Skin Adhesives with Controlled Adhesion by Polymer Chain Mobility

ACS Appl. Mater. Interfaces , 2019, 11 (1), pp 1496–1502. DOI: 10.1021/acsami.8b18947. Publication Date (Web): December 18, 2018. Copyright © 2018 ...
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Skin Adhesives with Controlled Adhesion by Polymer Chain Mobility Zhen Gu,†,‡ Xizi Wan,†,§ Zheng Lou,∥ Feilong Zhang,§ Lianxin Shi,†,§ Siheng Li,† Bing Dai,†,§ Guozhen Shen,∥ and Shutao Wang*,†,§

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†

CAS Key Laboratory of Bio-inspired Materials and Interfacial Science, CAS Center for Excellence in Nanoscience, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China ‡ Department of Chemistry and Biological Engineering, University of Science and Technology Beijing, Beijing 100083, P. R. China § University of Chinese Academy of Sciences, Beijing 100049, P. R. China ∥ State Key Laboratory for Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, P. R. China S Supporting Information *

ABSTRACT: Wearable devices have attracted a lot of attention because of their importance in the biomedical and electronic fields. However, as one of the important fixing materials, skin adhesives with controlled adhesion are often ignored. Although remarkable progress has been achieved in revealing the natural adhesion mechanism and biomimetic materials to complex solid surfaces, it remains a great challenge to explore nonirritant, controlled skin adhesives without surface structure. Herein, we present skin-adhesive patches of polydimethylsiloxanes (SAPs) with controlled adhesion by simply modulating polymer chain mobility at the molecular level. The controlled adhesion of SAPs strongly depends on the proportion of polymer chains with different mobility exposed to the solid surface, including free chains, dangling chains, and cross-linking chains. As a proof of concept, we demonstrate that the SAP can act as a skin-friendly fix to monitor the human pulse by integrating with the poly(vinylidene fluoride-trifluorethylene)/ reduced graphene oxide (P(VDF-TrFE)@rGO) nanofiber sensor. This study provides a clue to design durable and skin-friendly adhesives with controlled adhesion for wearable devices. KEYWORDS: skin adhesives, chain mobility, cross-linking, polydimethylsiloxane (PDMS), wearable devices the toe pad of tree frog,21 and adhesive patches with protuberances in suction cups inspired by octopus.19 However, most of these biomimetic adhesive materials inevitably depend on ingenious surface micro/nanostructures. It remains a great challenge to develop an alternative strategy which is independent of surface structure to prepare a kind of skinfriendly adhesive tape with controlled adhesion. Herein, we report a skin-adhesive patch of polydimethylsiloxane (SAP) with controlled adhesion by merely tuning polymer chain mobility, without any elaborate micro/nanosurface structure. We also demonstrate that SAP can act as a skin-friendly fixer to adhere a wearable strain sensor. PDMS is a kind of elastomer commonly used in the fields of microfluids devices,22 stretchable electronics sealants,23 as well as biomedical implants24 for its good transparency, chemical inertia, softness, and stretchability. Usually, Sylgard 184 (Dow Corning) is the widely used PDMS-based elastomer. (Hereafter, the PDMS or PDMS-based elastomer refers to Sylgard 184 except for specific situations.) We reveal that the skin

1. INTRODUCTION Wearable devices are of paramount importance in human healthcare,1,2 including real-time sensing of sweat,3−5 articular thermotherapy,6 drug delivery,7,8 human motion monitoring,9 self-powered biomedical systems,10 and piezoelectric nanogenerators.11 Generally, those devices are fixed to the skin via wrist bands, wearable straps, and acrylic tapes.12,13 However, bands and straps make closely conformal contact with the skin surface difficult,12 and conventional acrylic tapes often result in red spots and pain during adhesive removal,14 especially for the elderly and children.15 It is strongly demanded to develop skinfriendly adhesives with controlled adhesion for wearable devices. Some unique adhesion phenomena in nature inspire us to design artificial adhesive materials. For example, gecko achieves controlled adhesion through hierarchical fibrillar microsetae and nanospatula;16 tree frog achieves controlled adhesion through micro/nanochannel structured toe pads;17,18 and octopus achieves controlled adhesion through the dome-like protuberances in the suction cups.19 These unique natural adhesion phenomena brought out a series of adhesioncontrolled materials, including gecko-inspired hierarchical polymer nanosetae,20 channel-patterned surface mimicking © 2018 American Chemical Society

Received: October 31, 2018 Accepted: December 18, 2018 Published: December 18, 2018 1496

DOI: 10.1021/acsami.8b18947 ACS Appl. Mater. Interfaces 2019, 11, 1496−1502

Research Article

ACS Applied Materials & Interfaces

Figure 1. Design of skin-adhesive patches of PDMS (SAPs) by regulating the polymer chain mobility. (a, d) Un-cross-linked PDMS adhesive liquid with free chains can spread on the substrate because of its high mobility. (b, e) Low cross-linked PDMS with free chains and dangling chains can tightly adhere to the surface. (c, f) Highly cross-linked PDMS can be easily peeled off from the substrate because of its less free chains and dangling chains. Note that the PDMS was dyed red to make the interface clear.

Figure 2. Skin adhesion properties of PDMS tape with different amounts of cross-linking agent. (a) Representative peeling force versus displacement curves for low and highly cross-linked PDMS (cross-linking agent amount of 2.0 and 9.1 wt %, respectively) on a male volunteer’s forearm skin. Compared with highly cross-linked PDMS, the low cross-linked PDMS shows higher adhesion and is more difficultly peeled off from the skin. (b) The adhesion force of PDMS tape on the skin of a forearm decreases with increasing cross-linking agent amounts. 2.2. Skin Adhesion Force Experiments. The skin adhesion force of the SAP was measured using a tensile tester (Mark-10 ESM301) based on the peeling model. The long strip samples were sticked on the skin of a volunteer’s forearm. The end of the strips was clamped and pulled upward at a constant speed of 15 mm/s (inset, Figure 2a). The angle between the strip and the forearm skin was maintained at 90° until the strip was completely detached from the skin. The long strip sample was prepared by laminating a PDMS prepolymer on a plastic film with a film applicator (PA2041, BYK) and then cut to the size of 80 mm × 15 mm × 200 μm for the test. The maximum force read during the peel test is the skin adhesion force. 2.3. Normal Adhesion Tests on the PDMS Adhesives. Adhesion measurements were performed on a universal material tester (CETR-UMT-2, Bruker) with a sapphire sphere indenter with diameter of 5 mm. The load range is 5 mN−500 mN, and the load resolution is 50 μN. The procedure of the test was as follows: the indenter was applied to the PDMS sample with a prepressure of 10 mN for 20 s and then was lifted up at a rate of 50 μm/s, and the force−distance curves were recorded throughout the process. For each sample, the same process was repeated 20 times to test the durability of PDMS adhesion. The samples were also subjected to cyclic normal indentation by a sphere at different retraction rates of

adhesion strongly depends on the proportion of polymer chains, including free chains, dangling chains, and cross-linking chains,25,26 as shown in Figure 1. Note that free chains are trapped in the polymer network without fixation; dangling chains are connected to the network by one end; and crosslinking chains are connected to the network by both ends. The un-cross-linked PDMS adhesive liquid with plenty of free chains was highly mobile and easy to spread on the substrate because of its high mobility (Figure 1a,d). Compared with the highly cross-linked PDMS (Figure 1c,f), the low cross-linked PDMS (Figure 1b,e) with more free chains and dangling chains adhered more tightly to the substrate.

2. EXPERIMENTAL SECTION 2.1. Fabrication of SAPs. PDMS base was mixed with crosslinking agent (Sylgard 184, Dow Corning, USA) under mechanical stirring at various amounts of curing agent (2.0 wt %, 2.4 wt %, 3.2 wt %, 4.8 wt %, 9.1 wt %). Degassed PDMS mixtures were cured at 80 °C for 0.5 h. After cooling to room temperature, the PDMS samples were used as skin adhesives. 1497

DOI: 10.1021/acsami.8b18947 ACS Appl. Mater. Interfaces 2019, 11, 1496−1502

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ACS Applied Materials & Interfaces

Figure 3. Adhesion properties and chain mobility of PDMS with different cross-linking density. (a) We compressed SAP to the value of Fl (loading force) and then measured the resultant maximum separation force, named as adhesion force Fad. δmax: the maximum relative displacement. (b) Maximum relative displacement (δmax) and adhesion work (Wadh) for PDMS decrease with increasing cross-linking agent amounts. Wadh: the adhesion work denoted as the shadow area. (c) Adhesion force (Fad) versus retract speed (ν) of PDMS with various cross-linking agent amounts. The inset shows the slope (β) of the Fad−ν curve decreases with increasing cross-linking agent amounts. (d) The dangling chains (green strands) and free chains (yellow strands) decrease with the cross-linking agent amounts. (e, f) Compared with PDMS with higher cross-linking agent amount, the network with low cross-linking agent amount has more free chains and dangling chains. 2.6. Biocompatibility Tests. The biocompatibility of SAP and acrylic medical patches (2733−25, 3M, Japan) was tested by attaching the same size of two adhesives (3 cm × 3 cm) on the forearm skin of a volunteer for the time period up to 1 day. Then, some known side effects such as redness were compared. 2.7. Fabrication and Characterization of the Skin-Attachable Sensor. The skin-attachable sensor was fabricated by following the procedure of Lou and Shen.27 P(VDF-TrFE)@rGO nanofibers are sandwiched between the upper and lower SAP layers, and the two ends are connected by copper electrodes to external copper wires for testing. When testing the pulse, the sensor was directly attached to the arm, and then a copper wire is connected to a Keithley 6487 picoammeter (Keithley Instruments, Cleveland, OH) to test the resistance change over time during the pulse beat. The main dimensions of the sensor are as follows: the sensor size is 10 mm × 1.2 mm × 1.3 mm, and the distance between the two copper

indenter displacement. Experiments were conducted on each sample with retraction speeds ranging from 2 to 500 μm/s. 2.4. Mechanical Property Experiments. The tensile test was carried out on an electric test bench (Mark-10 ESM301) at a speed of 100 mm/min. The test sample was a dog-bone shape obtained by cutting. 2.5. NMR Experiments. The nuclear magnetic test of PDMS was performed on VTMR20−010 V-T (Shanghai Niumag Electronic Technology Co., Ltd., China). The parameters of the test were set as follows: test temperature 60 °C, magnet temperature 35 °C, magnet strength 0.5 T, and resonance frequency 21.31 MHz. The selection of the test temperature of 60 °C is mainly based on the following two considerations: on the one hand, it is advantageous to distinguish the difference between samples of different cross-linking densities, and at the temperature of 60 °C, the sample is not subjected to thermal damage. 1498

DOI: 10.1021/acsami.8b18947 ACS Appl. Mater. Interfaces 2019, 11, 1496−1502

Research Article

ACS Applied Materials & Interfaces electrodes is 1 cm. The size of the sample can be adjusted depending on the occasion. The preparation process of the 500 μm SAP was as follows: the PDMS prepolymer with cross-linking agent content of 2.0 wt % was spin-coated on a silicon substrate at 700 rpm by a spin coater (MYCRO) and cured at 80 °C for 0.5 h, then peeled off from the silicon substrate, and cut to the required size.

fraction Fs and the swelling ratio Q decreased with increasing cross-linking agent amounts, which indicates PDMS becomes more cross-linked.31 To further investigate the relevance between adhesion force and different cross-linking density, we tested the evolution of adhesion force (F) under different retract speeds (Figure 3c). When the cross-linking agent content decreased, the increase of retract speed (ν) during high retract speed (F ∝ νβ) can cause more increase of the adhesion force (F), from ν15.9 for the PDMS with 9.1 wt % of cross-linking agent to ν50.4 for the PDMS with 2.0 wt % of cross-linking agent as the inset shows. These results indicate that the mobility of PDMS chains becomes easier with decreasing restrictions of the chemical cross-linking network. To explore the restriction effect of cross-linking agent on the mobility of PDMS chains, molecular network structures of PDMS with varying amounts of cross-linking agent were analyzed by the nuclear magnetic resonance spectrometer (NMR).32,33 The PDMS networks above the glass transition temperature (Tg ≈ −120 °C) are composed of three main different kinds of chains: free chains, dangling chains, and cross-linking chains. The three different molecular chains produce distinguishable relaxation signals in NMR experiments.25 The total transversal magnetization decay (M(t)) can be described as follows34

3. RESULTS AND DISCUSSION To explore the influence of cross-linking agent amounts on the adhesive property of SAP, we measured the peeling forces of a series of SAPs in a 90° peel test (Figure 2a). When the crosslinking agent amount was as low as about 2.0 wt %, a strong adhesion force between SAP and a volunteer’s forearm skin was observed to be 1.27 N/cm. With the increasing amounts of cross-linking agent, the adhesion force decreases apparently (Figure 2b). When the cross-linking agent amount reached 9.1 wt %, a weak adhesion force of SAP was only 0.02 N/cm, leading it to be easily peeled off (Figure 2a). Furthermore, to analyze the intrinsic adhesion of the as-prepared SAP, we carried out a sphere-on-plane test because the spherical indenter is insensitive to misalignment,28,29 As shown in Figure 3a, the spherical indenter slowly approaches and contacts the surface of SAP until reaching a certain loading force (Fl). Upon retracting, the force reaches an equilibrium point at point a1 where there is no deformation between SAP and the indenter. Subsequently, the maximum adhesion force is reached at point a2 when the crack initiates. At point a3, the relative displacement reaches the maximum δmax, and then the indenter completely detaches from SAP. The shaded area from point a1 to point a3 represents the adhesion work Wadh needed to separate the indenter from the PDMS surface. The shaded area (adhesion work) and maximum relative displacement increased with decreasing cross-linking agent amounts (Figures S1a and 3b). These results indicate that the PDMS with less amount of cross-linking agent dissipates more energy. The amount of cross-linking agent also influences the mechanical properties of SAP. When the cross-linking agent reduces from 9.1 wt % to 2.0 wt %, the elongation at break increases from 172% to 304%, and Young’s modulus decreases from 0.82 MPa to 11 kPa (Figure S2). This shows that the compliance and ductility of SAP increase with decreasing cross-linking agent amounts. Furthermore, there is no notable decay in adhesion force for all SAP samples with cross-linking agent contents ranging from 2.0 wt % to 9.1 wt % after 20 repeated sphere-onplane tests (Figure S1b), suggesting a durable adhesive property of SAP. To further understand the adhesion mechanism of SAP, we investigated the influence of cross-linking agent amounts on the cross-linking density by swelling tests. Herein, toluene was used for swelling and sol extraction. Samples were weighed (Mi) and then immersed in the toluene for 36 h. The samples reached swelling equilibrium state and were weighed (Ms). The swollen samples were dried to constant weight (Md) over 36 h under vacuum at 85 °C. The soluble fraction Fs and the swelling ratio Q are calculated from the relations as follows30 M − Md Fs = i Mi (1) Q=1+

ρpolymer M − M s d ρtoluene Md

ij t yz ij t yz M(t ) = A expjjj− − 0.5qM 2t 2zzz + B expjjj− zzz j T z j Tz k 2 { k 2{ ij t yz + C expjjj− zzz + A 0 j T2s z k {

(3)

where M(t) is the detected signal; t is decay time; A, B, C are the part signal of the cross-linking chains, dangling chains, and free chains to the total signal content, respectively; T2 is the transversal relaxation time of cross-linking chains and dangling chains signal; and T2s is the transversal relaxation time of the free chain signal. qM2 is the average remaining part of dipolar magnetic coupling, where M2 is the second moment of the rigid lattice and q is the ratio factor between the second moment above the glass transition temperature to the second moment of the rigid lattice. A0 is used to compensate for possible signal offsets.34 NMR spectra can give us a series of data and the signal intensity of PDMS (M(t)) at various crosslinking levels as a function of decay time (t) (Figure S4). After we brought the NMR test data into eq 3, we got the parameters A, B, and C and the proportion of cross-linking, dangling chains, and free chains. As shown in Figure 3d, the proportion of dangling chains (green strands) and free chains (yellow strands) decreases, and cross-linking chains (blue strands) increase with increasing cross-linking agent amounts. These results could reveal that low cross-linked PDMS with more free chains and dangling chains show higher adhesion than highly cross-linked PDMS with less free chains and dangling chains (Figure 3e,f), which is consistent with the adhesion force trend in Figure 2b. The adhesion force between PDMS and substrate is mainly derived from van der Waals force.17 The main reason for the high adhesion of PDMS with low cross-linking degree is that it contains more free chains and dangling chains. The highly mobile chains allow the PDMS to easily and conformally contact the substrate, increasing van der Waals force. Therefore, we chose low cross-linked PDMS as skin-adhesive tape.

(2)

in which ρpolymer and ρtoluene are the density of the PDMS and the toluene, respectively. Figure S3 shows that the soluble 1499

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Figure 4. Adhesion and skin-compatible performance of SAP and the SAP-based strain sensor. The SAP can tightly attach a coin onto the back of the hand (a), and the coin cannot be detached even though the hand was shaken violently from side to side (b). For the SAP, there is almost no red spots or injuries (c). For commercial acrylic adhesives, they usually generate red spots and pain after one-day adhesion on the skin (d). A P(VDFTrFE)@rGO-based strain sensor was tightly stuck on the skin of the wrist through the SAP (e), which can monitor pulse characteristics (f, g).

Surface adhesion of SAP strongly depends on the proportion of polymer chains with different mobility. Free chains and dangling chains with high mobility facilitate surface adhesion. Low cross-linked PDMS with more free chains and dangling chains shows higher surface adhesion than highly cross-linked PDMS with less free chains and dangling chains. The controlled skin adhesion of SAP is independent of any surface structure, and the skin-friendly SAP has been successfully applied to fix wearable sensors to monitor the human pulse. It is anticipated that the SAP can facilitate the development of epidermal electronics and biomedical adhesives. This strategy based on molecular mobility will provide a general principle to regulate surface adhesion and design adhesive patches based on current materials, such as hydrogel, elastomer, and so on.

To demonstrate the skin adhesion and biocompatible properties of SAP, we adhered one piece of coin (25 mm in diameter) onto the back of a hand through low cross-linked PDMS (Figure 4a). The coin attached firmly onto the skin even when the hand was violently rocking from side to side as shown in Figure 4b and Movie S1 (Supporting Information), which illustrates the strong skin adhesion of SAP. We compared the biocompatibility of commercial acrylic adhesive patches with SAP by observing the forearm skin of volunteers after 24 h adhesion.15,35 For the SAP, there was scarcely any red spots or injuries (Figure 4c). However, the skin under the patch became apparently red for commercial acrylic adhesive patches (Figure 4d). It is probably caused by the damaged skin after removal and a small amount of residual adhesive.15,35 As a proof of concept, the SAP was used to construct a wearable pulse sensor as a skin-friendly fixer (Figure 4e). In brief, this wearable pulse sensor was fabricated via layer-by-layer sandwiched structure. P(VDF-TrFE)@rGO nanofibers were between two SAPs (Figure S5). This sensing mechanism can be explained by the force-induced contact changes in the sandwiched sensor structure.27 To demonstrate the practicality, the wearable pulse sensor was stuck onto the skin of the volunteer’s wrist just above the radial artery, as shown in Figure 4e. The sensor can continuously record the pulse beats for several cycles (Figure 4f), and one pulse pressure wave has two characteristic peaks (Figure 4g).36 These results confirm that the SAP is capable of adhering wearable devices closely onto the skin surface as a friendly fixer.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b18947. The contact curves and durable adhesive property of SAPs (Figure S1). The stress−strain curves for the SAPs with varying amounts of cross-linking agent (Figure S2). Evolution of the soluble fraction Fs and swelling ratio Q for PDMS with various cross-linking agent amounts (Figure S3). The signal strengths of the PDMS with various cross-linking agent amounts were detected with the changes in decay time (Figure S4). Schematic diagrams of the preparation of the skin-attachable sensor (Figure S5). The adhesion forces of PDMS tape on different substrates (Figure S6) (PDF) A coin sticks firmly to the back of the volunteer’s left hand through the SAP (thickness: 500 μm, area: 1 × 1

4. CONCLUSION In conclusion, we developed a molecule mobility-based strategy to prepare a skin-friendly adhesive patch with controlled adhesion toward potential wearable applications. 1500

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cm2) even though the hand is violently rocking from side to side (AVI)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Shutao Wang). ORCID

Guozhen Shen: 0000-0002-9755-1647 Shutao Wang: 0000-0002-2559-5181 Author Contributions

Z. Gu and X. Z. Wan contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is supported by the National Research Fund for the National Natural Science Foundation (21425314, 21434009, and 21421061), National Program for Special Support of Eminent Professionals, and Fundamental Research Funds for the Central Universities (FRF-TP-17-051A1, FRFBR-17-002B).

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DOI: 10.1021/acsami.8b18947 ACS Appl. Mater. Interfaces 2019, 11, 1496−1502

Research Article

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DOI: 10.1021/acsami.8b18947 ACS Appl. Mater. Interfaces 2019, 11, 1496−1502