Heat-stimulated lifetime-controllable encapsulation for transient electronics

Abstract

Transient electronic systems have attracted considerable attention because of their ability to degrade after a specific period. The development of flexible encapsulant materials with hydrophobic and stimulus-responsive characteristics is necessary for the operation and lifetime control of transient electronics. Although several stimulus-responsive encapsulating materials have been developed over the past few years, harsh stimuli and degradation products limit the application of encapsulants. In this study, composite films of biodegradable polymers (e.g., polycaprolactone) and fatty acids (e.g., lauric acid), which are biodegradable and biocompatible, were used as encapsulants for transient electronics. Composite films were formed using a simple solvent-casting technique. The water permeation and stimulus responsiveness of the composite films were studied under biological conditions, in which the lifetime of the encapsulated electronic component was controlled by manipulating the film composition and applying heat for a short period. The development of composite films of polymers and fatty acids facilitated the precise control of the lifetime of the composite material, that is, the dissolution rate. Moreover, the application of the composite material as an encapsulant achieved facile lifetime control of transient electronic devices, thereby significantly accelerating the commercialization of transient electronics.

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Introduction

Transient electronics are a class of electronics that completely disintegrate after operating for a specific duration.¹ The application of transient materials in medical devices, environmental monitors, and energy-related devices has drawn attention owing to the increasing awareness of electronic waste.^2,3 Among various applications, the development of bioelectronic systems integrated with transient electronics is being continuously studied to address the increasing demand for personalized healthcare and medication. The use of transient devices in the biomedical fields includes sensors for biomarkers, electrical stimulators, drug delivery devices, and transducers for powering implanted devices, which require different device structures and lifetime controls.^4–7 Consequently, suitable encapsulating materials that consider both the structure and lifetime control of electronic devices are imperative for the practical implementation of transient electronics.

The performance of single-component encapsulants depends on the permeation of water vapor or hydrolytic degradation of the encapsulant. Without active control over the water permeation of the encapsulating material, the lifetime of the encapsulated device is determined based on the lifetime of the encapsulant.^8,9 Therefore, the combination of stimulus-responsive materials and encapsulating materials for the lifetime control of transient electronics has drawn attention (Table S1, ESI†). Park et al. developed a wax-based heat-stimulated encapsulation system, in which thermal stimuli internally released an encapsulated acid (i.e., methanesulfonic acid) to dissolve the polymer substrate (i.e., cyclic poly(phthalaldehyde)) via acidic depolymerization.¹⁰ Zhong et al. developed a photoresponsive hydrogel/metal-oxide bilayer encapsulation layer. With the application of UV light, the hydrogel structure disintegrated through the cleavage of the azo bonds, thereby increasing the water penetration rate.¹¹ Sim et al. applied an electrical current as the stimulus for the lifetime control of transient electronics with a silicon nitride (Si₃N₄) membrane to protect the device layer against the encapsulated acidic solution. When an electrical current was applied to the acidic solution, the gas generated within the solution exerted pressure on the Si₃N₄ membrane, causing the membrane to fracture, leading to the released acidic solution dissolving the electronic components.¹²

Specific stimuli are suitable for implantable electronic applications. In particular, extreme changes in the chemical (e.g., pH and ion concentration) and physical (e.g., high temperature) conditions inside the human body can damage various tissues or organs in the surrounding environment, which defeats the purpose of implantable medical devices.¹³ Only a small number of stimuli are applicable to the human body. Heat stimulus can be applied to the human body through various means (e.g., hyperthermia, infrared radiation, and laser ablation) in a precise area with a determined temperature.^14–16 Moreover, it is widely used in various medical treatments ranging from pain relief and inflammation control to cancer treatment. Therefore, we selected mild heat as the activating stimulus for the lifetime control of the encapsulating material in this work.

Various inorganic and organic materials have been used as encapsulation materials in transient electronics. Inorganic materials, such as Si, Si oxide, nitride, and various metal oxides, have been studied as encapsulants because of their slow degradation and low water vapor permeation.¹⁷ However, the rigidity of these materials poses challenges when applied to implantable devices because of the dynamic movements of biological tissues. Therefore, organic materials, such as biodegradable polymers, have been investigated as encapsulants because they can conform to the surrounding biological environment.^18–20 In this study, polycaprolactone (PCL) was selected as the polymer matrix or encapsulating material. PCL is a biocompatible and biodegradable polymer with excellent mechanical properties; thus, it can be applied to medical implants.^21–23 Moreover, PCL is hydrophobic, which is a critical property for encapsulating materials in transient electronics. In particular, hydrophilic polymers exhibit high water vapor transmission rates and short lifetimes when applied to encapsulating materials.²⁴ Thus, the application of PCL prolongs the lifetime of the resulting material, which is crucial for the long-term application of transient electronics.

Fatty acids are organic materials that are abundant in nature. The melting point (T_m) of fatty acids varies according to the length and saturation degree of their aliphatic chain, which can be controlled by forming eutectic mixtures of two or more fatty acids.^25,26 Previous studies analyzed the thermoresponsive characteristics of fatty acids in medical and drug delivery applications. Zhang et al. developed a near-infrared (NIR)-triggered thermoresponsive microneedle drug delivery system, in which drug-loaded mesoporous silica encapsulated with polydopamine and fatty acids was applied to poly(vinylpyrrolidone) microneedles. The NIR radiation of a microneedle photothermally induced heat owing to polydopamine, liquified the fatty acid, and released the drug component.²⁷ Rehman et al. developed a drug delivery system based on solid lipid nanoparticles loaded with drugs through the hot-melt encapsulation method. The drug was released when the temperature of the surrounding environment was increased.²⁸ Lu et al. developed a mesoporous carbon nanoparticle/fatty acid-based drug delivery system, in which the carbon structure acted as the drug carrier and photothermal component, enabling the delivery of hydrophobic drug molecules with NIR radiation.²⁹ The application of fatty acid as oleogel encapsulant has been recently reported, where the application of fatty acid enhanced the lubricity and anti-biofouling properties of the composite material.³⁰ To the best of our knowledge, the application of fatty-acid-incorporated polymeric composite to transient electronic devices is yet to be reported.

Herein, we report the development and characteristics of a polymer/fatty acid composite film for the encapsulation of transient electronics. Without any external stimuli, the transport of water molecules through the composite film slowly degraded the electronic components of the encapsulated devices. Lauric acid (LA) was applied as the fatty acid. LA has a T_m of 43–45 °C and dermal exposure to this temperature range is considered safe for short periods. The fatty acid incorporated in the polymer film matrix acted as a phase change material. When exposed to mild heat (43–45 °C), the fatty acid transformed into a liquid. Water or body fluids then passed through the voids created within the polymer film, thereby degrading the internal devices. The development of polymeric encapsulating materials with actively controlled lifetime can advance the practical use of implantable, biodegradable, and transient electronic devices.

Experimental

Materials

PCL (average M_n = 80 000) was purchased from Sigma-Aldrich (USA). LA (99%) and ethyl acetate (99.5%) were purchased from Daejung Chemical and Metals Co. Ltd (Gyeonggi-do, Republic of Korea). Phosphate-buffered saline (PBS, 1X, pH 7.2) was provided by WelGENE, Inc. (Daegu, Republic of Korea).

Preparation of stimulus-responsive polymer films

PCL film was fabricated using a solvent casting method.³¹ PCL pellets (0.5 g) were added to 10 mL ethyl acetate (5% w/v) and sonicated for 2 h to promote the pellet dissolution. The polymer solution was stirred for 30 min before being cast in a glass Petri dish (diameter: 80 mm) and covered for slow evaporation. The polymer film did not shrink to the center, and no air bubbles were formed inside the film during drying owing to the slow evaporation. The solution was then dried in a fume hood at 25 °C for 48 h, and further dried in a vacuum oven at 40 °C to completely dry the resulting film (thickness of ∼40 μm). The amount of initial polymer solution determines the resulting film thickness. In detail, 5 and 20 mL polymer solutions yielded films with the thickness of ∼20 and ∼100 μm, respectively. For the fabrication of the PCL/LA films, LA was added to ethyl acetate at specific weight ratios to the PCL pellets (PCL : LA ratios of 5 : 1, 10 : 1, 15 : 1, 25 : 1, 50 : 1, and 100 : 1). The remaining steps were the same as that of the preparation of the PCL films. All the films were stored in a desiccator until further use. Unless otherwise noted, the films with a thickness of 40 μm were used for the experimental procedures.

Degradation test of the PCL and PCL/LA films

The as-prepared PCL and PCL/LA (PCL : LA ratios of 5 : 1, 10 : 1, and 15 : 1) films were cut into rectangular specimens (10 mm × 20 mm). The rectangular samples were immersed in PBS and kept at 37 and 45 °C, respectively, under constant stirring for three weeks. Every 3 d, the solution in the reaction container was decanted and refilled with fresh PBS. After immersion for 7, 14, and 21 d, the films were retrieved and dried under ambient condition for further analysis.

Water penetration test of the PCL and PCL/LA films

Mg resistor patterns (thickness: 300 nm; serpentine length: 1.45 mm; width: 150 μm; turns: 4) were formed on a Si wafer using electron beam evaporation with a deposition speed of 1.5 Å s⁻¹. Cu wires were connected to the Mg resistor patterns using a conductive Ag epoxy paste for external electrical data acquisition. The PCL and PCL/LA films prepared with various thicknesses (20, 40, and 100 μm) and compositions (PCL : LA ratios of 5 : 1, 10 : 1, and 15 : 1) were adhered to the Mg-patterned wafer by heating the substrate above the T_m of PCL (e.g. 70 °C). A poly(dimethyl siloxane) (PDMS, Sylgard™ 184; part A : part B = 4 : 1) chamber was loaded onto the polymer film by heating the substrate above the T_m of PCL. The PDMS chamber was filled with deionized (DI) water or PBS and sealed with the PDMS lid to avoid evaporation. The wafers were then placed in an oven preheated to 37 or 45 °C and stored until the infiltrated water dissolved the Mg resistors and the electrical resistance was no longer measurable. The aqueous solution in the PDMS chamber was replaced daily to avoid changes in the water penetration rate associated with the possible dissolution of PCL or LA.

Stimulus responsiveness test of the PCL/LA films

Two Mg resistor patterns were prepared for each PCL/LA film (PCL : LA ratios of 5 : 1, 10 : 1, and 15 : 1) using the same experimental settings as those used in the water penetration tests. The PDMS chamber was filled with DI water and kept at 25 °C for one week. After one week, one of the Mg resistors was placed on a hotplate, heated to 45 °C, kept for 5 min, and was let cooled to 25 °C. The electrical resistances were measured for both Mg resistor patterns until the Mg patterns were dissolved in the infiltrated water.

Characterization

The thermogravimetric analysis (TGA) of the PCL and PCL/LA films was performed under N₂ atmosphere at 25–800 °C at a heating rate of 10 °C min⁻¹ using a thermogravimetric analyzer (TGA N–1000, Sinco). Differential scanning calorimetry (DSC; DSC N-650, Sinco) was used to obtain the DSC curves under N₂ atmosphere at 25–400 °C with a heating rate of 10 °C min⁻¹. Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectrometer (FT/IR-4100, Jasco) was used to collect the spectra in the region of 600–4000 cm⁻¹. The surface morphologies of the PCL and PCL/LA films were observed using scanning electron microscopy (SEM; JSM-6380, JEOL). A contact profilometer (XP-200, Ambios Technology) was used to measure the thickness profiles of the films on glass substrates. The water contact angle of the films was measured using a drop shape analyzer (DSA100, KRÜSS). The mechanical properties of the PCL and PCL/LA films before and after degradation were measured using a universal testing machine (3343, Instron).

Results and discussion

TGA was performed to confirm the composition of the PCL/LA films (Fig. 1). All PCL/LA films exhibit weight loss in the temperature range of 150–250 °C, which is attributed to the thermal degradation of LA. At 275 °C, the PCL/LA films demonstrated a weight loss proportional to the LA content, whereas pure PCL showed no significant weight loss. A second major weight shift occurred at 300–425 °C owing to the thermal decomposition of polymer chains.^32,33 The TGA curve for the film with a PCL : LA ratio of 100 : 1 was also measured, whereby no noticeable difference was noted compared to the PCL film (Fig. S1, ESI†).

Fig. 1 (a) TGA curves and (b) magnified view of the TGA curves of the PCL and PCL/LA films with various compositions (PCL : LA ratios of 5 : 1, 10 : 1, 15 : 1, 25 : 1, and 50 : 1) at 100–400 °C.

The thermal responsiveness of the PCL and PCL/LA films was determined using DSC (Fig. 2). The pure PCL film exhibited a single endothermic peak at 63 °C, corresponding to the T_m of PCL. The PCL/LA films with PCL : LA ratios of 5 : 1, 10 : 1, and 15 : 1 showed two endothermic peaks at 60–62 °C, assigned to the T_m of PCL, and at 43–45 °C, assigned to the T_m of LA.^34,35 The PCL/LA films with lower LA content (PCL : LA ratios of 25 : 1 and 50 : 1) did not exhibit any peaks related to the melting of LA, indicating the lack of thermal response at 43–45 °C, which is unsuitable for the development of thermoresponsive films at the T_m of LA in this study.

Fig. 2 DSC curves of the PCL and PCL/LA films with various compositions (PCL : LA ratios of 5 : 1, 10 : 1, 15 : 1, 25 : 1, and 50 : 1, respectively).

The compositions of the PCL and PCL/LA films (PCL : LA ratios of 5 : 1, 10 : 1, and 15 : 1) were further examined using ATR-FTIR spectroscopy (Fig. 3). The peaks common to all four spectra were observed mainly in three regions, namely at 3000–2800 cm⁻¹ for the C–H stretching, 1800–1600 cm⁻¹ for the C=O stretching, and 1350–1050 cm⁻¹ for the C–O stretching. For the PCL, –CH₂– asymmetric and symmetric stretching peaks were observed at 2944 and 2864 cm⁻¹, respectively. The PCL/LA films with the PCL : LA ratios of 10 : 1 and 15 : 1 showed broader peaks due to the presence of LA, whereas that with the PCL : LA ratio of 5 : 1 film achieved a shift in the –CH₂– asymmetric and symmetric stretching peaks to 2944 and 2864 cm⁻¹, respectively. In addition, new peaks corresponding to –CH₃ asymmetric and symmetric stretching appeared at 2954 and 2870 cm⁻¹, respectively, owing to the presence of LA (Fig. 3b). For the PCL film, a single peak was observed at 1720 cm⁻¹, corresponding to the carbonyl C=O stretching. When LA was added, the corresponding peak gradually shifted from 1720 cm⁻¹ to 1696 cm⁻¹ with slight broadening (Fig. 3c). For the peaks related to the C–O stretching, two characteristic peaks corresponding to the C–O–C asymmetric and symmetric stretching were observed at 1240 and 1160 cm⁻¹, respectively.^36,37 However, with the film with a PCL : LA ratio of 5 : 1, both peaks disappeared (Fig. 3d). In addition, ¹H NMR analysis was performed to confirm the interaction between PCL and LA (Fig. S2, ESI†). The NMR spectra of the PCL/LA films showed peaks both from PCL and LA, and extra peaks were not visible. Through this, PCL and LA are considered to be mechanically mixed together, and do not exhibit any chemical interactions.

Fig. 3 (a) ATR-FTIR spectra of the PCL and PCL/LA films with the PCL : LA ratios of 5 : 1, 10 : 1, and 15 : 1. The highlighted regions denote the characteristic peaks of the C–H stretching (blue), C=O stretching (green), and C–O stretching (orange), respectively. (b)–(d) Magnified views of the highlighted regions in (a), respectively.

The height profiles of the PCL and PCL/LA (PCL : LA ratios of 5 : 1, 10 : 1, and 15 : 1) films were characterized using a stylus profilometer (Table 1 and Fig. S3, ESI†). The average thickness of all four films was measured as ca. 37 μm. Meanwhile, the average roughness depends on the composition of the films. The roughness of the PCL/LA film with the PCL : LA ratio of 5 : 1 was substantially larger than that of the other films, which was further investigated using SEM.

Table 1 Film thickness and average roughness of the PCL and PCL/LA films with various compositions (PCL : LA ratios of 5 : 1, 10 : 1, and 15 : 1)^a

Sample	Film thickness (μm)	Roughness (Ra, nm)
a The average thickness and mean surface roughness of the films were measured by a profilometer (XP-200, Ambios Technology) with a stylus radius of 2.5 μm and applied stylus force of 10 mg.
PCL	36.88	699.8
5 : 1	35.87	1594.7
10 : 1	36.21	621.3
15 : 1	39.33	918.5

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The PCL and PCL/LA films (PCL : LA ratios of 5 : 1, 10 : 1, 15 : 1) were immersed in PBS solution, heated to 37 and 45 °C, respectively, and kept for three weeks to investigate their degradation under biological conditions (Fig. 4 and Fig. S4, ESI†). On day 0, the PCL film exhibited a smooth surface, whereas the PCL/LA films showed an irregular surface. The surface morphology of the PCL film stored at 37 °C did not change considerably after three weeks, whereas surface erosion was observed on day 21 in the films stored at 45 °C. For the PCL/LA films, more surfaces were exposed to the solution as LA was exposed to the surface dissolved over time. The PCL/LA films with the PCL : LA ratios of 5 : 1 and 15 : 1 kept at 37 °C showed degradation with visible cracks, pores, and surface erosion after three weeks. The PCL/LA film with the PCL : LA ratio of 5 : 1 exhibited the most cracks and pores because LA incorporated within the PCL matrix weakened the mechanical strength of the film. At 45 °C, cracks and pores appeared in all PCL/LA films on day 14, indicating the notable deterioration of the film when exposed to temperatures above the T_m of LA. In addition, the crack formation in the PCL/LA films with the PCL : LA ratios of 5 : 1 and 10 : 1 resulted in the collapse of the film structure at day 7 and 14, respectively. The low-magnification image of the PCL/LA film with the PCL : LA ratio of 5 : 1 film at day 21 at 45 °C depicts the complete degradation of the film with cracks propagating within the film structure. PCL and PCL/LA films were subjected to an acceleration test at 37 °C, 45 °C, and 55 °C while being immersed in pH 9 buffer solution to observe the bulk degradation of the films (Fig. S5, ESI†). The PCL and PCL/LA films at 37 °C did not show noticeable changes in morphology, while films exposed to 45 °C and 55 °C showed deformation even after 1 h, as elevated heat and pH actively enhanced the hydrolysis of ester groups in the polymer chain.

Fig. 4 SEM images of PCL (a–d) and PCL/LA films with various compositions (PCL : LA ratios of 5 : 1 (e–h), 10 : 1 (i–l), and 15 : 1 (m–p), respectively) during degradation in PBS (pH 7.2) at 37 °C over three weeks. The arrows show the formation of cracks and pores, whereas the dashed-line circle shows the surface erosion of the films.

The surface-water contact angles were measured for the PCL and PCL/LA (PCL : LA ratios of 5 : 1, 10 : 1, and 15 : 1) films after degradation in PBS (Fig. 5). For the films immersed in PBS at 37 °C, all films on day 0 exhibited a contact angle of 80° or higher, indicating the hydrophobic nature of the PCL and PCL/LA films. The PCL/LA films exhibited higher contact angles because of the hydrophobicity of LA and the irregular surface structure induced by the excessive LA content.³⁸ The immersion of the films in PBS stripped the excess LA from the surface, revealing the actual film surface, which resulted in a decrease in the contact angle measured for each film over time. For the PCL/LA film with a PCL : LA ratio of 5 : 1 film, the contact angle sharply decreased on days 14 and 21, as observed by SEM, owing to the formation of cracks and pores. Meanwhile, the contact angle of the PCL film did not decrease remarkably because there was no notable change in the surface morphology after immersion in PBS. The degradation test at 45 °C yielded similar results, whereby the initial contact angle of the PCL/LA films was higher than that of the PCL film, and the contact angle of each film decreased throughout the test.

Fig. 5 Water contact angle of the PCL and PCL/LA films (PCL : LA ratios of 5 : 1, 10 : 1, and 15 : 1) as a function of the degradation time while immersed in PBS (pH 7.2) at (a) 37 °C and (b) 45 °C.

The composition of the PCL/LA film, particularly with a PCL : LA ratio of 5 : 1, during the degradation test in PBS at 37 °C for three weeks was observed using ATR-FTIR (Fig. 6). On day 0, a dominant LA spectrum was noted for the C–H, C=O, and C–O stretching regions of the PCL/LA film, compared to that of the PCL film. The spectrum at day 7 showed similar results to that at day 0, whereas the effect of LA started to diminish at day 14. The C=O stretching peak shifted from 1696 to 1720 cm⁻¹, and a distinct C–O stretching peak of PCL appeared at 1720 cm⁻¹, denoting the removal of excess LA on the surface of the PCL/LA film. The transition continued on day 21, and peaks assigned to the C–H stretching, particularly –CH₃ stretching (2954 and 2870 cm⁻¹), were no longer visible. When the PCL/LA films were exposed to biological conditions, the LA content decreased over time, resulting in cracks and surface erosion, which are visible through SEM.

Fig. 6 (a) ATR-FTIR spectra of the PCL/LA film with a PCL : LA ratio of 5 : 1 on days 0, 7, 14, and 21 of immersion in PBS (pH 7.2) at 37 °C. Enlarged regions of the characteristic peaks of the (b) C–H stretching, (c) C=O stretching, and (d) C–O stretching.

The mechanical properties of the PCL and PCL/LA (PCL : LA ratios of 5 : 1, 10 : 1, and 15 : 1) films were determined using uniaxial tensile tests before and after the degradation test (Fig. 7, Table 2). The PCL and PCL/LA (PCL : LA ratios of 10 : 1 and 15 : 1) films exhibited ductile behavior and elastic elongation, followed by yielding and necking due to the plastic deformation of the films, drawing, strain hardening, and finally, breaking.³⁹ Comparing these three films, the maximum tensile strength and elongation decreased as the LA content increased. This phenomenon was noted for the PCL/LA films with a PCL : LA ratio of 5 : 1 exhibiting a brittle behavior compared to the other three films. For the PCL film, ductile behavior was maintained after three weeks of degradation with a gradual decrease in the overall mechanical properties. Unlike the PCL film, the PCL/LA films exhibited brittleness even after one week of degradation, which was most likely ascribed to the formation of defects in the film structure due to the loss of LA, resulting in poor mechanical properties. For the degradation test at 45 °C, the deterioration of the mechanical properties occurred earlier than that at 37 °C (Fig. S6, ESI†). In order to further study about the mechanical properties of the PCL and PCL/LA composite films, the stress–strain curves were obtained after each film was exposed to 1000 bending cycles. The tensile strength and the elongation of each film after bending cycles remained close to those of the films without bending cycles (Fig. S7, ESI†).

Fig. 7 Stress–strain curves of the (a) PCL film and PCL/LA film with PCL : LA ratios of (b) 5 : 1, (c) 10 : 1, and (d) 15 : 1 after immersion in PBS (pH 7.2) at 37 °C for three weeks.

Table 2 Mechanical properties of the PCL and PCL/LA films on days 0, 7, 14, and 21 of immersion in PBS (pH 7.2) at 37 °C^abcd

Sample		Maximum tensile stress (MPa)	Elongation at break (mm mm^−1)	Young's modulus (MPa)
a The mechanical properties of the prepared films were measured using a universal testing machine (3343, Instron) equipped with a static load cell of 1 kN. b The rectangular (1 cm × 2 cm) films were used to measure the mechanical properties. c The elongation speed was set at 50 mm min^−1 for each measurement. d The Young's modulus of all samples are expressed as mean ± standard deviation, and each samples were measured three times.
PCL	Day 0	114.46	13.54	446.58 ± 19.02
	Day 7	85.13	9.36	385.80 ± 6.93
	Day 14	56.40	8.94	365.06 ± 8.03
	Day 21	45.58	6.42	312.53 ± 8.87
5 : 1	Day 0	49.52	2.53	399.94 ± 17.06
	Day 7	49.39	0.77	82.64 ± 22.05
	Day 14	38.29	1.87	53.66 ± 11.79
	Day 21	31.82	0.13	112.25 ± 129.19
10 : 1	Day 0	56.05	5.87	420.93 ± 6.27
	Day 7	32.35	0.40	359.11 ± 7.72
	Day 14	39.39	0.24	333.37 ± 6.65
	Day 21	40.91	0.26	278.39 ± 19.04
15 : 1	Day 0	50.28	8.62	430.06 ± 10.06
	Day 7	25.20	2.78	370.94 ± 16.16
	Day 14	31.27	0.53	301.92 ± 16.82
	Day 21	29.74	0.64	268.55 ± 19.18

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The water permeability tests of the PCL and PCL/LA (PCL : LA ratios of 5 : 1, 10 : 1, and 15 : 1) were performed by measuring the change in the electrical resistance after exposing the Mg resistor encapsulated in the prepared film to water (Fig. 8). The Mg resistor patterned on the Si wafer was sealed with a film via thermal bonding, and a PDMS chamber containing water was placed on it. As the PCL and PCL/LA films were applied as the encapsulation layers of the Mg pattern, water molecules gradually permeated the film structure, eventually dissolving the Mg patterns and increasing the electrical resistance. Thus, the sharp increase in the electrical resistance of the Mg resistor indicated water permeation through the film. The water barrier properties of all films were first assessed at 25 °C using DI water. The Mg resistor encapsulated in the pristine PCL film maintained its electrical resistance for 25 d, whereas that in PCL/LA films with PCL : LA ratios of 5 : 1, 10 : 1, and 15 : 1 maintained their resistance for 21, 28, and 33 d, respectively. PCL films of different thicknesses were subjected to a water permeation test at 25 °C using DI water. The Mg resistor maintained its function for 9 and 38 d for the PCL films with thickness of 20 and 100 μm, respectively. Because water molecules penetrated the PCL and PCL/LA films, the permeation rate was affected by the film composition, structure, and thickness. As observed by SEM, the LA addition during the film casting resulted in an excess layer of LA on the film surface, and its irregular surface structure increased the hydrophobicity of the film.³⁸ However, a high LA content inhibited the PCL matrix to maintain its structural integrity, thereby increasing the water permeability despite the hydrophobic surface of the film with a PCL : LA ratio of 5 : 1. The film thickness also affected the water permeability of the films as the length of the water permeation pathway increased, resulting in a longer lifetime for the Mg resistor patterns.

Fig. 8 Water permeability of the PCL and PCL/LA encapsulation layers. (a) Resistance changes of the Mg resistor patterns (thickness: ≈ 300 nm) encapsulated in PCL and PCL/LA (PCL : LA ratios of 5 : 1, 10 : 1, and 15 : 1) films while immersed in DI water at 25 °C (encapsulation layer thickness ≈ 40 μm). (b) Resistance changes of the Mg resistor patterns encapsulated in PCL films with different thicknesses (∼20 and ∼100 μm). The curves show the resistance changes of the Mg resistor patterns encapsulated in PCL and PCL/LA films while immersed in (c) DI water at 37 °C and (d) PBS at 37 °C.

Stimulus responsiveness was demonstrated for the PCL/LA (PCL : LA ratios of 5 : 1, 10 : 1, and 15 : 1) films using conditions similar to that in the water permeability test (Fig. 9). Two sets of Mg resistor patterns encapsulated in each film were prepared. Both resistors were exposed to DI water at 25 °C; one was kept until the electrical resistance of the Mg pattern increased and was no longer measurable and the other was kept for 7 d and exposed to heat at 45 °C for 5 min on day 7. As shown in Fig. 8, the lifetime of the Mg resistor patterns exposed to DI water at 25 °C depends on the film composition, where the Mg resistor encapsulated in the film with a lower LA content functioned longer. When the Mg patterns were exposed to heat for 5 min, the resistance of all samples immediately increased, implying the hydrolytic degradation of the Mg pattern due to the penetration of DI water through the PCL/LA film. Regardless of the LA content, all the films exhibited a fast response to heat, where the liquefaction of LA instantly formed channels for the water molecules to pass through. Thus, the lifetime control of the PCL/LA composite films through heat exposure was confirmed. The lifetime control of transient electronic devices can be extended through the application of an additional stimuli-responsive film. Polyacrylic acid (PAA) films were applied underneath the PCL/LA composite film, forming a double layer of encapsulation. PAA is responsive to basic conditions, where the polymer chains expand due to the ionization of the abundant carboxylic groups. With this setup, the Mg resistor pattern was exposed to two buffer solutions with pH of 1 and 8, respectively, while being exposed to mild heat of 45 °C. The Mg resistor pattern that was exposed to pH 1 buffer solution remained active even after 1 h, but the Mg resistor pattern exposed to pH 8 buffer solution showed a sharp increase in resistance, exhibiting faster degradation when exposed to basic conditions (Fig. S8, ESI†). The effect of applying polymer film encapsulation on electronic devices was tested with a piezoresistive pressure sensor (Fig. S9, ESI†). The response of pressure sensor to changes in pressure was measured with changes in the electrical resistance of the sensor. The pressure sensor without encapsulation showed sensitivity of 2.57 Ω mm Hg⁻¹, and the pressure sensor with PCL/LA encapsulation showed sensitivity of 2.51 Ω mm Hg⁻¹. Through this, it was possible to evaluate the impact of applying the PCL/LA encapsulant, and the results showed that the PCL/LA encapsulant has only a minimal effect on the performance of the encapsulated electronic device.

Fig. 9 Stimulus responsiveness test of the PCL/LA films with PCL : LA ratios of (a) 5 : 1, (b) 10 : 1, and (c) 15 : 1. Two Mg resistor patterns (thickness ≈ 300 nm) encapsulated in PCL/LA films were immersed in DI water at 25 °C for 7 d. One of the resistors was exposed to heat at 45 °C for approximately 5 min on day 7.

Conclusion

Biodegradable polymer/fatty acid composite films were developed for heat-stimulated lifetime-controllable encapsulation in transient electronics. The application of PCL and LA as encapsulant materials offered the advantages of biocompatibility and hydrophobicity. Controlling the composition of the composite film affected the surface morphology, where excess LA was embedded on the film surface. The hydrophobicity of LA promoted the hydrophobic characteristics of the surface; however, after immersion under biological conditions, excessive LA content formed cracks and pores, lowering the structural integrity and water repellency. The degradation of the tensile strength of the films was measured using contact angle measurements and uniaxial tensile tests. The water permeabilities of the PCL and PCL/LA films were measured using Mg resistor patterns. The addition of LA to the PCL matrix improved the water resistance while maintaining the long lifetime of the PCL films for up to three weeks. The stimulus response of the composite films was confirmed by applying heat to the composite films before the failure of the encapsulated electrical component. All the composite films showed a rapid response to the application of heat (45 °C) within 5 min, where the dissolution of LA prompted the water permeation through the film structure, dissolving transient components. The development of composite encapsulant films that respond to mild thermal stimuli could promote the development of transient electronic devices for medical and environmental applications.

Author contributions

Hyukjoon Gwon: data curation, formal analysis, investigation, methodology, writing – original draft. Seungae Lee: conceptualization, resources, supervision, funding acquisition, methodology, project administration, writing – review & editing.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2022R1C1C1008249) and the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. RS-2023-00222078).

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Footnote

† Electronic supplementary information (ESI) available: (1) Comparison of stimuli-responsive encapsulating materials for transient electronics, (2) TGA curve of PCL/LA (PCL : LA ratio of 100 : 1) film, (3) NMR spectra of PCL, LA, and PCL/LA films, (4) thickness profile of PCL and PCL/LA (PCL : LA ratios of 5 : 1, 10 : 1, and 15 : 1) films, (5) SEM images of the PCL and PCL/LA films under degradation in PBS at 45 °C, (6) photographs of PCL and PCL/LA films after accelerated degradation testing, (7) stress–strain curves of PCL and PCL/LA films after immersion in PBS at 45 °C, (8) stress–strain curves of PCL and PCL/LA films after multiple bending cycles, (9) dual-stimuli responsiveness test with an additional layer of polyacrylic acid (PAA) film, and (10) effect of encapsulating material on the performance of piezoresistive pressure sensor. (PDF). See DOI: https://doi.org/10.1039/d4tc02138j

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