 36.9 15.8 135 3.02 46.1 24.6 185 4.96 63.5 40.5 240 6.52 74.3 53.3 305 7.87 84.1 64.2 370 9.26 95.2 75.6 425 10.62 103.9 86.7 485 11.90 115.6 97.5 In terms of the SAR, the PBAT@CoFe2O4 foil exhibits a reasonable conversion efficiency, with a SAR of 97.5 W/g. Considering that the specific heat capacity of the composite is https://doi.org/10.26434/chemrxiv-2025-46k18 ORCID: https://orcid.org/0000-0001-9417-6514 Content not peer-reviewed by ChemRxiv. License: CC BY 4.0 18 relatively low (1.22 J/gK), the rapid increase in heating speed is not surprising, as a moderate nanoparticle energy input will be necessary to heat the overall system efficiently. However, it should be noted that the low thermal conductivity of the PBAT matrix (0.2 W/mK)42 may affect the uneven spatial heat distribution, leading to the formation of heat spots, especially in the case of thick composite foils or upon non-homogeneous nanoparticle dispersion. The additional advantage of using inorganic fillers in low-conductivity polymers like PBAT is the possibility of enhancement of thermal conductivity and dielectric properties. Both features are important in advanced applications such as EMI (electromagnetic interference shielding materials) and overheating protection in critical electronic devices43. Alternating magnetic field stimulation of the PBAT@CoFe2O4 foils (Fig. 9) toward heat induction was measured as a function of the three field frequencies, namely 496 kHz, 226 kHz, and 145 kHz for the maximal possible field intensities of 22 kA/m, 27 kA/m, and 32 kA/m (manufacturer presets), respectively. Figure 10. Heating curves of the PBAT@CoFe2O4 foil under AMF stimulation and as a function of the field frequency (145 kHz – 496 kHz). As one can note, the efficiency of the AMF conversion is relatively low with ΔT ≈ 4 °C in the best case (145 kHz). We were able to generate Tmax of around 25 °C (starting temperature 21 °C) with the dT/dt of 0.4 °C/s. While the heating speed is not that slow, the temperature achieved for the CoFe2O4 nanoparticles embedded in the PBAT matrix is not striking, with a SAR value of 3.25 W/g. This efficiency of heating is very low and characterizes materials that https://doi.org/10.26434/chemrxiv-2025-46k18 ORCID: https://orcid.org/0000-0001-9417-6514 Content not peer-reviewed by ChemRxiv. License: CC BY 4.0 19 are not applicable for hyperthermia applications. We believe that such a low ability to heat upon AMF stimulation will be caused by the several factors: (1) non-optimal particle size for efficient generation of power losses, (2) immobilization of the CoFe2O4 particles within polymer matrix blocking the Brownian relaxation (particle physical rotations) mechanism as well as (3) not optimal particle concentration to compensate for lack of physical rotations with Néel internal relaxation process, (4) associated with (3) and sought as almost two fold reduction of the magnetic saturation compared to stock CoFe2O4 particles. However, while AMF induced heating was seriously limited, the magnetic functionality of the PBAT@CoFe2O4 foil offers an alternative application via magnetic separation of waste materials, especially during the end-of-life product recycling process. We demonstrated (see Fig. 11 and short video file in the supplementary files) that it is possible to retrieve magnetic polymer composites from the non-magnetic waste mixture (paper, other polymers) using a simple laboratory magnet. This particular feature may hold value in waste stream segregation or material recycling scenarios, especially in the context where automatic magnetic separation systems are in place, which is important for the sustainable use of resources. Figure 11. Magnetic segregation and separation of the PBAT@CoFe2O4 composite foils from the waste mixture. The foil potential evaluation for smart heating material, specifically pork skin with underlying muscle tissue, was used as a well-established and accepted model in ex-vivo experiments for thermal testing, wound healing, burn modeling, skin interaction studies, heat diffusion, or surface damage effects. Moreover, pork skin closely mimics human skin anatomy, is easily accessible as a by-product without the need to sacrifice additional animals, and complies with ethical regulations. The primary goal was to assess the heating behavior of the PBAT@CoFe2O4 foil in contact with soft tissue under NIR laser irradiation, a relevant scenario for modern applications in localized thermal therapies, skin-interfacing smart materials, food safety materials, or smart contact surfaces in food safety and related areas. This relatively https://doi.org/10.26434/chemrxiv-2025-46k18 ORCID: https://orcid.org/0000-0001-9417-6514 Content not peer-reviewed by ChemRxiv. License: CC BY 4.0 20 simple experiment enabled visualization of the temperature rise and observation of the thermal effects on biological tissue, which is crucial for safety assessment and potential future applications. The results of the PBAT@CoFe2O4 pork skin heating are shown in Fig. 12. Generally, the irradiation time with 808 nm laser was extended for approximately 150 s. Since the foil was put directly on pork skin, which works as a heat receiver, it was decided to extend the range of the laser power that is necessary to heat the sample to a comparable temperature range (up to 1100 mW) as in the case of the foil itself. Under NIR radiation (135 – 1100 mW), the composite foil exhibited strong and controllable heating ability, allowing for reaching 115 °C with the maximum laser power used of 1100 mW. Moreover, the temperature increase is very rapid, and the range of temperatures achieved for certain laser powers is sufficient to denature proteins, disrupt bacterial biofilms, as well as to inactivate pathogens. Figure 12. NIR heating effects of the PBAT@CoFe2O4 composite foil (left) placed on pork skin under 808 nm laser stimulation as a function of laser output power (distance 50 mm). Photographs of the pork skin ex-vivo tissue model (right panel), before NIR irradiation, and after. It is evident that during NIR exposure, the foil quickly heats, which leads to partial protein coagulation on the pork skin surface (discoloration) at the contact site under the composite (thermal denaturation typically occurs at temperatures above 60 °C). The observed change supports the effective heat transfer from PBAT@CoFe2O4 foil to biological tissue. These https://doi.org/10.26434/chemrxiv-2025-46k18 ORCID: https://orcid.org/0000-0001-9417-6514 Content not peer-reviewed by ChemRxiv. License: CC BY 4.0 21 preliminary results provide evidence for the controlled thermal interaction with soft matter, which is relevant for biomedical or smart surface applications44. 3.3. Cytotoxicity of the PBAT@CoFe2O4 foil To assess the degree of toxicity of PBAT@CoFe2O4, we measured the level of intracellular metabolism of NIH/3T3 cells in the MTT assay in response to the extracts. Since serum concentration during the extraction of metallic degradable materials can affect ion release45– 47, we performed extraction separately in medium with 10% NCS and 5% NCS. Since serum proteins can bind to extracted substances by masking their toxic effects on cells, we also used two concentrations of NCS, 10% and 5%, during the testing. A 24 h exposure of NIH/3T3 cells to an extract prepared with PBAT@CoFe2O4 at 100% and 50% concentrations resulted in a significant decrease in the metabolic activity of cells relative to a negative control, which were cells treated with vehicle for extract (DMEM) (Figure 13a). The metabolic activity of NIH/3T3 cells exposed to 100% extract was 84.8 ± 2.1% and 85.1 ± 2.5% compared to the negative control for medium with 10% and 5% NCS content, respectively. Upon comparison to the negative control extract prepared from the reference sample for all concentrations did not significantly alter the metabolic activity of the cells (Figure 13b). At the same time, the serum content of the culture medium did not affect the degree of toxicity of the extracts tested. According to accepted standards, if the relative viability of cells at the highest extract concentration is ≥ 70%, the material is considered non-toxic. Figure 13. Relative metabolic activity of NIH/3T3 cells (MTT assay) after 24 h of incubation with extracts prepared with (a) PBAT@CoFe2O4 and (b) PBAT reference foils. The results are presented as mean ± SEM; ***p < 0.001; ** p < 0.01; n=6. https://doi.org/10.26434/chemrxiv-2025-46k18 ORCID: https://orcid.org/0000-0001-9417-6514 Content not peer-reviewed by ChemRxiv. License: CC BY 4.0 22 Analysis of the reference sample showed that NIH/3T3 cells were able to proliferate in direct contact with the analyzed sample. After 24 and 48 h of incubation, no morphological changes were observed compared to the negative control. Cells showed normal fibroblast morphology with no signs of cell death (Fig. 14). The results indicate the absence of a cytotoxic effect in direct contact. Evaluation of the degree of reactivity in the direct contact test for the reference sample indicates that there is no reactivity: grade 0; no detectable zone around or under the specimen (see supplementary file, Fig. S1). Cell morphology confirmed the quantitative results obtained in the MTT test, indicating the absence of the cytotoxic effect of the reference sample. The morphology of cells cultured with PBAT@CoFe2O4 foil showed changes of cells in direct contact with the analyzed sample. After 24 h, single round and detached cells from the substrate were observed (Fig. 14). After 48 h, a slight reduction in the density of cells in direct contact with the analyzed sample was observed. Cells were characterized as long, thin, spindle-shaped, and loosely attached to each other (Fig. 14). Evaluation of the grade of reactivity in the 48 h direct contact test for the PBAT@CoFe2O4 foil indicated a grade 2: reactivity mild; zone limited to the area under the specimen (see Fig. S1). According to the accepted standard, the tested material meets the requirements of the test and is considered non-toxic. https://doi.org/10.26434/chemrxiv-2025-46k18 ORCID: https://orcid.org/0000-0001-9417-6514 Content not peer-reviewed by ChemRxiv. License: CC BY 4.0 23 Figure 14. Morphology of NIH/3T3 cells in the direct contact assay after 24 and 48 h of culture - negative control; reference PBAT foil, and PBAT@CoFe2O4 composite. 4. Conclusions The PBAT@CoFe2O4 foils were prepared by using the solvent evaporation casting technique. As a polymer matrix filler and functional agent, small 6.3 nm superparamagnetic nanoparticles of CoFe2O4 prepared by the decomposition technique were used. It was shown that the magnetic component integration within the PBAT polymeric matrix resulted in strong interaction of the nanoparticles with PBAT, as evidenced by the vibration mode shift assigned to the CoFe2O4 toward higher energies. The ferrite nanoparticles significantly affect the thermal stability of the PBAT, as reflected by the multi-step decomposition processes and its temperature decrease. The presence of the CoFe2O4 resulted in a higher glass transition temperature, lower crystallization temperature, as well as pronounced melting enthalpy increase, indicating that magnetic ferrite acts as an effective nucleating agent promoting https://doi.org/10.26434/chemrxiv-2025-46k18 ORCID: https://orcid.org/0000-0001-9417-6514 Content not peer-reviewed by ChemRxiv. License: CC BY 4.0 24 polymer crystallization. The magnetic characterization confirmed superparamagnetic behavior of stock CoFe2O4 ferrite and PBAT@CoFe2O4 foil. The incorporation of the magnetic particles into PBAT resulted in weaker interparticle interactions. Moreover, PBAT@CoFe2O4 foil shows high responsiveness toward NIR808 light exposure, resulting in significant and rapid heating (Tmax between 37 – 115 °C, depending on the laser power). In the case of the efficacy of the composite material calculated SAR value for the maximum possible laser power used was 97.5 W/g with a heating speed of 11.9 °C/s. Cytotoxic characterization of the PBAT@CoFe2O4 foils in contact with the NIH/3T3 cell line shows that the fabricated composite material can be considered safe and non-toxic. To estimate the potential for future applications of fabricated NIR converting composite foil as a smart material, pork skin as an ex-vivo model was used. Upon NIR exposure, the foil induced visible heating and partial protein coagulation on the skin surface, mimicking the photothermal surface-cleansing effect. Such performance underscores the potential of the material for applications in antimicrobial surfaces, smart packaging, heat-assisted self-cleaning applications for food contact materials, and bio-interactive surfaces. Localized photothermal heating could provide a non-chemical, on-demand method for surface sterilization in packaging films, helping to reduce bacterial load on items. By combining flexibility, biocompatibility, and photothermal activity, PBAT@CoFe2O4 composite foil could be integrated into reusable surfaces that undergo periodic NIR-triggered disinfection, especially in settings where chemical cleaning is undesirable. Moreover, rapid and localized heating under Nir light may open future paths in wound care/healing where controlled thermal pulses could assist with sterilization or drug release. In addition, while the AMF stimulation was not effective (SAR below 4 W/g), the polymeric composites with embedded magnetic particles enable remote manipulation and post-recovery from waste materials via effective magnetic separation. In the context of the sustainable development, i.e. use of resources this particular feature could facilitate selective waste sorting and material recycling, contributing to the sustainable lifecycle of the composite. 5. Acknowledgements M.K.G. is grateful for the financial support provided by the UR within the Young Scientist internal feedback. D.Z. acknowledges the assistance provided Advanced Multiscale Materials for Key Enabling Technologies project, supported by the Ministry of Education, Youth, and Sports of the Czech Republic Project No. CZ.02.01.01/00/22_008/0004558, co-funded by the European Union, as well as Charles University Research https://doi.org/10.26434/chemrxiv-2025-46k18 ORCID: https://orcid.org/0000-0001-9417-6514 Content not peer-reviewed by ChemRxiv. License: CC BY 4.0 25 Centre program No. UNCE/24/SCI/010. We would like to thank Patrycja Skała for the SEM and EDS characterization. 6. Authors contribution statement E.Z. preparation of foils, TGA, DSC, manuscript writing, and editing. A.T. synthesis of CoFe2O4 nanoparticles, XRD, and manuscript editing. M.K.-G. FTIR measurements, TEM imaging, energy conversion measurements, and manuscript editing. D.Z. magnetic measurements, data analysis, manuscript writing, and editing. M.R.M. cytotoxicity tests, data analysis, manuscript writing, and editing. R.P. idea, conceptualization, data analysis, energy conversion analysis, manuscript writing, and editing. 7. References 1 S. B. Borrelle, J. Ringma, K. L. Law, C. C. Monnahan, L. Lebreton, A. McGivern, E. Murphy, J. Jambeck, G. H. Leonard, M. A. Hilleary, M. Eriksen, H. P. Possingham, H. De Frond, L. R. Gerber, B. Polidoro, A. Tahir, M. Bernard, N. Mallos, M. Barnes and C. M. Rochman, Science (1979), 2020, 369, 1515–1518. 2 L. Lebreton and A. Andrady, Palgrave Commun, 2019, 5, 6. 3 M. Dokl, A. Copot, D. Krajnc, Y. Van Fan, A. Vujanović, K. B. Aviso, R. R. Tan, Z. Kravanja and L. Čuček, Sustain Prod Consum, 2024, 51, 498–518. 4 N. Singh and T. R. Walker, npj Materials Sustainability, 2024, 2, 17. 5 A. K. Mohanty, F. Wu, R. Mincheva, M. Hakkarainen, J. M. Raquez, D. F. Mielewski, R. Narayan, A. N. Netravali and M. Misra, Nature Reviews Methods Primers, 2022, 2, 46. 6 F. V. Ferreira, L. S. Cividanes, R. F. Gouveia and L. M. F. Lona, John Wiley and Sons Inc, 2019, preprint, DOI: 10.1002/pen.24770. 7 J. Jian, Z. Xiangbin and H. Xianbo, Advanced Industrial and Engineering Polymer Research, 2020, 3, 19– 26. 8 U. Witt, T. Einig, M. Yamamoto, I. Kleeberg, W.-D. Deckwer and R.-J. M Uller, Chemosphere, 2001, 44, 289–299. 9 L. Li, L. Han, H. Hu and R. Zhang, Mater Adv, 2022, 4, 726–746. 10 P. Phadtare, V. Viswapriya, V. Shinde, D. Mathure and M. R. Salunke, Hybrid Advances, 2025, 8, 100365. 11 H. Zhou, H. Li, L. Li, T. Liu, G. Chen, Y. Zhu, L. Zhou and H. Huang, Mater Today Energy, 2022, 24, 100924. 12 M. Rezoanur Rahman, A. Bake, A. Jumlat Ahmed, S. Md Kazi Nazrul Islam, L. Wu, H. Khakbaz, S. FitzGerald, A. Chalifour, K. L. Livesey, J. C. Knott, P. C. Innis, S. Beirne and D. Cortie, J Magn Magn Mater, 2023, 579, 170859. 13 M. Kulpa-Greszta, A. Tomaszewska, E. Zachanowicz, P. Krzemiński and R. Pązik, J Alloys Compd, 2022, 898, 162840. 14 M. Kulpa-Greszta, R. Pązik, P. Kłoda, A. Tomaszewska, E. Zachanowicz, K. Pałka, G. Ginalska and A. Belcarz, Materials Science and Engineering C, 2021, 118, 111360. 15 L. Gao, X. Cui, C. D. Sewell, J. Li and Z. Lin, Royal Society of Chemistry, 2021, preprint, DOI: 10.1039/d0cs00962h. 16 X. Cui, S. Lei, A. C. Wang, L. Gao, Q. Zhang, Y. Yang and Z. Lin, Nano Energy, 2020, 70, 104525. 17 L. Gao, X. Cui, Z. Wang, C. D. Sewell, Z. Li, S. Liang, M. Zhang, J. Li, Y. Hu and Z. Lin, PNAS, 2021, 118, e2023421118. 18 M. A. Yousefi, D. Rahmatabadi, M. Baniassadi, M. Bodaghi and M. Baghani, Macromol Rapid Commun, 2025, 46, 2400661. 19 M. Rincón-Iglesias, M. Salado, S. Lanceros-Mendez and E. Lizundia, Polymer (Guildf), 2022, 249, 124804. 20 H. Shen, B. Gao, S. Pan, L. Liu and G. Yang, J Magn Magn Mater, 2023, 587, 171310. 21 F. da Silva Lima, A. R. Medeiros, A. G. Rosenberger, E. Ballmann, D. C. Dragunski, E. C. Muniz and J. Caetano, Mater Chem Phys, 2023, 309, 128278. 22 J. Yang, H. Wang, Y. Zhang, H. Zhang and J. Gu, Nanomicro Lett, 2024, 16, 31. 23 M. A. A. Mohammed, Z. Chen, X. Du, B. Li, J. Zhang and T. Kang, Colloid Polym Sci, DOI:10.1007/s00396025-05391-x. 24 I. Bilecka, I. Djerdj and M. Niederberger, Chemical Communications, 2008, 886–888. https://doi.org/10.26434/chemrxiv-2025-46k18 ORCID: https://orcid.org/0000-0001-9417-6514 Content not peer-reviewed by ChemRxiv. License: CC BY 4.0