Mechanically strong and stretchable polyurethane–urea supramolecular hydrogel using water as an additional in situ chain extender

Abstract

Mechanically strong hydrogels have attracted much interest as a result of their potential applications as biomaterials. However, it is still a challenge to produce mechanically strong supramolecular hydrogels because of the inherently weak characteristics of non-covalent interactions. A novel polyurethane–urea supramolecular hydrogel with excellent mechanical properties was developed in our laboratory by chance during the preparation of a water-borne dispersion of polyurethane with an excess amount of –NCO groups. Subsequent studies showed that this mechanical strength was because of the slow formation of multi-urea linkages and further chain extension because of the reaction of water with the excess –NCO groups in the isocyanate prepolymer chains or the free diisocyanate, or both. The mechanical properties of the polyurethane–urea supramolecular hydrogels obtained can be adjusted by simply altering the diisocyanate content. The following ranges of properties were obtained: shear modulus, 0.2–0.8 MPa; elongation at breakage, 970–2420%; tensile strength, 3.3–34 MPa; and compression stress, up to 38 MPa. Further analysis showed that the elongation ratio and tensile stress at breakage linearly decreased and increased, respectively, with an increase in the ratio of the hard segment.

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Introduction

Hydrogels are chemically or physically crosslinked three-dimensional networks which may absorb water from 10–20% up to thousands of times their dry weight.^1,2 Since the pioneering work of Wichterle and Lim³ in 1960 on covalently crosslinked polyhydroxyethylmethacrylate hydrogels, synthetic hydrogels have been thought to have great potential for use in artificial implants, biomedical devices, tissue engineering and regenerative medicine as a result of the similarity of their softness and water content to natural soft tissues, their high permeability for water-soluble nutrients and metabolites, and their potential biocompatibility.^2–16

However, the actual utilization of classic hydrogels has been seriously impeded by their poor mechanical properties. To solve this problem, several types of chemical hydrogels with high mechanical strength have been developed over the last few decades, including covalently crosslinked sliding hydrogels,¹⁷ double-network hydrogels,^18,19 macromolecular microsphere composite hydrogels,²⁰ tetra-poly(ethylene glycol) PEG hydrogels,²¹ covalent bond and hydrogen bond crosslinked hydrogels,²² inorganic clay crosslinked nanocomposite hydrogels,²³ and polyacrylamide–alginate hybrid hydrogels.¹³ However, in these chemical hydrogels, the networks are connected by covalent bonds to yield a “permanent” hydrogel and its shape is set at the time of gelation.

In contrast, supramolecular hydrogels or physical hydrogels are greatly extending our views about new functional hydrogels, with their abilities to be processed and recycled, their self-healing properties and their injectability.^7,24–26 This is because, in supramolecular hydrogels, the networks are held together by molecular entanglements and/or reversible non-covalent interactions, such as hydrogen bonds, hydrophobic forces, metal–ligand interactions and host–guest interactions. Such features, especially those with reversible and dynamic characteristics, are more similar to those of natural soft tissues and, thus make supramolecular hydrogels more attractive for use in biomedical applications. However, the problem of mechanical strength remains. Most reported supramolecular hydrogels have a low mechanical strength and are often jelly- or paste-like in consistency, partially due to the inherently weak characteristics of the non-covalent interactions.

It is well known that polyurethane (PU) or polyurethane–urea (PUU) elastomers are ideal candidates for fabricating tissue engineering scaffolds with mechanical properties similar to strong and resilient soft tissues.^27,28 There has recently been an upsurge in interest in PU or PUU hydrogels^29–32 because of the wide variety of properties of the final products. As with many other supramolecular hydrogels, however, most reported PU or PUU supramolecular hydrogels are very weak or brittle. Recently, Wu et al.³¹ developed a mechanically strong PEG–POSS hybrid PU supramolecular hydrogel with a shear modulus ranging from 0.3 to 4.0 MPa, much higher than previously reported hydrogels, however, the tensile behaviour of the hydrogel was not reported.

We recently found that, while making water-dispersible PU adhesives in our laboratory, very strong and stretchable gels were formed after the samples had been left exposed to air for a period of time ranging from a couple of days up to 1 week. Careful examination showed that an excess amount of isophorone diisocyanate (IPDI) had been added. As a result of this, in the water-dispersible PU dispersions prepared, urea linkages were slowly formed due to the reaction of water with the excess –NCO groups in the PU chains or the free IPDI, or both. We later established that this process is consistent with a recently developed synthetic approach, i.e., the formation of urea linkages by in situ chain extension using water, for the preparation of very strong PUU elastomers.^33–35

We report here the development of this novel, tough (i.e., requires work for extension) and stretchable PUU supramolecular hydrogel (PUUS gel), which has the advantages of other supramolecular materials, such as its capacity to be easily processed. The aim of this work was to demonstrate the possibilities of this new and simple synthetic approach to preparing strong and stretchable supramolecular hydrogels using the inherently strong hydrogen bonding interactions of urea groups.

Experimental section

Materials

All reagents were purchased from commercial sources and used without further purification. Poly(ethylene glycol) (M_w = 2000 g mol⁻¹, PEG2k) and biodegradable hydrophobic poly(butylene adipate) (M_w = 2000 g mol⁻¹, PBA2k) were dried under vacuum at 80 °C overnight in the presence of P₂O₅ before use. Acetone was dried with a 4 Å molecular sieve before use. CT-E229, an environmentally friendly catalyst, was kindly provided by Newtop Chemical Materials Co. Ltd (Shanghai).

Synthesis

The PUU was synthesized according to the procedure shown in Scheme 1. In this procedure, 2.5 mmol PEG2k, 2.5 mmol PBA2k, 4.0 mmol dimethylol butanoic acid (DMBA), X mmol IPDI and 15.0 mL acetone were added into a three-necked flask. The catalyst CT-E229 (0.1 g) was then added to the flask and stirred for 10 min at 60 °C; the reaction mixture was then stirred for 3 h at 75 °C. After 3 h, 5.0 mmol 1,4-butanediol (BDO) was added and the reaction mixture was stirred for another 1 h at 70 °C before cooling to 60 °C. Finally, (X–14) mmol of H₂O were added to the flask with stirring until the conversion of the –NCO groups was complete, as verified by FTIR. This took 18–60 h depending on the total amount of IPDI used. A solvent (acetone) can be added to adjust the viscosity and to facilitate the formation of the high molecular weight product.³³ For simplicity, the feed ratios of all the raw materials, except IPDI and H₂O, were kept constant and the total amount of all the diol molecules was fixed at 14 mmol (Table 1). For comparison, a PU hydrogel (PU gel) without urea linkages was also prepared using 14 mmol of IPDI.

Scheme 1 The synthetic procedure used to produce the PUU copolymer using water as an additional chain extender. In the second step, BDO may also first react with free IPDI and then with the growing chain. In the last step, H₂O may also first react with IPDI, yielding one amine group which immediately reacts with any –NCO group attached to the prepolymer chain end or IPDI.

Table 1 Chemical compositions of the PU and PUU copolymers

Sample [NCO]/[OH]	Soft segments		Hard segments
	PEG2k (mmol)	PBA2k (mmol)	DMBA (mmol)	BDO (mmol)	IPDI (mmol)	H2O^b (mmol)
a PU hydrogel prepared under equal molar amounts of –NCO and –OH groups.b The molar amount of water added in the last step was equal to that of the excess amount of IPDI.
14/14^a	2.5	2.5	4.0	5.0	14.0	0.0
17/14	2.5	2.5	4.0	5.0	17.0	3.0
19/14	2.5	2.5	4.0	5.0	19.0	5.0
22/14	2.5	2.5	4.0	5.0	22.0	8.0
26/14	2.5	2.5	4.0	5.0	26.0	12.0
32/14	2.5	2.5	4.0	5.0	32.0	18.0

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Hydrogel preparation

The final reaction mixture was poured into a Teflon™ mould, where the solvent evaporated to yield dry films. Acetone, an easily evaporated solvent, was used to facilitate the dry film formation instead of dimethyl formamide (DMF), which was used as the solvent to prepare the PUU elastomers with water as the in situ chain extender. After fully swelling in a large amount of water, hydrogel films were obtained. For the compression measurements, dry rod samples were first prepared by a thermo-compression method in a Teflon™ mould at 120 °C; after cooling to room temperature and fully swelling in a large amount of water, rod hydrogel samples with diameters of 24 mm and thicknesses of 21.5 mm were obtained.

Characterization

The water content of the hydrogel samples was calculated according to the following equation:

Water content (wt%) = 100 × (M_w − M_d)/M_w

where M_w is the weight of the equilibrium water swelling samples and M_d is the dry weight of the samples.

The hard segment ratio was defined as the weight ratio of IPDI and DMBA, BDO and H₂O (in the case of PUUS) to that of the copolymer; the weight of the CO₂ gas formed in the last step of Scheme 1 was excluded. Infrared (IR) spectra were recorded on a Nicolet 6700 FTIR spectrometer by co-adding 32 scans with a resolution of 4 cm⁻¹. Rheology measurements were performed on a HAAKE Rheometer (RS 6000) with a parallel plate accessory (20 mm in diameter). All the tensile tests were carried out in air, at room temperature, using a universal tensile testing machine (KJ-1065B, Kejian-tech, China) with a 50 N loading cell. The cross-head speed of the tensile measurements was 50 mm min⁻¹. Compression measurements were carried out on a universal tensile machine (WDT-2000, Kaiqiangli Testing Instruments Co.) with a 20 kN loading cell and a cross-head speed of 5 mm min⁻¹. Both the tensile and compression stresses were obtained by the relation: stress = F/S₀, where F is the load applied on the specimen and S₀ is the original cross-sectional area of the specimen. An Alpha Technologies RPA 2000 rubber process analyser was used for the rheological temperature sweeps (25–150 °C, 3 °C min⁻¹) at a frequency of 1 Hz and a strain amplitude of 3%. Thermogravimetric analysis was carried out using a Pyris 1 instrument (PerkinElmer). Disc samples cut from the films were heated at 10 °C min⁻¹ under a nitrogen atmosphere.

Results and discussion

As shown in Scheme 1, the synthetic procedure was a simple one-flask method with a three-step reaction without any purification processes. Each step was monitored by determining the amount of the isocyanate group present using the di-n-butylamine-acetone method associated with IR spectroscopy until a desired stoichiometry was reached (Fig. S1†).³⁶ Hydrophilic PEG2k and biodegradable PBA2k were used as the soft segments and DMBA and BDO as the normal chain extenders. Water was added during the last step as an additional in situ chain extender for the generation of urea linkages, which are stronger hydrogen bonding units than the urethane groups.³⁵ The synthesis strategy was to first create an isocyanate end-capped prepolymer by allowing diol-terminated molecules to react with IPDI; the isocyanate-terminated prepolymers were then further chain extended by adding water in the last step.

All the diol molecules were completely consumed in the first two steps. Therefore, when water was added in the last step, the corresponding amine group was formed by the reaction with the remaining isocyanate groups; the amine group then reacted with other isocyanate groups in the prepolymer chain end or free IPDI, resulting in a urea linkage. These reactions were repeated until all the –NCO groups were transformed to urea linkages. Fig. 1 shows the normalized FTIR spectra of the PU ([NCO]/[OH] = 14/14) and PUU ([NCO]/[OH] > 14/14) samples. It can be observed that, as the [NCO]/[OH] ratio was increased while adding water as the in situ additional chain extender, the weak shoulder centred around 1650 cm⁻¹ increased in intensity. At the same time, the intensity of the N–H stretching region between 3100 and 3500 cm⁻¹ also increased. This shows that increasing numbers of urea linkages were formed with increasing [NCO]/[OH] ratios as the total amount of diol molecules was kept constant (14 mmol, Table 1) for all the samples. Previous studies have shown that the urea–urea hydrogen bond is much stronger than the corresponding urethane–urethane hydrogen bond.^33–35 It is generally accepted that one of the driving forces for the phase separation of PU or PUU is strong hydrogen bonding interactions between the urethane or urea groups, or both. Thus, it may be safe to presume that the higher hydrogen bonding strength and higher driving force for phase separation of the shorter hard segments would result in improved mechanical properties of the resulting supramolecular hydrogels.

Fig. 1 Normalized FTIR spectra of the various PU ([NCO]/[OH] = 14/14) and PUU samples ([NCO]/[OH] > 14/14).

Fig. 2 shows photographs of the PU hydrogel ([NCO]/[OH] = 14/14) and PUUS ([NCO]/[OH] > 14/14) hydrogels with various ratios of [NCO]/[OH]. We can see that the PU hydrogel is opaque and its surface is very coarse with plentiful cracks. It is also too weak and brittle to perform any mechanical measurements, such as the rheology or tensile tests. In contrast, all the PUUS hydrogels are transparent and their surfaces are smooth and compact. They also showed unexpected mechanical properties. For example, Fig. 3 shows photographic images of the biaxial stretching and loading–unloading behaviour of a PUUS ([NCO]/[OH] = 22/14) hydrogel film. As shown in Fig. 3a and Movie S1 in the ESI,† the transparent 60 × 60 × 1.5 mm hydrogel film can be stretched to several times its original size without fracture. A 75 × 15 × 1.5 mm hydrogel strip can withstand 6.2 kg of loading and return to its original size immediately after unloading (Fig. 3b and Movie S2†). More surprisingly, a hydrogel rod with a diameter of 24 mm and a thickness of 21.5 mm can bear more than 1.6 tons of loading (Fig. 3c and S2†) without breaking and can then almost return to its original state 10 min after unloading (Fig. 3c).

Fig. 2 Photo images of the PU hydrogel ([NCO]/[OH] = 14/14) and PUUS hydrogels ([NCO]/[OH] > 14/14).

Fig. 3 Photographic images of the (a) stretching, (b) loading–unloading and (c) compression of the hydrogel films with [NCO]/[OH] = 22/14. In (c), the hydrogel was dyed with Rhodamine B for clear observation.

To obtain the detailed mechanical properties of the PUUS hydrogels with various [NCO]/[OH] ratios, dynamic oscillation rheology measurements were carried out. The existence and extent of the linear viscoelastic regime were determined by measuring the dynamic shear storage modulus (G′) and loss modulus (G′′) as a function of strain (0.01 < γ < 10) at an angular frequency of 6.28 rad s⁻¹. All the measurements were carried out within the linear viscoelastic range, where G′ and G′′ were independent of strain.

Fig. 4a shows the log–log plots of the shear modulus G′ (the corresponding loss modulus G′′ is plotted in Fig. S3†) as a function of frequency ω at a fixed strain of γ = 1%. The shear modulus of the PUUS hydrogel increased from ∼0.2 to ∼0.8 MPa, a range comparable with most previously reported tough hydrogels, as the [NCO]/[OH] ratio increased from 17/14 to 32/14. The sample with the highest [NCO]/[OH] ratio (32/14) had the largest G′ of ∼0.8 MPa.

Fig. 4 (a) Rheology, (b) tensile-test and (c) compression measurements of the PUUS hydrogels with various [NCO]/[OH] ratios as shown in the insets.

The tensile tests, however, showed a slightly different trend. As shown in Fig. 4b and Table 2, with an increase in the [NCO]/[OH] ratio from 17/14 to 26/14, the elongation at break decreased from 2420 to 1450%, a range comparable with previously reported highly stretchable hydrogels. However, the tensile stress at breakage increased from 3.3 to 34 MPa, a range much higher than that of previously reported high mechanical strength hydrogels, including sliding hydrogels (<20 kPa),³⁷ double-network hydrogels (<3.8 MPa),³⁸ tetra-PEG hydrogels (<200 kPa),^39,40 hydrogen bond and covalent bond crosslinked hydrogels (<1.8 MPa),^22,41,42 and inorganic clay crosslinked nanocomposite hydrogels (<1 MPa).^23,43 With a further increase in the [NCO]/[OH] ratio to 32/14, the elongation and tensile stress at breakage decreased, but the tensile stress in the experimental range (strain from 0 to 970%, Fig. 4b) was still always higher than all the other samples.

Table 2 Hard segment ratio of the PUU copolymers, and the stress and elongation at break and water content of the respective PUUS hydrogels

Sample [NCO]/[OH]	Hard segment ratio^a (wt%)	Stress at break (MPa)	Elongation at break (%)	Water content (wt%)
a The hard segment ratio was defined as the weight ratio of IPDI and DMBA, BDO and H2O (in the case of PUUS, [NCO]/[OH] > 14/14) to that of the respective copolymer; and the weight of CO2 gas formed in the last step was excluded.b The sample is full of cracks and too brittle to obtain any data.
14/14	29	Unavailable^b	Unavailable^b	66.0
17/14	32	3.3	2410	61.7
19/14	34	11.0	1990	53.8
22/14	36	21.0	1700	52.3
26/14	39	34.0	1450	46.0
32/14	43	22.7	970	45.5

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The mechanical properties under compression are also very important for the utilization of hydrogels. For simplicity and to protect the machine, the PUUS hydrogel with [NCO]/[OH] = 22/14, i.e., the sample with a mid-range mechanical strength, was chosen for the compression measurements. As shown in Fig. 4c, the hydrogel can bear a compression stress of 36.8 MPa with a strain of 90.7% and a compression force of 1.65 tons (16.5 kN, Fig. S2†) without breaking. This stress is much higher than that of all the strong hydrogels reported previously at the same strain (<17 MPa with a strain of 90% (ref. 18)). Of special note, the compression force used here was very close to the upper limit of the machine (20 kN) and thus, to protect the machine, we did not determine the strain and stress or force at break.

All these excellent mechanical properties were dependent on the soft/hard segment ratio of PU and PUU in the hydrogel and corresponded well with previously reported thermoplastic elastomers using water as the in situ chain extender.^33,34 It is well known that PU and PUU elastomers always contain soft segments and hard segments. The soft segments give the material its elastic properties, while the hard segments provide physical strength.³⁵ As shown in Table 1, the molar amounts of the soft segments (PEG2k and PBA2k) and normal chain extenders (DMBA and BDO) were constant for all the samples and all the hydroxyl groups were completely consumed in the first two steps. Thus, with an increasing [NCO]/[OH] ratio, although the hard segment fraction formed from the urethane linkages was constant, the hard segment fraction formed from urea linkages increased. This is also confirmed by the results shown in Fig. 1. Therefore, we conclude that the higher fraction of hard segment formed from urea linkages, which have stronger hydrogen bond groups than the urethane groups, should be the main contributor to the dramatically increased mechanical properties with increasing [NCO]/[OH] ratio.

Compared to the PU supramolecular hydrogel obtained in this study and other conventional supramolecular hydrogels formed from linear PUU copolymers, the excellent mechanical properties of the present PUU hydrogels are amazing. As shown in Table 2, the PUUS hydrogel with 32 wt% hard segment ratio could be elongated to 24 times its original length, whereas the PU sample with 29 wt% hard segment ratio was too brittle to be cut and clamped to perform the tensile tests. As the hard segment ratio increased from 32 to 39%, a narrow hard segment ratio range of only 7% difference, the stress at breakage increased 10 times (from 3.3 to 34 MPa). We also tried to use isophorone diamine (a small diamine molecule with a similar chemical structure as IPDI) or 1,2-ethanediamine as the chain extender instead of water in the last step, but only a viscous water solution was obtained after the dry PUU film was immersed in large amounts of water, indicating the dissolution of the diamine chain extended PUU copolymers in water. Although more experiments and characterizations are needed to clarify the detailed mechanism, we may obtain some preliminary clues from Scheme 2. Traditional linear PUU copolymers are often prepared by a one-flask, two-step reaction procedure, as shown in Scheme 2a. In this case, –NCO end-capped PU prepolymers are obtained in the first step, then the prepolymers are further chain extended by small diamine molecules to yield the final PUU copolymers. These are, in fact, bis-urea and bis-urethane alternately segmented linear copolymers (Scheme 2a). Our example was totally different. As shown in Scheme 1 and 2b, although only similar –NCO end-capped PU prepolymers and free IPDI existed in the last step before the addition of water, the formation of urea units after the addition of water was complicated. Any –NCO group attached at the prepolymer chain end or free IPDI can slowly react with water, yielding one –NH₂ group. The –NH₂ group formed can then immediately react with any other –NCO group attached to the prepolymer chain end or IPDI, yielding one urea group. Thus, in the present example, the resultant PUU polymer was in fact a bis-urethane, urea, bis-urea and multi-urea segmented linear copolymer. With increasing [NCO]/[OH] ratios, more or longer multi-urea linkages were formed.³⁵ We may therefore conclude that the formation of these multi-urea linkages should have a profound influence on the improvement in the mechanical properties of the PUUS hydrogels.

Scheme 2 Schematic representation of the formation of (a) normal PUU and (b) PUU with multi-urea linkages in the backbone.

A detailed analysis of the data presented in Table 2 reveals another interesting behaviour of the present PUUS hydrogels, as shown in Fig. 5. Fig. 5a and b show, the effect of the hard segment ratio of the PUU copolymers on the elongation and the tensile stress at breakage of the PUUS hydrogels obtained, respectively. Fig. 5a clearly shows that the elongation at breakage decreased almost linearly with increasing hard segment ratio. For the tensile stress, however, the stress at breakage increased linearly as the hard segment ratio increased from 32 to 39%. As far as we know, this may be the first supramolecular hydrogel whose mechanical properties can be quantitatively tuned and therefore it may provide a useful reference for the rational design of other PUUS hydrogels with exceptional mechanical properties.

Fig. 5 Effects of hard segment ratio on the (a) elongation and (b) tensile stress at breakage of the PUUS hydrogels. Note: linear fitting was conducted using Origin Pro 8 software (OriginLab, USA) and the first four data points were linear-fitted in the case of tensile stress (b).

In addition to the previously mentioned excellent mechanical properties, another advantage of the present PUUS hydrogel is its ability to be easily processed because of its dynamic and reversible crosslinking character. The as-prepared viscous PUU solution (Fig. S4†) can be stored for several months without any obvious change. Thus, the films can be easily prepared by a solution-casting method and fibres can also be obtained by wet-spinning, such as the electro-spinning technique (Fig. 6).

Fig. 6 Micrograph of the electro-spinning fibres of a PUU sample with an [NCO]/[OH] ratio of 22/14 obtained from its acetone solution.

Furthermore, as shown in Fig. 7a, the temperature-dependent rheology experiment for a dry PUU film with an [NCO]/[OH] ratio of 22/14 showed that both G′ and G′′ decreased with increasing temperature. A cross temperature was observed at about 90 °C, which was much lower than its degradation temperature (∼250 °C, Fig. S5†). Below the cross temperature, the behaviour of the PUU dry film was mainly elastic (G′ > G′′) and above this temperature the behaviour changed to viscous (G′′ > G′).⁴⁴ Similar behaviour can also be observed for PUU dry films with other [NCO]/[OH] ratios (Fig. S6†). These results demonstrate that the typical thermoplastic nature of the present PUU material will greatly facilitate the manufacture of hydrogel products by injection, extrusion and compression moulding. At the same time, in contrast with most other PU or PUUS hydrogels which generally turn into a sol state at higher temperatures, the present hydrogel showed high thermal stability by keeping its gel state (G′ > G′′) up to temperatures as high as 95 °C (Fig. 7b).

Fig. 7 Temperature-dependent rheology experiments of (a) dry and (b) hydrogel films.

Conclusions

In summary, a novel type of transparent polyurethane–urea supramolecular hydrogel (PUUS gel) with excellent mechanical properties and the ability to be easily processed has been developed using water as the additional in situ chain extender to form urea linkages. The mechanical properties, in terms of shear modulus (0.2–0.8 MPa), elongation at breakage (970–2420%), tensile strength (3.3–34 MPa) and compression stress (up to 36.8 MPa) of the PUUS gel could be controlled by simply altering the [NCO]/[OH] ratio. The elongation and tensile stress at breakage can even be quantitatively tuned. The superior mechanical properties of this PUUS hydrogel can be ascribed to the formation of unique multi-urea linkages in the last step of the synthesis. These linear PUU polymers can be dissolved in many organic solvents, such as acetone, methanol, ethanol and DMF, and thus could be produced in different forms by various manufacturing methods including solution casting, electro-spinning, injection, extrusion and compression moulding. So far, no other supramolecular hydrogel has been reported to possess all these features, making these supramolecular hydrogels unequalled. This study provides a route to the simple synthesis of mechanically strong and stretchable PUUS hydrogels, which could be used to promote their applications in various fields. The results may also provide useful theoretical references for the rational design of other PUUS materials with designed mechanical properties. Our current efforts are focused on increasing the equilibrium water content of the resultant supramolecular hydrogels, exploring their applications and realizing the other functionalities of these supramolecular hydrogels, such as their self-healing and shape-memory functions.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (NSFC, no. 21274102 and 21074087).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Fig. S1–S5 and Movie S1 and S2. See DOI: 10.1039/c4ra02597k

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