Organic–inorganic shape memory thermoplastic polyurethane based on polycaprolactone and polydimethylsiloxane

Abstract

In this paper, we report a hybrid organic–inorganic thermoplastic shape memory polymer (SMP) that exhibits extremely fast-response time, with the switching temperature at body temperature and a thermoplasticity that allows for solvent processing. The polymers show good shape recovery (>95%) and shape fixity (>95%). The combination of organic–inorganic hybrid polymers in a linear chain could form a physical network structure that allows for exceptionally fast response upon being triggered at 40 °C.

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Introduction

Shape memory polymers (SMPs) are a class of smart polymeric materials that have attracted much attention in the field of polymeric and biomedical research in recent years due to their versatility and functionality.¹ SMPs are stimuli-responsive materials with the capability to change and recover either their permanent or programmed temporary shapes when exposed to an external stimulus,^2–4 which can be temperature,^5–8 light,^9–11 magnetic fields,^12–15 electrical currents^16–19 etc. SMPs are lightweight, solid-state materials that are suitable for actuators, sensors, microfluidic systems, biomedical applications^20,21 such as vascular grafts and cardiovascular stents,^22–24 textiles,²⁵ consumer care products,²⁶ as well as in aerospace technology^27–29 and harsh condition applications.^30,31

Thermo-responsive SMPs which display fast response with a transition temperature (T_trans) near the human body temperature and are also thermoplastic would be very useful. The ability of SMPs to recover their programmed shapes quickly is of particular importance, especially in microfluidic systems, sensor and actuator applications.^32–34 For example, a fast-response SMP can enhance the channel valves in microfluidic systems to trigger the lock rapidly for high-precision assays (hydrogel beads are currently used). Additionally, fast-response SMPs may also find value in robotic systems and artificial muscles.^35,36 While slower response SMPs are useful in applications like medical implants to prevent post-implant shocks, speed is a key indicator for good actuating performance, making fast-response SMPs viable for artificial muscle applications. Thermo-responsive SMPs for use in biomedical applications would greatly benefit from having a switching temperature in a safe range near to the human body temperature to prevent potential tissue damage caused by prolonged exposure to high temperature.^2,37 Current SMPs response times generally range from 3 s to 120 min.^{5,23,32,38–43} State-of-the-art SMP reported by Xu and Song was a POSS-PLA network cross-linked polyurethane that showed fast response (3 s) at 51 °C.⁴³ However, it is a thermosetting material and requires high temperature for shape recovery. Grunlan's group reported a few inorganic–organic SMP^44–47 thermoset networks/foam with good shape recovery (91–96%) and shape fixity (∼99%) at 60 °C. The copolymers were prepared by rapid photocuring of solutions of diacrylated PCL-PDMS-PCL macromers or blending PCL with polymethylvinylsiloxane (PMVS). Despite the ability to achieve high shape fixity in such thermoset PCL-PDMS-PCL triblock networks, the covalent crosslinks in a thermoset network do not allow the polymer to be melted or heat processed. As such, thermosetting polymers are generally only processable by means of chemical reactions.⁴⁸ In contrast, physically crosslinked thermoplastic SMPs such as poly(PCL/PDMS urethane)s investigated in this study are activated by the melting transition (T_m) of the switching segments⁴⁹ (in this case, PCL). This makes the SMP easily processable by heat, solvent and many conventional processing methods such as thermoforming, injection molding and extrusion. Solvent processability of poly(PCL/PDMS urethane)s allows for potential spray applications such as functional thin films and membranes,⁵⁰ and further improves its appeal as a smart polymeric material. The ease of processing these poly(PCL/PDMS urethane)s make them highly versatile and reconfigurable to suit various design and application requirements.¹

The shape memory ability of the polymers relies on the entropy-driven randomly coiled configuration of the polymer chains. For use in potential biomedical and actuating applications such as artificial muscles, the switching of long-range motion of polymer chains from frozen to flexible (or activated) has to be fast upon being triggered at temperature close to body temperature (37 °C). SMPs utilising a melting transition (T_m) as T_trans generally exhibits faster response times. As such, the use of a T_m as T_trans would be favourable as the transition occurs over a much smaller temperature range as compared to a glass transition. We hypothesize that a physically crosslinked thermoplastic system from an organic–inorganic polyurethane based on poly(ε-caprolactone) (PCL) and polydimethylsiloxane (PDMS) could achieve fast switching performance at body temperature. PCL crystallites and hydrogen bonding⁵¹ between urethane groups could act as physical netpoints holding the shape (Fig. 1a); the melting temperature (T_m) of the PCL segments could act as T_trans. Polydimethylsiloxane (PDMS) is a superior and effective soft segment due to its extremely low glass transition temperature (T_g), which could improve the stretchability and flexibility (strain-at-break) of the SMP. With an inorganic siloxane component in its polymer backbone, it offers special properties including good biocompatibility, hydrophobicity, elasticity, gas permeability and possesses good oxidative, chemical and thermal stability.⁵² In this paper, we report a hybrid organic–inorganic thermoplastic SMP that exhibits extremely fast-response time, T_trans at body temperature and thermoplasticity that allows for solvent processing.

Fig. 1 (a) Schematic diagram depicting the shape programming and recovery process of a single-shape memory SMP. (b) Synthesis of poly(PDMS/PCL urethane)s.

Experimental section

Materials

Poly(ε-caprolactone)-diol (PCL-diol) with M_n of ca. 2000 (PCL2000) and PCL-diol with M_n of ca. 1250 (PCL1250) were purchased from Sigma-Aldrich and dihydroxyl-terminated polydimethylsiloxane (PDMS-diol) with M_n of ca. 2000 was purchased from Scientific Polymer Products Inc. Dibutyltin dilaurate (95%) and 1,6-hexamethylene diisocyanate (HDI) (98%) were purchased from Sigma-Aldrich, and were used as received. Solvents including chloroform, toluene and n-hexane are of ACS grade and were used as received.

Synthesis of poly(PCL/PDMS urethane)s

Poly(PCL/PDMS urethane)s were synthesized from the diols of PCL and PDMS with various weight ratios of PCL/PDMS using HDI as a coupling reagent. GPC profile showed the M_n and M_w of PCL2000 diols were 2700 and 3420 g mol⁻¹, respectively. The M_n and M_w of PCL1250 diols were found to be 1980 and 2820, respectively. The M_n and M_w of PDMS diols were found to be 4050 and 6860, respectively. The amount of HDI added was equivalent to the reactive hydroxyl groups in the solution.

The synthesis method is generally similar to that of previous work done by our group.^53–59 Typically, 18 g of PCL-diol (M_n = 2700, 6.7 × 10⁻³ mol), and 2 g of PDMS-diol (M_n = 4050, 4.9 × 10⁻⁴ mol) were dissolved in a round bottom flask in 100 mL of anhydrous toluene at 60 °C. Trace amount of water in the mixture was removed through azeotropic distillation by rotary evaporation until only 1 mL of toluene was being left in the mixture. This process is repeated two times. The flask was then heated to 110 °C, and 1.16 mL (7.2 × 10⁻³ mol) of HDI and two drops of dibutyltin dilaurate (∼8 × 10⁻³ g) were added sequentially. The reaction mixture was stirred at 110 °C under nitrogen atmosphere for 24 h. The resultant copolymer was precipitated from n-hexane. The yield was 75% and above after isolation and purification. This is illustrated in Fig. 1b. ¹H NMR (CDCl₃) of poly(PCL/PDMS urethane)s: δ (ppm) 0.07 (–CH̲₃ of PDMS segment), 1.38 (OCH₂CH₂CH̲₂CH₂CH₂CO), 1.49 (OOCNHCH₂CH̲₂CH₂CH₂CH̲₂CH₂NHCOO), 1.64 (OCH₂CH̲₂CH₂CH̲₂CH₂CO), 2.30 (OCH₂CH₂CH₂CH₂CH̲₂CO), 3.15 (OOCNHCH̲₂CH₂CH₂CH₂CH₂CH̲₂NHCOO), 3.68 (OOCNHCH₂CH₂CH₂CH₂CH₂CH₂NHCOO), 4.06 (OCH̲₂CH₂CH₂CH₂CH₂CO), 4.23 (OOCNH̲CH₂CH₂CH₂CH₂CH₂CH₂NH̲COO).

Molecular characterisation

Gel permeation chromatography (GPC) analysis was carried out with a Waters 2690 system equipped with three Phenogel 5μ 500 Å, 10⁴ Å and 10⁶ Å columns (size: 300 × 7.80 mm) in series and a Waters 2420 ELS detector. HPLC grade tetrahydrofuran (THF) was used as eluent at a flow rate of 1.0 mL min⁻¹ at 40 °C. Monodispersed poly(methyl methacrylate) standards were used to obtain a calibration curve.

The ¹H NMR (400 MHz) spectra were recorded on a Bruker AV-400 NMR spectrometer at room temperature. The ¹H NMR measurements were carried out with 16 scans. Chemical shift was referred to the solvent peaks (δ = 7.3 ppm for CDCl₃).

Fourier transform infrared (FTIR) spectra of the polymer dissolved in acetone and coated and dried on potassium bromide pellets were recorded on a Perkin Elmer Spectrum 2000 FT-IR Spectrometer; 16 scans were signal-averaged with a resolution of 4 cm⁻¹ at room temperature.

Thermal analysis

Thermogravimetric analyses (TGA) were carried out on a TA Instruments TGA Q500. Samples were heated at 20 °C min⁻¹ from room temperature to 700 °C in a dynamic nitrogen atmosphere (flow rate = 40 mL min⁻¹).

Differential scanning calorimetry (DSC) measurements were performed using a TA Instruments DSC Q100 differential scanning calorimeter equipped with an autocool accessory and calibrated using indium. The following protocol was used for each sample: the first heating run started from room temperature to 170 °C at 20 °C min⁻¹, with an isothermal step, holding at 170 °C for 2 min, the first cooling run started from 170 °C to −30 °C at 5 °C min⁻¹, and finally the second heating run took place from −30 °C to 170 °C at 5 °C min⁻¹. Data were collected from the second heating curve. Melting temperatures were taken as peak maxima.

Thermo-mechanical properties analysis

Linear viscoelastic thermo-mechanical properties of the copolymers were determined using dynamic mechanical analysis (DMA). A TA Instruments DMA Q800 apparatus was employed in tensile mode with a preload force of 0.01 N, an oscillation amplitude of 20 μm, static stress/dynamic stress amplitude ratio (“force tracking”) of 150%, and an oscillation frequency of 1 Hz. Poly(PCL/PDMS urethane) films were prepared by casting a 7 × 10⁻² g mL⁻¹ chloroform solution onto a poly(tetrafluoroethylene) (PTFE) dish. After slow evaporation of the chloroform, remaining solvent in the films was removed by drying the films in high vacuum at 40 °C for 1 day. Samples were cut from the cast films in dimensions of 30 mm (length) × 6.0 mm (width) × 0.5 mm (thickness). After loading each film specimen at room temperature under tensile test, they were cooled to T = −100 °C, thermally equilibrated, and then ramped to 100 °C at a rate of 3 °C min⁻¹.

Dynamic mechanical analysis and shape memory characterisation

Shape memory properties of the materials were determined using dynamic mechanical analysis (DMA). A TA Instruments DMA Q800 was employed in controlled force mode with a preload force of 0.001 N. Poly(PCL/PDMS urethane) films were prepared by casting a 7 × 10⁻² g mL⁻¹ chloroform solution onto a poly(tetrafluoroethylene) (PTFE) dish. After slow evaporation of the chloroform, remaining solvent in the films was removed by drying the films in high vacuum at 40 °C for 1 day. Samples were cut from the cast films in dimensions of 30 mm (length) × 6.0 mm (width) × 0.5 mm (thickness). In a typical run, for shape memory characterisation, samples were equilibrated at 45 °C, elongated to a force of 2.0 N at a rate of 0.03 N min⁻¹ (this strain was recorded as ε_m), cooled to 5 °C at a rate of 2.50 °C s⁻¹, with an isothermal time of 2.0 min, unloaded to 0.001 N at a rate of 0.1 N min⁻¹ (this strain was recorded as ε_u), then heated to 45 °C at a rate of 2.5 °C min⁻¹, with an isothermal duration of 15.0 min to observe the recovery (this strain was recorded as ε_r). This process was repeated three more times on the same sample in order to obtain multiple cycles. The shape recovery ratio (R_r) and the shape fixity ratio (R_f) were calculated by the following equations:^60,61

Results and discussion

Synthesis and characterisation of poly(PCL/PDMS urethane)s

A series of random multiblock poly(PCL/PDMS urethane)s with different PCL and PDMS compositions were synthesized, and their molecular weights and molecular weight distributions were determined by GPC. A typical GPC chromatograph of one of the poly(PCL/PDMS urethane)s, ED1-1, shows a unimodal peak (Fig. 2). The non-overlapping nature of the copolymer chromatographs with that of their PCL-diol and PDMS-diol precursors indicates that a complete polymerisation reaction took place with no unreacted precursor remaining. Molecular weights of the poly(PCL/PDMS urethane)s and their polydispersity indices are tabulated in Table 1. The copolymers can be melt processed into different shapes and they are soluble in common organic solvents such as chloroform, tetrahydrofuran, acetone, N,N-dimethylformamide and 1,4-dioxane.

Fig. 2 GPC diagrams of poly(PCL/PDMS urethane), ED1-1, and its PCL-diol and PDMS-diol precursors: (a) ED1-1 (M_n: 96.1 × 10³, M_w: 169 × 10³, M_w/M_n: 1.76); (b) PCL2000: (M_n: 2700, M_w: 3420, M_w/M_n: 1.27); (c) PDMS-diol (M_n: 4050, M_w: 6860, M_w/M_n: 1.69).

Table 1 Molecular characteristics of poly(PCL/PDMS urethane)s

Copolymer^a	Composition in copolymer^b (wt%)		Copolymer characteristics
	PCL	PDMS	Mn^c (×10^3)	Mw/Mn^c
a Poly(PCL/PDMS urethane)s are denoted ED, E for PCL and D for PDMS. A prefix of 1 is used if the PCL used is of Mn ca. 2000 and prefix of 2 is used if the PCL used is of Mn ca. 1250. The Mn of PCL2000, PCL1250 and PDMS used for the copolymer syntheses were 2700, 1980 and 4050, respectively.b Calculated from ^1H NMR results.c Determined by GPC.
ED1-1	93.2	6.8	96.1	1.76
ED1-2	83.4	16.6	53.7	1.61
ED2-1	81.9	18.1	73.2	1.87

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The chemical structure of poly(PCL/PDMS urethane)s was verified by ¹H NMR and FTIR spectroscopy. Fig. 3 shows the ¹H NMR spectrum of ED2-1 in deuterated chloroform (CDCl₃), in which all proton signals belonging to PCL and PDMS segments are confirmed according to references from previous work.^53,55,62,63

Fig. 3 400 MHz ¹H NMR spectroscopy of poly(PCL/PDMS urethane), ED2-1, in CDCl₃.

Fig. 4 shows the FTIR spectra of ED1-1 and its PCL2000, PDMS-diol and HDI precursors. From the spectrum of ED1-1, the peak arising from –NCO– stretching (at 2200 cm⁻¹) in HDI was not observed, indicating that the isocyanate groups have reacted and are absent in the polymer product. The peak attributed to the urethane bond (at 1530 cm⁻¹) developed in the spectrum of ED1-1, proving the occurrence of the polymerisation process.

Fig. 4 FTIR spectra of poly(PCL/PDMS urethane), ED1-1, and its precursors: (a) PCL2000: (M_n: 2700, M_w: 3420, M_w/M_n: 1.27); (b) PDMS-diol (M_n: 4050, M_w: 6860, M_w/M_n: 1.69); (c) ED91 (M_n: 96.1 × 10³, M_w: 169 × 10³, M_w/M_n: 1.76); (d) HDI.

Thermal properties

Crystallization behaviour of PCL segments in the poly(PCL/PDMS urethane)s were studied using DSC, under a dynamic nitrogen atmosphere to prevent oxidative degradation. Fig. 5a shows the DSC thermogram of the ED1-1 obtained in the second heating run at a rate of 5.0 °C min⁻¹ after the removal of thermal history in the first heating and cooling run, displaying a T_m of 37.9 °C. Evaluation of the DSC thermograms using the second heating cycle allows for the observation of the intrinsic thermal properties of the poly(PCL/PDMS urethane)s. The PCL homopolymer is a semi-crystalline polymer with a melting temperature range from 45–60 °C with increasing M_n,⁶⁴ while PDMS is a non-crystalline polymer and does not show a melting peak. Crystalline content and the enthalpy of melting of the polymers are tabulated in Table 2.

Fig. 5 (a) DSC and (b) TGA curves of poly(PCL/PDMS urethane), ED1-1, and its precursors: (a) PCL2000 (M_n: 2700); (b) PDMS-diol (M_n: 4050); and (c) ED1-1 (M_n: 96.1 × 10³). (c) Time-dependent strain profile for poly(PCL/PDMS urethane), ED1-2, in five-cycle thermocyclic test after the first conditioning cycle.

Table 2 Transition temperatures, corresponding enthalpies, crystallinity and decomposition temperatures of poly(PCL/PDMS urethane)s

Sample	Tm^a (°C)	ΔHm^b (J g^−1)	Xc^c (%)	Td^d (°C)
a Melting point determined in the DSC second heating run.b Enthalpy change during melting determined in the DSC second heating run, where ΔHm is the area of the endothermic peak for melting read from the DSC curves.c Crystallinity calculated from melting enthalpies. Xc = [ΔHm]/[ΔH^0m], where ΔH^0m is the reference value for a completely crystallised sample. Reference value of 142.0 J g^−1 for completely crystallized PCL was used.^60d Temperature at which 10% mass loss has occurred from TGA curves.
ED1-1	37.9	37.8	26.6	323.0
ED1-2	37.1	25.3	17.8	321.2
ED2-1	37.8	27.2	19.2	327.4

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T_m of the poly(PCL/PDMS urethane)s synthesised were observed to be between 35 and 38 °C, which is close to the human body temperature. In our experiments, T_m of the crystalline PCL segments is used as a switching temperature for shape recovery; therefore achieving a T_m close to the human body temperature is desirable and makes the copolymer system a viable choice for use as a biomedical material.⁶⁵

Thermal stabilities of the poly(PCL/PDMS urethane)s were evaluated using TGA. Fig. 5b shows the TGA curve for ED1-1 compared with its PCL-diol and PDMS precursors. The thermal degradation of pure PCL2000 occurs at 289.33 °C, while degradation of pure PDMS-diol occurs at 357.18 °C. ED1-1 undergoes a single step thermal degradation (Fig. 5b).

Thermomechanical properties

The change in the tensile storage modulus was measured by means of dynamic mechanical analysis. The storage modulus values at 25 °C are tabulated in Table S2.† The copolymer samples show two thermal transitions. The first transition corresponds to the glass transition of PCL segments, and the second transition can be attributed to the melting of the PCL crystals. When the temperature is low at about −60 °C (T < T_g), the copolymer films are glassy. The storage moduli of the copolymers are in the MPa range. When the temperature is higher than T_g, the storage modulus of the polymer films drop drastically. The storage modulus drops further when the temperature is further increased (Fig. S1†). This can be attributed to the melting of the PCL segments within the copolymer which further softens the polymer. The polymers did not become fully molten when the temperature is raised beyond the melting temperatures determined by DSC up to 100 °C. This can be attributed to the presence of hydrogen bonding and hexamethylene groups between the urethane groups which hold the polymer chains together.

Shape memory characterisation

Thermocyclic shape memory experiments were carried out for the polymer films using dynamic mechanical analysis (DMA). Fig. 6a shows the one-way shape memory cycle for poly(PCL/PDMS urethane). The asterisk marks the start of the thermocycle and the arrows denote the different stages of the cycle: (a) loading and elongation at 45 °C; (b) shape fixation and cooling to 5 °C; (c) unloading to evaluate shape fixity; and (d) heating to 45 °C to induce and evaluate shape recovery.

Fig. 6 (a) Stress–strain–temperature curve for poly(PCL/PDMS urethane) in the second thermocycle. (b) Stress–strain–temperature curve for poly(PCL/PDMS urethane), ED1-2, in a five-cycle thermocyclic test. (c) Frame-by-frame analysis of shape recovery response time of poly(PCL/PDMS urethane), ED1-1, in a 40 °C water bath. (d) Shape fixity ratio (R_f) of poly(PCL/PDMS urethane), ED1-2. (e) Shape recovery ratio (R_r) of poly(PCL/PDMS urethane), ED1-2.

The polymers were deformed at 45 °C, above their T_m (as determined by DSC experiments). Shape fixation was carried out by cooling the temperature to 5 °C to allow crystallisation to take place. The load was removed to allow for evaluation of shape fixity by measuring the extent of strain recovery before heating. Shape recovery was carried out by elevating the temperature back to 45 °C.

Good shape fixity was observed as evident in the small elastic recovery observed upon removal of the applied static force. Shape fixity ratios were generally 95% and above for the ED-1 series of polymers. Shape recovery was also observed to be good, with general shape recovery ratios of above 95% for the ED-1 series of polymers, after an initial conditioning cycle (cycle 0). Good shape fixity and shape recovery in poly(PCL/PDMS urethane)s are mainly due to a sufficient degree of crystallinity (≥17.8%) of the switching PCL segments.⁶⁶ Good shape fixity is essential for applications such as medical devices as it is crucial for the material to be able to maintain its temporary shape well. The calculated shape fixity and shape recovery ratios are tabulated in Table 3. The shape fixity ratio (R_f) (Fig. 6d) and shape recovery ratio (R_r) (Fig. 6e) of poly(PCL/PDMS urethane), ED1-2, with the dotted line indicating ideal fixity and recovery ratio of 100%. The sample was observed to display R_f and R_r that were close to 100%.

Table 3 Shape recovery and shape fixity ratio of poly(PCL/PDMS urethane)s

Sample		Cycle 0	Cycle 1	Cycle 2	Cycle 3
a Sample was strained to a length beyond the measurement limits of the DMA equipment in cycle 3.
ED1-1	Rr	62.1	82.2	86.5	^a
	Rf	93.1	97.2	88.5	^a
ED1-2	Rr	34.5	91.3	94.1	95.6
	Rf	94.4	95.2	96.5	96.2
ED2-1	Rr	70.8	89.7	93.1	95.0
	Rf	74.6	79.0	81.0	82.7

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Shape recovery and shape fixity ratios of the samples were observed to have an increasing trend with each thermocycle, with the values becoming more similar with an increasing number of cycles⁶¹ (Fig. 5c and 6b). The increasing trend in shape recovery ratio with each thermocycle has been reported in previous studies on SMPs.^5,47,67 It was observed that shape recovery ratios generally increased greatly after cycle 0, and ED-1 samples generally displayed R_r values of more than 95% in cycle 3. As such, poly(PCL/PDMS urethane)s should be pre-conditioned with one thermocycle prior to use in any applications, in order to maximise the shape recovery potential of the material. The need for pre-conditioning has also been reported in prior studies on PCL/PDMS-based shape memory polymers.^44–46

Frame-by-frame analysis was employed to investigate the shape recovery response time. The hybrid poly(PCL/PDMS urethane) system examined in our study exhibits fast response shape recovery, with a recovery time of less than 0.5 s (Fig. 6c). In current literature, typical response time for shape recovery of SMPs range between 10 seconds to several hours. Even with samples with similar dimensions, our SMPs display remarkably faster response times. The fast-response shape recovery of the poly(PCL/PDMS urethane)s synthesised in this study displays a marked improvement in response time over the PCL-PDMS-PCL triblock polymer networks studied by Zhang et al.⁴⁴ as well as similar studies of other polymer and composite systems (Table 4).^{5,23,32,38–42,45,46}

Table 4 Comparison of sample thickness, T_trans and shape recovery time of SMPs in various studies

	Sample thickness (mm)	Ttrans (°C)	Shape recovery time
a Sample diameter cited for specimens with spherical cross-section.b Sample thickness was not provided.
Poly(PCL/PDMS urethane)s	0.6	35–38	<0.5 s
Photo-crosslinked poly(PCL/PDMS/PCL) tri-block copolymers^45	1.1	60	2 s
Poly(PCL/PDMS) block copolymer foams^46	15^a	55	8 s
Poly(PCL/PHBV urethane)s^23	0.21	40	25 s
Poly(OCL/ODX urethane)s^5	1.0^a	40	20 s
MM-3520 polyurethane^38	1.5^a	40	75 min
Crosslinked polytriazoles^39	^b	92–145	40–42 s
Thermosetting SMP/boron nitride/CNT composites^40	2.5	120	60 s
Polyurethane/micro-Ni powder^32	1.0	55	90 s
Thermoplastic polyurethane with embedded CuCl2 (ref. 41)	1.0^a	70	120 min
Thermosetting SMCP modified with PBAN^42	1.0	40	15–30 s

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Conclusion

As a summary, we have successfully optimised the properties of a series of thermoplastic SMPs to achieve extremely fast response time (<0.5 s), T_trans at body temperature, good shape recovery (>95%) and shape fixity (>95%). The combination of organic–inorganic hybrid polymers in a linear chain could form a physical network structure that allows for exceptionally fast response upon being triggered at 40 °C. From a practical point of view, this is a promising smart material that can be easily tuned by varying the composition of PCL and PDMS in the system. Mechanical properties considerations of the SMPs, such as SMP-tissue moduli matching and SMP load-bearing strengths, are also important factors to consider for potential practical applications.⁶⁸ Future works including studies on mechanical properties and degradation behavior of the SMPs, as well as biocompatibility evaluation will be reported in a separate study.

Acknowledgements

The authors acknowledge the financial support from Institute of Materials Research and Engineering, A*STAR, Singapore (IMRE/13-2P0806). The authors thank S. Y. Chan for proofreading the manuscript and C. Owh for her assistance with designing the schematic diagram in the manuscript. B. Q. Y. Chan would like to acknowledge the A*STAR Graduate Scholarship from A*STAR.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra04041a

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