Gradient Polydopamine Coating: A Simple and General Strategy

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Gradient Polydopamine Coating: A Simple and General Strategy towards Multi-Shape Memory Effects Yuan Wei, Xiao-dong Qi, Shiwen He, Shihao Deng, Dingyao Liu, and Qiang Fu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b13134 • Publication Date (Web): 31 Aug 2018 Downloaded from http://pubs.acs.org on September 3, 2018

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ACS Applied Materials & Interfaces

Gradient Polydopamine Coating: A Simple and General Strategy towards Multi-Shape Memory Effects

Yuan Wei, Xiaodong Qi, Shiwen He, Shihao Deng, Dingyao Liu, Qiang Fu* College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, China *Corresponding author, E-mail: [email protected] (Q. Fu)

Abstract Multi-shape memory polymers (multi-SMPs) exhibits many potential applications such as aerospace, soft robotics and biomedical devices due to their unique abilities. Although many works are done to broaden the preparations of multi-SMPs, the desire to a simple and versatile strategy as well as more complex shapes still exists. Moreover, a light-induced SMP shows more advantages than a thermal-induced one in many practical working circumstances. Herein, inspired by strong adhesion and efficient photo-thermal conversion of poly-dopamine (PDA) coating, we report a more simple and facile approach to prepare light-induced multi-SMPs by introducing a gradient PDA coating on a dual-SMP through time controlled dipping. Photo-thermal converting properties with varying thickness of PDA under the tunable near-infrared (NIR) light source are investigated. Then light-induced multi-SMEs based on gradient PDA coatings are illustrated, three designs of multi-SMPs - rectangle, triangle and cross are prepared and demonstrated.

Also,

the evolutions of coating morphology during shape shifting are carefully studied. Finally, we present few complex designs of patterns and shapes as well as a design of potential application for the highly controllable smart devices. This strategy demonstrates a very simple and general strategy to design and prepare the light-induced multi-SMPs with complex shapes based on any thermal-responsive dual-SMPs. Keywords: Multi-shape memory polymer, polydopamine, coating, dipping, light-actuation

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1. INTRODUCTION Shape memory polymers (SMPs) are a category of stimuli-responsive materials which can be manipulated to a stable temporary shape and revert to the permanent shape upon external stimulus such as heat,1-3 electricity,4, 5 light,6 solvent7, 8 or magnetic field.9, 10 Such properties of SMPs have generated ultimate potential applications covering biomedical materials, deployable space devices, actuators, smart surfaces and packaging for instance.11-13 However, the number of temporary shapes that a SMP can memorize in a single shape memory cycle still limits their further practical usages, since most SMPs can only exhibit one shape during recovery process, which are also called dual-shape memory effect (Dual-SME).3 So far, many works have been done to explore the SMPs which are capable to show more than one distinctive temporary shapes in each recovery process2. Current approaches can be generalized as either introducing two or more transition units (or a broad transition unit) or incorporating different stimuli-responsive fillers into the SMP’s matrix. Lendlein et al firstly reported a triple-shape memory polymer (Triple-SMP) with two switching temperature (Ts) by crosslinking two reversible transition phase into one set of network.14 And for decades, many kinds of transition

units

such

hydrogen-bonding,19, through chemical2,

20

23

as

glass/melting

transitions,15-17

isotropic-mesogen

metal-ligands,21 host-guest interactions,22, or physical approaches16,

17

23

transitions,18

etc. are applied in SMPs

. However, introducing several reversible

transition phases into achieve multi-shape memory effect (Multi-SME) seems to be challenging, since synthesizing a polymer containing two or more separately controllable and strongly bonded switch phases will be extremely difficult. In 2010, Xie reported a commercial polymer Nafion with a broad glass switching temperature range, which also exhibits triple- even multi-shape memory effect.24 The broad glass transition can be regarded as a collection of infinite sharp transitions at each temperature. Therefore, SMPs equipped with a reversible broad thermal transition are likely to exhibit similar tunable multi-SME, for instance, a broad melting transition.25 Nevertheless, designing and synthesizing such molecular structures exhibits many limitations and difficulties. Intriguingly, some graded shape memory polymers are reported, which contain a gradient transition temperature and also exhibits tunable multi-SMEs.26, 27 Researchers demonstrated several interesting methods to realize such SMPs, for instance, by curing the SMP in a linear temperature gradient28 or by constructing a graded metallosupramolecular network in the


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SMP.29 However, the preparation of such matrix is difficult or requires special equipment. On the other hand, triple- or multi-SME can also be realized based on single sharp thermal transitifon by introducing different functional fillers into a dual-SMP. Some multi-composite SMPs are reported, for example, that epoxy is embedded with Fe3O4/Carbon nanotubes (CNTs)30, p-aminodiphenylimide (p-Ap)/CNTs31 or Fe3O4/p-Ap/CNTs32 through a multi-staged cure. Different regions of the composite contain different fillers and can be stimulated by the different stimulus, thus for multi-SMEs can be realized. Moreover, Lendlein’s group33 has realized a triple-SMP by constructing a SMP encapsulating two identical magneto-sensitive composites with a same volume but different interface area. Under an alternative magnetic field, two magneto-sensitive composites generates different heat simultaneously and results in a temperature difference in SMP, which can trigger a triple-SME. However, surface modifications of introduced nanoparticles are usually required to assure the good dispersion of filler in polymer, and a stage curing strategy is necessary which complicates the fabrication of SMPs and limits the choices of matrix. Another limitation of SMPs is that many of them only exhibit thermal-responsive, which may be hard to manipulate under practical working circumstances. On the contrary, light is expect to be a more ideal stimulus since it can be remotely, spatially and accurately controlled as well as rapidly switched. So far, two strategies have been employed to achieve most light-induced SMPs as either incorporating photo-isomeric functional group34, 35 or introducing light-absorbing triggers with photo-thermal effect6, 36, 37. Nevertheless, synthesizing of the photo-isomerisable groups or modification of photo-absorbing additives remains challenging. Interestingly, a coating strategy is developed to prepare light-induced SMPs in a more direct and simple way. Ji et al reported a solar light-induced SMP by introducing perovskite coating on SMP’s surface38. Dickey et al exhibited a photo-actuating sequential self-folding polymer sheet by laserjet-printing colored inks which absorbs different wavelength of light39. However, some problems such as the stability and adhesion abilities of coatings also limits their design and fabrication. Polydopamine (PDA) coating, a mussel inspired surface chemistry, reported firstly by Messersmith et al in 200740, is an ideal approach to induce photo-absorbing triggers on surface of SMPs because of its strong adhesive ability and efficient photo-thermal effect41. Due to the catechol group, PDA coating exhibits firmly adhesion on almost every organic and inorganic



material. It’s worth nothing that PDA coating can be synthesizing by dipping material into mild base solution (pH = 8.5) of dopamine (DA) at ambient temperature which is considered mild, controllable42 and simple. More importantly, PDA shows strong photo-thermal effect under stimulus of near-infrared (NIR) light43. Also, reports suggest that PDA coating is capable to generate heat under stimulus of NIR light and triggers shape recovery of deformed SMPs44-46. Herein, inspired by those works, we introduce a gradient PDA coating on the surface of crosslinked poly-(ε-caprolactone) (cPCL) in order to realize a tunable light-induced multi-SMP based on a single sharp thermal transition, by simply dipping different region of the SMP in weak base solution of dopamine for varying time. An increased thickness of PDA coating results an increment in apparent temperature when stimulated. Under a tunable NIR (808 nm) light source, a temperature gradient is generated on horizontal direction of SMP surface. Hereby, a multi-SME should occur if the difference of surface temperature is high enough where higher surface temperature (TH) is similar or higher than the Ts of cPCL and triggers the recovery while lower surface temperature (TL) is still beneath the Ts. For example, through adjusting the light intensity (LI) of NIR light source, TH and TL can be flexibly controlled from Ts > TH > TL (non-recover) to TH > Ts > TL (partly recover) and to TH > TL > Ts (fully recover), and therefore a triple-SME is achieved. Interestingly, by introducing a more refined PDA coating on one substrate of cPCL, quadruple- even penta-SME is also realized, and various shapes are prepared. An illustration of the design of Multi-SMP and its recover process is displayed in Scheme 1. This strategy provides a very simple and versatile way to prepare light-induced multi-SMPs which is suitable for almost all SMPs. Many fascinating shape recovery processes and complex shapes can be achieved by such a strategy. Moreover the morphological evolutions of PDA coatings during shape deformation are also investigated in detail by Scanning Electron Microscope (SEM) and Fourier Transform Infrared Spectroscopy (FTIR). The result suggests that PDA gradients exhibit strong adhesion and high stability. Further, some results in this work also show the introduced PDA coating will not affect most properties of the matrix of SMP, which suggests the mechanical properties, degradation capability and other properties of the matrix can be freely chosen to fulfil practical applications. Mussel-inspired PDA coating on SMPs also shows green chemical, mild reaction, easy fabrication and high versatility compared with many other synthesizing of photo-active additives or coatings. Our works may have potential applications in light-induced actuator,


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biomedical devices and robotics.

Scheme 1. Illustrations of: (a) the preparation of a deformed multi-SMP with the PDA gradient. (b) The recovery process of the gradient multi-SMP. The surface of a multi-SMP is divided to 3 regions corresponding to the different thickness of the PDA coating, and the surface temperature of each region is marked as T1, T2 and T3, respectively. Firstly the light intensity is set to “Light Intensity 1” (LI1), under this light only the surface of region 1 (T1) reaches Ts while T2 and T3 is lower than Ts, resulting in recovery of region 1 only. Then the light intensity is increased to LI2, when T2 excesses Ts and T1 further increases but T3 is still lower than Ts, resulting in recovery of region 2. Finally, by further improving the light intensity to LI3, T3 is higher than Ts, (T1 and T2 is further increased), resulting the full recovery.

2. EXPERIMENTAL SECTION 2.1. Materials. PCL (Capa6800, Mw = 120 kDa, Mw/Mn = 2.6) was purchased from Perstorp (UK). Dicumyl peroxide (DCP) was purchased from Sinopharm Chemical Reagent Co.,Ltd. (Shanghai, China). Dopamine hydrochloride (DA) was purchased from Best-reagent (Chengdu, China). Tris(2-aminoethyl)amine (Tris) was purchased from Best-reagent (Chengdu, China). Ammonia solution (98%) was purchased from Chron Chemicals (Chengdu, China). 2.2. Preparation of cPCL. PCL is crosslinked with different content of DCP to prepare shape memory substrates. PCL and DCP were firstly added into an inner mixer (XSS-300, Shanghai, China) at 100 oC for 6 min. Then the master batch was hot pressed using a plate vulcanizer (Qingdao, China) at 160 oC under 10 MPa for 6 min.



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2.3. Preparation of PDA Coating. Uniform PDA coatings were prepared by simply dipping cPCL10 into mild base solution of DA (0.02mM Tris, pH = 8.5) at ambient temperature. The impact of the concentrations of dopamine and the dipping time on thickness of PDA are systemically studied. The prepared samples were washed by deionized water and dried overnight at 40 oC in vacuum oven. 2.4. Preparation of Gradient PDA Coating. Approaches to achieve gradient PDA coatings on surface of cPCL10 are shown in Scheme 2. The concentration of DA is 2 mg/mL with 0.02mM Tris (pH = 8.5 at ambient temperature). In such method, different regions of specimens can be immersed in the solution for varying time. Then a PDA gradient can be created on the surface of cPCL10. 2.5. Characterizations of Thickness of PDA coatings. Fourier Transform Infrared Spectroscopy (FTIR) (Nicolet 6700, Thermo Fisher Scientific, USA) was firstly employed to investigate the thickness of PDA coatings using the ATR and transmission mode. Moreover, X-ray Photoelectron Spectroscopy (XPS) (XSAM800, Kratos, UK) was employed to scan the C 1s, O 1s and N 1s peak on the surface of cPCL10 and cPCL10 coated PDA. Also, Scanning Electron Microscope (SEM) (Nova NanoSEM450, FEI, USA) was used to observe the morphology of PDA coatings in different thickness. 2.6. Shape Memory Properties Tests. Dynamic Mechanical Analyzer (DMA) (Q800, TA, USA) was firstly used to measure the Ts of cPCL10, in which the temperature was raised from 0 o

C to 85 oC with the heat rate of 3 oC/min. Quantitative dual-shape memory properties of cPCL10

was also carried on DMA under a force control mode. A four step procedure was designed and employed as follows: (1) Sample tested was stretch to a strain (εm) at Ts by a constant force. (2) Stretched sample was quenched to ambient temperature while maintaining the constant force. (3) The external force was removed, and recorded the temporary strain (εt). (4) Sample was reheated to Ts and maintain for 20 min to fully recover, and remaining strain (εr) was recorded. Shape fixing ratio (Rf) and shape recovery ratio (Rr) of cPCL10 were calculated based on Equations (1) and (2):

= =

(1)

(2)


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Light-induced shape memory properties were evaluated by a bending method. cPCL10 samples coated different thickness of PDA were firstly put under an oven and bended to a “U” shape. After deformation, samples were rapidly dipped into liquid nitrogen to fix. The angle between two bent after fixity was recorded as θf. Then samples were put under a tunable NIR light source (808m) (LASEVER, China) to recover. PDA coatings could generate heat and triggered shape recovery when surface temperature reached similar or higher to Ts. The recovery processes of samples with different thickness of PDA coating under different LI of light were observed. The angel of two bent during recovery was recorded as θr. The recovery ratio (Rr) were calculated via Equation (3):

=

(3)

2.7. Design and Tests of Light-induced Multi-SME. A rectangular shape (20 mm (l) × 5mm (w) × 0.5 mm) of cPCL10 coated gradient PDA was firstly fabricated. Sample tested was put under the oven, deformed to “wave” shape, in which every regions with different thickness of PDA coating were banded to “U” shape, and then quenched in liquid nitrogen. Next, entire deformed sample was put under the spot of the tunable NIR light simultaneously. Triple- or multi-SME could be realized by gradually increasing the LI. Also, triangular-shaped, cross-shaped coated gradient PDA were also fabricated and tested. The size of prepared samples are also shown in Scheme 2. 2.8. Other Characterizations. Differential Scanning Calorimetry (DSC) (Pyris1, PerkinElmer, USA) was employed to investigate the crystallization behaviors of PCL, cPCL and cPCL10 coated PDA. The thermal program of samples tested included following steps, which firstly heated samples from 30 oC to 140 oC at rate of 30 oC/min and isothermal for 5 min, then followed by a cooling procedure to 0 oC at 5 oC/min and a secondary heating run was employed to 140 oC at 10 oC/min. Thermogravimetric Analyzer (TGA) (Q500, TA ,USA) was employed to investigate the thermo-stability of cPCL10 and cPCL10 coated PDA. Tests were carried out in a Platinum crucibles under nitrogen atmosphere, samples were heated to 600 oC at 10 oC/min in nitrogen atmosphere. 2.9. Names designating for specimens in this work. Prepared crosslinked PCL are named as cPCLx, where x represents content of DCP (phr, per hundred resin). Samples coated with different thickness of PDA in 2mg/mL DA for different time are named as cPCL10/PDAx, where



x represents the dipping hour. Prepared rectangular multi-SMP are noted as cPCL10/PDAx-y-z, where x, y, z represents different immersion time. Moreover, prepared triangular and cross-shaped multi-SMPs are designated as cPCL10(t)/PDAx-y-z and cPCL10(c)/PDAx-y-z-n, respectively, where x, y, z, n represents different immersion time and (t) and (c) represents their permanent shapes.

Scheme 2 Illustrations of the preparation of gradient PDA coatings on (a) rectangular specimens (cPCL10/PDA24-6-0); (b) triangular specimens (cPCL10(c)/PDA24-12-6) and (c) cross-shaped specimens (cPCL10(t)/PDA 24-18-12-6). Regions of a specimen are covered by adhesive tapes. The entire specimen is immersed in DA solution simultaneously. The PDA gradient can be realized on the substrate by removing the tapes in sequence as designed. Thickness of prepared samples: 0.5 mm.

3. RESULT AND DISCUSSION 3.1. Preparation of the shape memory substrate. Due to the strong adhesion of PDA, this strategy could be achieved on any thermal-responsive dual-SMP. Therefore, considering the switching temperature range and a proper mechanical properties for flexible deformation, we chose the crosslinked PCL as matrix, and PCL crosslinked with varying content of DCP (1~10 phr)


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were prepared. The crystalline behavior and dynamic mechanical properties were also investigated to seek a satisfactory substrate (Figure S1). During multi-shape recovery, when the lowest temperature of the temperature gradient triggers the recovery of relative region of the dual-SMP, the regions bearing higher temperature must still be stable, for instance, still showing enough modulus and enough resilience. Thus for, cPCL10 was chosen to be the matrix of multi-SMP due to the relatively high modulus at the melt transition. Dynamic mechanical analysis possesses a glass transition of cPCL10 at about -40 oC and a melt transition at from ~60 to 80 oC (Figure 1a). Quantitatively shape memory properties of cPCL10 was carried on DMA at stress controlled mode (switching temperature, Ts = 70 oC). cPCL10 shows very high Rr (all > 95%) (Figure 1b), as the results of the crosslinking network, and highly Rf due to the crystallization of PCL molecular chains.

Figure 1. (a) Dynamic mechanical analysis curve cPCL10, (b) dual-shape memory cycle of cPCL10 under a stress-controlled deformation condition.

3.2. Thickness control and characterizations of PDA coatings. To achieve the gradient PDA coatings, experiments were done to investigate the impact of the concentration of DA and the immersion time to the thickness of PDA. Samples immersed in 2 mg/mL DA for different time and immersed in different concentration of DA for 12 h are illustrated in Figure 2a. By eyes, a thicker PDA layer results in a darker surface. Interestingly, the increased immersion time in 2 mg/mL DA results in obvious surface color change (darker). However, after immersion for 12 h, when the concentration of DA > 2 mg/mL, increasing the concentration leads to negligible changes of surface color. SEM photographs of substrate coated different thickness of PDA are shown in Figure 2b. By varying the immersion time in 2 mg/mL DA, different thickness and



morphology of PDA coating can be formed on the SMPs. After immersion for 12 h, many discrete PDA aggregates adhere to the surface of substrate, showing non-uniform located and cannot cover whole surface. As for the substrate immersed for 24 h, a dense coating is formed, covering the whole surface of substrate. Moreover, immersion for 36 h results in a thicker and rougher PDA layer on substrate’s surface, which may be due to the random stacking of PDA on the substrate in air atmosphere42, while the whole surface is covered by a PDA layer. Research suggests that DA self-polymerizes, in situ forming PDA and deposits on substrates, which may result the change of morphology of PDA coating while increasing the immersion time. Also, XPS was done to investigate the chemical formation of the surface to confirm the formation of PDA layers (see Figure S2). The results suggests the formation of PDA according to the reference47.

Figure 2. (a) Photos of cPCL10 coated PDA using different conditions, (b) SEM images of cPCL10’s surface coated for varying time, the concentration of DA is 2 mg/mL. Scale bar: 20 µm.

FTIR was employed to further investigate the difference between the concentration of DA and the immersion time. Figure 3a shows the FTIR/ATR spectra of samples immersed in 2 mg/mL DA for varying times. Compared to cPCL10, cPCL10/PDAx presents a new broad absorption peak at about 3200 ~ 3700 cm-1 that is assigned to ν(N-H) and ν(O-H); stretching modes of νring(C=C) (aromatic ring) at 1590 cm-1 and an absorption peak at 1515 cm-1 that is assigned to


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νring(C=N), which indicates the presence of PDA layers. FTIR spectra under transmission mode of cPCL10, DA, PDA spheres and cPCL10/PDA6 present same results (Figure S3). Intriguingly, cPCL10/PDAx also exhibits a weakened signal of ν(C=O) of cPCL10 (ester group) at 1722 cm-1. One can see that with increased immersion time, the absorption peaks of ν(N-H)/ν(O-H) at 3200 ~ 3700 cm-1, νring (C=C) at 1590 cm-1 and νring(C=N) at about 1515 cm-1 increase (Figure 3a), indicating a thicker PDA layer is formed47. Figure 3a’ shows the amplified spectra at 1400 ~ 1900 cm-1. Despite the increased absorption of νring(C=C) and νring(C=N), the absorption of ν(C=O) (ester groups of cPCL10) results in decline, suggesting an increased thickness of PDA layer also. On the other hand, by increasing the concentration of DA, the peak of ν(N-H), ν(O-H), νring(C=C) and νring(C=N) also become stronger, as shown in Figure 3b. However, the amplified spectra in Figure 3b’ show a negligible increment in νring(C=C) as well as νring(C=N) and a slowly decreased absorption of ν(C=O) at 1722 cm-1 when the concentration of DA is higher than 2 mg/mL, which reveals a negligible increment in the thickness of PDA layer. Such results suggest that the immersion time shows a higher controllable manner than the concentration of DA to adjust the thickness of PDA. In consequence, in order to realize a gradient PDA coating which can generate appropriate temperature gradient on the substrate’s surface, the concentration of DA is set at 2 mg/mL (at ambient temperature), and the thickness of PDA is only controlled by the immersion time.



Figure 3. FTIR/ATR spectra of (a) pristine cPCL10 and cPCL10 coated with different thickness of PDA. The concencration of DA is set at 2 mg/mL with alternative coating time. (a’) Amplified Figure 3a at 1900 cm-1 ~ 1400 cm-1 (ν(C=C) of PDA and ν(C=O) of PCL). (b) Pristine cPCL10 and cPCL10 coated with different thickness of PDA. The coating time is set at 12 h while changing the concentration of DA. (b’) Amplified figure 3b at 1900 cm-1 ~ 1400 cm-1 (ν(C=C) of PDA and ν(C=O) of PCL).

3.3. Photo-thermal conversion of PDA coating and light-induced SME tests. Further, we investigated the photo-thermal converting properties of PDA coatings dipped on substrates of cPCL10. A tunable 808 nm NIR laser was employed to quantitatively investigate the photo-thermal converting behaviors. The surface temperatures of cPCL10/PDAx under tunable NIR laser were recorded by a thermocouple thermometer. The surface temperature of all specimens increased with the escaping time and reached equilibrium finally. Interestingly, the equilibrium temperature also increases with a thicker PDA coating (Figure 4a) or a higher light intensity (LI) (Figure 4b). The thicker PDA or higher LI also results in a shorter time period for reaching equilibrium. Furthermore, a thermal infrared imager (Fluke Ti-26, USA) was utilized to observe the surface temperature of samples under NIR light, as shown in Figure 3c. The captured photos also suggest that the samples’ surface temperature reach equilibrium after 3 min irradiation.


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Hence, based on the hypothesis that the surface temperature will reach the balance after illumination of 5 min for all tested samples, the equilibrium temperature of each substrate coated with different thickness of PDA under different LI are measured and plotted as a temperature “map” (Figure 4d). In such a map, a thickness of PDA and a LI is corresponding to an equilibrium temperature. Therefore, by constructing a PDA gradient on the SMP’s substrate and switching the LI to the specific values stepwise when each region with different thickness of PDA can reach Ts in sequence, a triple- or multi-SME could be achieved.

Figure 4. (a) Surface temperature versus escaped time of different coated samples under 0.6 W/cm2 light. (b) Surface temperature versus escaped time of cPCL10/PDA12 under varying light intensities. (c) The infrared photos of samples under NIR light for different time. (d) The generated equilibrium surface temperature after illumination for 5 min under NIR light of cPCL10 with varying PDA thickness under different light intensities.

Moreover, light-induced SMEs of cPCL10/PDAx were also measured. In such samples, PDA coatings act as energy conversion units which transform NIR light into thermal energy and trigger the thermally-induced cPCL to recover. Hence, upon the substrates with same thickness, the thicker PDA or higher LI should result in faster recovery and higher Rr. Light-induce SME tests were conducted through bending tests (schematics is shown in Figure 5a). Figure 5b illustrates the



recovery ratio of cPCL10/PDA12 with the escaping time under different LI. One can see that under lowest LI (0.25 W/cm2), cPCL10/PDA12 cannot recover, while gradually increasing the LI, the recovery rate and Rr increases simultaneously. Such results suggest that more heat is generated in unit interval with the increased LI. When the sample is irradiated, the heat is generated from PDA coatings at substrate’s surface via photo-thermal conversion. As time escaped, the surface temperature is increased while the heat is conducted to the bulk, and shape recovery is triggered when the bulk temperature is similar or higher than Ts. At the early stage of recovering, the recovery ratio is slowly increasing, which is due to the relatively lower bulk temperature. Few second later, the recovery rate increases to a constant and maintains, because the surface and bulk temperature of matrix has reached balanced. In the late stage of recovery, because of the fact that resilience is almost released, the recovery rate shows gradually decreased. Interestingly, the increment in LI also results the higher Rr. For LI = 0.25 W/cm2, the surface temperature can’t reach Ts under irradiation, thus the shape recovery can’t be triggered. By increasing the LI, a higher surface and bulk temperature can be realized, melting more PCL crystalline domains in unit time while the highly crosslinked molecular networks still provide strong resilience, which results faster recovery and higher Rr. Herein, cPCL10/PDA12 takes 30 s to recover under 0.6 W/cm2 while it takes only 18 s under 1.0 W/cm2. Moreover, results of cPCL10/PDAx (x = 6, 12, 24, 36) under LI of 1 W/cm2 are plotted in Figure S4, which suggests that thicker PDA exhibits similar results to higher LI.

Figure 5. (a) Schematic shows the testing method of light-induced SME. (b) Photos of recovering process of deformed cPCL10/PDA12 under different light intensities. (c) Curves of recovery ratio versus irradiating time of


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cPCL10/PDA12 under different light intensities. The tested samples size 30 mm (l) × 5 mm (w) × 0.5 mm (t).

3.4. Tunable multi-SME based on gradient PDA coating. A specimen containing simplest design (designated as cPCL10/PDA12-0) was firstly constructed and the light-induced triple-SME was conducted, as shown in Figure 6a. The boundaries of the gradient coating are clear while the morphologies of those boundaries were further investigated by SEM (Figure S5). The rectangular shape only contains one PDA coating (12 h) on one side, and a simplest triple-SME can be achieved. cPCL10/PDA12-0 is firstly deformed to an “S” shape. Under irradiation of 0.6 W/cm2 NIR light, segments of cPCL10/PDA12 is heated. This leads to complete recovery of the region of such segment (see Movie S1). After heated in an oven, the specimen fully recovers. Moreover, the specimen with a more complex gradient coating (designated as cPCL10/PDA24-6-0) is prepared and shown in Figure 6b. cPCL10/PDA24-6-0 is deformed to a “W”-like shape and fixed. The programmed sample is firstly subjected to 0.4 W/cm2 light, where only the region of cPCL10/PDA24 fully recovers. Then increasing the LI to 1.0 W/cm2, the region of cPCL10/PDA6 gradually recovers to flat. Herein, if the deformed sample is firstly put under 1.0 W/cm2 light, two regions coated PDA will recover simultaneously, while the region of cPCL10/PDA24 recovers faster. Finally, thermal-actuation is employed to obtain full recovery of the specimen. Infrared-thermal images also show the surface temperature gradients of both specimens at each recovering step. The temperature gradient can be tuned by the LI, and the captured infrared images of temperature gradient is well matched to the Scheme 1. When under NIR illuminations, all PDA coatings generate heat from light energy simultaneously, however, different thickness of PDA coating generates different amount of heat which finally results a temperature gradient on substrate’s surface. Through switching the LI stepwise, the region of thickest PDA recovers earliest and the region with the thinnest (or none) PDA recovers latest (or none-recover), hence, triple- or multi-SMEs are achieved.



Figure 6. Schematic illustrations and photos of (a) triple-SME of cPCL10/PDA12-0, temporary shape 1 can be

achieved by heat deforming, and temporary shape 2 is realized by applying a 0.6 W/cm2 NIR light. Full sample sizes 20 mm (l) × 5 mm (w) × 0.5 mm (t). (b) Quadruple-SME of cPCL10/PDA24-6-0, temporary shape 1 can be obtained by directly deforming, temporary shape 2 can be achieved by applying 0.5 W/cm2 NIR light and temporary shape 3 can be realized by applying 1 W/cm2 NIR light. Both samples need to be heated to recover to its original shape. Full sample sizes 30 mm (l) × 5 mm (w) × 0.5 mm (t).

Then more designs of multi-SMPs were prepared and demonstrated. Figure 7 shows a triangular quadruple-SMP designated as cPCL10(t)/PDA24-12-6. The programming step is to fold each corner and fix. By putting the entire specimen under the tunable light simultaneously, multi-SME can be realized through altering the LI. When LI reaches 0.4 W/cm2, only the region of cPCL10/PDA24 recovers, exhibiting the temporary shape 2. Then switching the LI to 0.6 W/cm2, which leads to complete recovery of cPCL10/PDA12, resulting in the temporary shape 3. Further increasing the LI to 1.0 W/cm2, full recovery of the sample can be realized (also see Movie S2). Also, if the deformed sample is subjected to 1.0 W/cm2 light directly, entire matrix will recover simultaneously while the region of cPCL10/PDA24 recovers faster and cPCL10/PDA6 recovers slower, that is also considered as a gradient recovery behavior (also see Movie S3). Such multi-SMP can be actuated by light only, which shows potential applications in remotely controlled devices.


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Figure 7. Schematic illustrations and photos of triple-SME of cPCL10(t)/PDA24-12-6. After deformation, by

increasing the applied light intensity from 0 to 0.5 and to 0.6 W/cm2, two temporary shape (2 and 3) can be achieved step by step. And by tuning LI to 1 W/cm2, sample can recover to its permanent shape. Each edge is 8 mm long and thickness is 0.5 mm.

A cross-shaped sample is also demonstrated (Figure 8), it is firstly deformed through folding. The demonstrated shapes can be sequentially regulated by a photo controlled procedure, and a multi-stage recovery is presented. Through altering the LI, each temporary shape or permanent shape can be achieved. For instance, when the deformed shape (temporary shape 1) is subjected to 0.4 W/cm2 light, only the region of cPCL10/PDA24 recovers. Then increasing the LI to 0.5, 0.6 and 1.0 W/cm2 in sequence, the temporary shape 3, 4 and permanent shape can be realized stepwise. Moreover, several recovery routes could be realize, for instance, subjecting deformed sample to 0.6 W/cm2 light which will immediately result in the temporary shape 4, then tuning LI to 1.0 W/cm2 will trigger the full recovery. However, one should be noted that, despite such gradient PDA coatings realize the multi-SMPs, the primary condition to fulfil such multi-SME is that the difference between generated temperature in the temperature gradients is high enough, which suggests one cannot induce gradient PDA coatings arbitrarily, for instance, cPCL10/PDA16-14-12 is also prepared and tested for multi-SME (not show here). However, region coated 12 h and 14 h PDA is unable to recover independently by tuning the LI because the generated heat of such two regions is similar. Therefore, the equilibrium temperature of each region in such gradients should be distinct enough to support every unrecovered regions to recover independently, that a multi-SME can be realized, and few other multi-SMEs realized by PDA gradients are shown in Figure S6.



Figure 8. Illustration and photos of various shape recovery routes for cPCL10c/PDA36-24-12-6. The cross sample

is firstly deformed to a box-like specimen. Four recovery routes depending on applied light intensity are possible. Also, by directly heating or applying a LI of 1 W/cm2, original shape can be recovered from any temporary shape. Each faces sizes 5 mm (l) × 5 mm (w) × 0.5 mm (t).

3.4 Morphological evolution and stability of PDA coatings during shape shifting. In order to further investigate the morphological changes of those coatings during shape memory processes, several prepared samples were investigated by SEM and FTIR/ATR. To demonstrate the stability of PDA coatings, a more “rough” preparing method was employed, and the preparation of tested samples is shown in Figure S7. Few rectangular samples coated PDA for 24 h were firstly uniaxially stretched to 100%, 200% and 300% elongation respectively then fixed and observed by SEM. However, it’s worth mentioning that it’s hard to observe the morphologies of different stretched elongation using a single specimen. Thus, a single sample was applied to different elongation in sequence and investigated by FTIR/ATR stepwise to explore the consecutive changes of the PDA coating. The FTIR/ATR spectra are shown in Figure 9a and 9a’. Results indicate that the higher elongation, the higher transmittance of PDA was acquired, namely,


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lower PDA content existed in scanning areas. Nevertheless, each time the sample recovered, the signals of PDA coating recovered similarly to the origin, which indicates the sequential stretch negligibly affected the thickness of PDA coating. Figure 9b exhibits the morphology of non-deformed cPCL/PDA24, and the morphologies of samples stretched to 100%, 200% and 300% elongation are demonstrated in Figure. 9c, 9d and 9e, respectively. Intriguingly, the PDA coating were stretched to many discrete domains when the sample was elongated (Figure 9c). Moreover, as shown in Figure 9d, the increased elongation resulted in more refined PDA domains and more cracks. Further increased the elongation (Figure. 9e), the cracks between PDA domains grew, and many oriented surfaces were no longer covered by PDA. SEM images suggest that the PDA coating is considered to be broken and isolated when stretched, while the thickness of the coatings remain unchanged. Thereby, higher elongation leads to more refined and isolated PDA domains which explains the results of FTIR. Figure 9f and 9g shows the morphology of the sample recovered from 300% elongation, where the recovered morphology of PDA layer is interestingly identical to the origin in Figure 9b. Such phenomenon indicates the discrete PDA domains fused together when the sample recovered. Figure 9g also shows the stretch results in few small cracks in the PDA coating.

Figure 9. (a) The FTIR/ATR spectra of a single cPCL/PDA24 sample, the sample was stretched to 100%

elongation and recovered, then to 200% and recovered and finally to 300% and recovered. (a’) Amplified spectra of figure 9a to exhibit the signals of ν(C=C) (benzene ring) of PDA coatings. And SEM images of surface of (b) prepared cPCL/PDA24, (c) 100% elongated cPCL/PDA24, (d) 200% elongated cPCL/PDA24 and (e) 300%



elongated cPCL/PDA24, (f) and (g) SEM images of cPCL/PDA24 recovered from 300% elongation.

Furthermore, in order to explore the stable adhesion of PDA coatings on specimens, numbers of rectangular strips coated PDA (24 h) were stretched (100% elongation) or bended (180 degrees) deformation for 1, 10 and 100 times respectively, and then were subjected to SEM or FTIR/ATR. The results of stretching experiment are firstly demonstrated in Figure 10. A single specimen was also stretched, fixed and recovered for 100 times, and then was subjected to FTIR/ATR at first, 10th and 100th deformation and recovery. Resulted spectra are shown in Figure 10a and 10a’. The FTIR/ATR spectra suggest that after 10 cycles, the signal of PDA coatings remained identically to the origin. However, after 100 cycles, the signal of PDA coating attenuates, which indicates a slightly loss of PDA coating using the method in Figure S7. Moreover, Figure 10b and 10b’ demonstrates the morphology of PDA coatings of the cPCL/PDA24 stretched for the first time and recovered, respectively. SEM images exhibit that PDA coating is stretched to many discrete domains and fuses together when sample has recovered. Then, the PDA coating is stretched to more refined domains after 10 cycles as shown in Figure 10c. And then those domains fuses together when sample has recovered (Figure 10c’), returning to the original morphology, which is also concluded from FTIR. Further, 100 times of stretching leads to more refined PDA domains (Figure 10d), however, the surface morphology of the recovered sample in Figure 10d’ exhibits some residual cracks and broken PDA domains, which indicates the PDA coating does not fully recovers to origin after many times of deformation in a very short period, which also accords to the results of FTIR/ATR.

Figure 10. (a) FTIR/ATR spectra of a single cPCL/PDA24 sample, the sample was stretch to 100% elongation and


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recovered for 100 times totally. The sample stretched and recovered at first, 10th and 100th time was measured by FTIR/ATR. (a’) Amplified spectra to exhibit the signals of ν(C=C) (benzene ring) of PDA. And SEM images of the surface of (b) cPCL/PDA24 stretched for 1 time and (b’) recovered from the first stretch, (c) cPCL/PDA24 stretched for 10 times and (c’) recovered from 10 times of stretch, and (d) cPCL/PDA24 stretched for 100 times and (d’) recovered from 100 times of stretch. Scale bar: 20 µm.

In contrast to stretching, some of cPCL10/PDA24 samples were bended (180 degrees for each time), fixed then recovered for 1, 10 and 100 cycles respectively and were observed by SEM. Intriguingly, the bended samples showed two different morphologies at the fold and both sides of the fold. As shown in Figure 11a1, the PDA layer at the fold is stretched to many discrete domains, showing many small cracks simultaneously, which is similar to the stretching deformation. And the PDA domains also fuse together when the sample has recovered, as shown in Figure 11a1’. However, the PDA coating on both sides of the fold shows a totally different morphology. Figure 11a2 shows a wrinkled PDA layer, while the wrinkles are paralleled to the fold. Such morphology suggests that after being bended and fixed, the PDA coating near the fold overlapped, and resulted in such wrinkled layers of PDA. The PDA coating also recovers to the origin when the sample has recovered, as shown in Figure 11a2’. After 10 cycles, the PDA coating at the fold is breached to more refined domains (Figure 11b1), while denser wrinkles are formed at the sides of the fold, as displayed in Figure 11b2. Both morphologies returned to the origin when sample recovered to flat, as shown in Figure 11b1’ and 11b2’, respectively. Further, the surface morphology of the sample at the fold deformed for 100 times shows discrete domains and many small cracks as well (Figure 11c1) and recovered to the original morphology with recovery of the sample (Figure 11c1’). Further, more wrinkled morphology is formed at the fold after 100 times of bending (Figure 11c2), however, many times of deformation in a very short period results in an incomplete recovery of morphology at the side of the fold, where the residual wrinkles are obviously observed, as shown in Figure 11c2’.



Figure 11. SEM images of cPCL10/PDA24. (a1) Center morphology after bended 1 time, (a1’) center morphology

after bended 1 time and recovered; (b1) center morphology after bended 10 times, (b1’) center morphology after bended 10 times and recovered; (c1) center morphology after bended 100 times, (c1’) center morphology after bended 100 times and recovered. (a2) Side morphology after bended 1 time, (a2’) side morphology after bended 1 time and recovered; (b2) side morphology after bended 10 times, (b2’) side morphology after bended 10 times and recovered; (c2) side morphology after bended 100 times, (c2’) side morphology after bended 100 times and recovered. Scale bar: 20 µm.

Generally speaking, those morphological transitions of PDA coating when experiencing stretching or bending suggest that the PDA coatings are either broken into many discrete domains or overlapping together without thickness changes. Under higher elongation, the PDA domains become more separated, and many surface areas are no longer covered with PDA, while the increased time of stretching results in more refined PDA domains. Intriguingly, recovery of samples usually results in recovery of those PDA layers to the original morphology, but few cracks still remain in PDA layers. Moreover, despite bending tests exhibit some different morphological changes, the surface morphologies recover to origin as much as possible when samples have


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recovered. FTIR spectra of stretched samples also indicates that several times of deformation do not affect the thickness of PDA coatings. Despite many cycles of deformation and recovery in a very short period may lead to some permanent changes of coatings’ morphology or a slight loss of PDA coatings, the PDA coatings still exhibits a strong adhesion and stability on the substrates, which provides many advantages for shape design and potential applications. 3.5 Designs of patterns of coatings and complex shape changing. We further demonstrate several designs of PDA pattern and light-induced shape changings by the strategy reported in this work. As firstly shown in Figure 12a, different materials were chosen and were applied different gradient PDA coatings. Results suggest that those gradient PDA coatings can be freely designed and realized on those substrates with varying sizes. Therefore, one can deduce that PDA gradients may also be achieved on many substrates of SMPs to introduce light-induced multi-SMEs. Furthermore, PDA gradients can be imparted with fruitful designs, as shown in Figure 12b. Except the PDA coatings have realized characters, flexible designs of PDA gradients are also achieved on substrate with different shapes and sizes. Intriguingly, those design of PDA coatings are repeatable, and those gradient PDA coatings applied on the substrates with different size result identical (Figure S8). In order to demonstrate some light-induced shape changings using PDA coatings, a commercial PVC heat-shrinking film (Zhejiang, China) is incorporated. The PVC films are coated PDA with different designed patterns at different positions, as shown in Figure 12c. One can see that with different patterns introduced on the heat-shrink films, different shapes were be achieved under NIR light. Except those simple origami structures, pyramid- and cube-shape can also be achieved by designing the PDA patterns, which show some potential practical applications such as packaging and tissue engineering. Moreover, we managed to realize a consecutive PDA gradient coating on cPCL (Figure 12d) using the method reported elsewhere48, which also showed controllable light-induced triple-shape memory effect (see more in Figure S9 and S10).



Figure 12. (a) PDA gradients achieved on different materials. (b) Flexible designs of PDA patterns and gradients

on cPCL10 substrates with varying sizes. (c) Designs of light-induced shape changing, varying PDA patterns on the heat-shrinking films result in different shapes. (d) Illustrations of consecutive PDA gradients.

Furthermore, two samples capable of sequentially-controlled and light-induced shape shifts with a potential application were prepared and demonstrated. The preparation of the samples is displayed in Figure S11. Different output light intensities can be utilized to actuate specifically designed shape transitions for the programmed recovery. As shown in Figure 13a, the “T”-shape substrate (cPCL10) coated with gradient PDA are deformed into a box (sized 4 mm × 4mm × 4mm), and the subsequent stepwise recovery under the tunable light is also displayed. Under each light intensity (0.3, 0.4 and 0.5 W/cm2), the region coated different thickness of PDA (36, 24 and 12 h, respectively) are heated to Ts of the substrate, and the box unfolds in sequence. A similar design for the “+”-shape film is also demonstrated in Figure 13b, which exhibits the facile and simple design of gradient PDA coatings. Such results show the remote control and actuation of multi-step recovery, which may be applied in controllable deployable devices such as the packaging, medical implants, solar panels, etc.


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Figure 13. Illustrations and photos of the prepared samples and designed stepwise unfolding of the box of (a)

“T”-shape film. (b) A “+”-shape film. Each plane of both boxes sizes 4 mm (l) × 4 mm (w) × 0.5 mm (t).

We note that except the aforementioned advantages of PDA coatings, the introduced coatings will not affect most the properties of materials such as mechanical properties, bulk structure and shape memory properties (Figure S12). Meanwhile, more experiments were done to briefly investigate the difference of introducing PDA layers on the substrates between this in situ dipping strategy and other coating methods such as directly painting and spin-coating. Results also suggest that such in situ strategy leads to PDA layers showing higher controllable manner, stability and uniformity (Figure S13). Such results provides more versatility and advantages for practical applications, because the gradient PDA coating can be introduced regardless of the properties changes of the substrates, with complex designs of patterns as well as highly controllable manners, and the matrix can be freely decided according to the requirements of applications.

4. CONCLUSION Conclusively, a tunable light-induced multi-SMP by incorporating gradient PDA coatings on a typical dual-SMP via dipping is realized, such mussel inspired approach shows facile, simple and highly designable. Most dual-SMPs can be made into such multi-SMP in this way due to the strong adhesion and efficient photo-thermal performance of PDA coatings, which will further expand the design and fabrication of multi-SMPs. The thickness of PDA coatings can be easily



controlled by the dipping time, and the design of shapes and patterns results various. Generated surface temperature of different thickness of PDA coatings under alternative light intensities are systemically investigated. Results show that the thicker PDA coating or the higher light intensity leads a higher equilibrium surface temperature. Moreover, the morphological evolutions of coated PDA layers were carefully studied by SEM and FTIR. All results suggest that despite the PDA layers are broken or overlapped during stretching or bending respectively, the thickness of PDA remains unchanged and it will recover to the origin during sample’s recovery. Such results indicates a stable coating was achieved by this in situ dipping. Furthermore, multi-SMEs based on complex PDA gradients and various shapes are also achieved. Our work demonstrates a very general and facile way to prepare light-induced multi-SMPs on substrates of any dual-SMP, and many complex shapes and patterns are achieved through such strategy, which shows great potential in soft robotics, smart devices and biomedical implants.

SUPPORTING INFORMATION DMA and rheological properties of cPCL/DCP (Figure S1), XPS spectra of PDA coatings (Figure S2), FT-IR spectra of PDA coatings (Figure S3, Table S1), Light-induced shape memory effects (Figure S4), SEM photos of gradient PDA boundaries (Figure S5), Light-induced multi-shape memory effects (Figure S6), Testing methods of repeatable samples (Figure S7), Repeatability experiments of PDA gradients (Figure S8), Fabrication of consecutive PDA gradient (Figure S9), FT-IR tests of consecutive PDA gradient (Figure S10), Preparation of samples using in Figure 13 (Figure S11), DSC, DMA and TGA results of cPCL with or without PDA coatings (Figure S12), Comparison of different coating strategies (Figure S13). Movie S1. Video of light-induce multi-SME of cPCL10/PDA12-0. Movie S2. Video of light-induce multi-SME of cPCL10(t)/PDA24-12-6. Movie S3. Video of sequential recovery of cPCL10(t)/PDA24-12-6.

ACKNOWLEDGEMENTS We would like to express our sincere thanks to National Natural Science Foundation of China


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for financial support (51721091 and 51210005).

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Gradient Polydopamine Coating: A Simple and General Strategy

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