Design and Thermal Properties in-house Synthesised Polymer
In need of new thermoresponsive polymers, we designed and synthesised a triblock terpolymer which is based on a novel combination of methacrylate units. The triblock terpolymer has an ABC linear architecture, which has been shown to provide the best thermogelling properties, i.e. clear sol-gel transition without solubility issues, in the previously studied systems based on PEGMA-BuMA-DMAEMA;16, 19 where DMAEMA stands for 2-(dimethylamino)ethyl methacrylate. This novel polymer consists of two hydrophilic compartments based on PEGMA (A block) and DEGMA (C block), which is also thermoresponsive close to physiological conditions (CP at 30 °C, depending on the MM), and a hydrophobic central block based on BuMA. Thus, its general chemical structure is PEGMAx-b-BuMAy-b-DEGMAz, where x, y, and z are the DPs of PEGMA, BuMA, and DEGMA, respectively, as shown in Fig. 1b), along with the structure of Pluronic® F127. It is noteworthy that even though previous studies investigated polymers consisting of EG-based repeated units,44, 45 this is the first time that this novel combination of repeated units is reported (patent published46). We believe that the substitution of DMAEMA units with DEGMA units will provide thermoresponsiveness, by avoiding any undesired effect of electrostatic interactions. In addition, the incorporation of BuMA units is beneficial, as it will promote self-assembly, while the incorporation of PEGMA units will balance the hydrophilicity, and thus solubility in aqueous media, while providing well-hydrated bridges between the micelles upon thermoresponse of DEGMA.
To ensure well-defined structural parameters, i.e. well-defined MM, and composition, we have implemented the synthesis via GTP, as these properties are crucial for controlling the thermoresponsive properties, i.e. gelation temperature (Tgel) and Cgel.1 The gel permeation chromatography (GPC) traces of the triblock terpolymer (Supplementary Fig. S1) as well as the proton nuclear magnetic resonance (1H NMR) spectra (Supplementary Fig. S2) reveal that we have successfully synthesised the triblock terpolymer with narrow MM distribution, indicated by the low dispersity value (Ð<1.2), and controllable MM and composition. The theoretical and experimental structural properties, i.e. MM, Ð, and composition are listed in Table S1.
To investigate the macroscopic differences in aqueous media between PEGMAx-b-BuMAy-b-DEGMAz and Pluronic® F127, we visually inspected the samples across a range of temperatures and concentrations and constructed the relative phase diagrams; the ones in phosphate buffered saline (PBS) are presented in Fig. 1c). The stable gel state, which is defined visually as the temperatures at which the sample does not flow upon tube inversion, as indicated by the images in Fig. 1c), is of main importance. As extensively reported in the literature,47-50 and shown in Fig. 1c) (left panel), the Cgel of Pluronic® F127 is 15 w/w%, and it forms gels at room temperature at 20 w/w% and 25 w/w%, which makes handling of the samples challenging, as previously discussed. On the other hand, as visible in Fig. 1c) (right panel), the in-house synthesised polymer, PEGMAx-b-BuMAy-b-DEGMAz, is a runny solution at all concentrations tested at room temperature, which ensures homogeneity and ease in handling and injection. As the temperature is increased, a wide gelation area, shown approximately in black dashed line, is identified, with Tgel ranging from 32°C to 39°C, depending on the concentration. Most importantly, we report a novel polymer that gels at body temperature (denoted by black dotted line in Fig. 1c) at five times lower Cgel than Pluronic® F127. This is highly advantageous in terms of the cost-effectiveness of the application and low sample viscosity at room temperature upon administration. Interestingly, we observed a controlled decrease in Tgel by increasing the concentration from 2 w/w% to 25 w/w%, as opposed to the sudden decrease in Tgel for Pluronic® F127. Similar trends are observed in deionised water (see Fig. S3), with the Tgel (and Cgel for the PEGMAx-b-BuMAy-b-DEGMAz) being slightly higher compared to the PBS solutions, which is attributed to the absence of ions in solutions.47
As previously mentioned, the solutions of the PEGMAx-b-BuMAy-b-DEGMAz are runny at room temperature, regardless the concentration, while the solutions of Pluronic® F127 are highly viscous, or gels at room temperature, depending on the concentration. To this end, we evaluated the injectability of their solutions at 15 w/w% in PBS, and it is proven that the injection rate of the polymer solution of PEGMAx-b-BuMAy-b-DEGMAz is at least one order of magnitude higher than to the one of Pluronic® F127, when the same force is applied (see Fig. S4 in Supplementary). Therefore, we demonstrate the ease in administration of the solution of PEGMAx-b-BuMAy-b-DEGMAz, which is highly advantageous especially in the application of injectable gels, as injection of a sample of lower viscosity through narrow needles could be potentially less painful for the patient.
It is well-documented that DSC has been employed to monitor i) the micellisation, i.e. the self-assembly of unimers (free polymer chains) in micelles, the gelation, i.e. the formation of a 3-D network of interconnected micelles, and the cloud point, i.e. the phase separation, of polymer solutions.50-56 Therefore, we employed DSC to record the changes in the thermal behaviour of the polymer solutions at 15 w/w% in PBS (Cgel for Pluronic® F127), and the results are presented in Fig.2; the repeated thermograms can be found in Fig. S5. As visible, a broad endothermic peak is present on the DSC thermogram of Pluronic® F127 (Fig. 2a)), with an onset temperature (Tonset) at 11.6±0.1 °C, and a maximum temperature (Tmax) at 14.5±0.1 °C. This peak is indicative of the micellisation process of Pluronic® F127 caused by the de-hydration of the PG units, and it has been previously reported in the literature.51, 54 The enthalpy of micellisation (ΔHmicell.), calculated by the area of the endothermic peak, is 5.0±0.2 J/g ( equal to 63 kJ/mol). Both the Tmax and the ΔHmicell. for Pluronic® F127 are in a good agreement with previously reported values for similar systems.50-54 As previously stated, the gelation process is scarcely endothermic (i.e. almost athermal), and thus it has only been observed as a spike on the main peak at concentrations higher than the ones in the present study.51, 54, 55 Studies on different systems other than Pluronics® recorded the phase separation at higher concentrations via DSC.56 On the other hand, the DSC thermogram of PEGMAx-b-BuMAy-b-DEGMAz (Fig. 2 b)) shows no apparent changes up to 45°C, which could be due to the intrinsic micellisation, caused by the incorporation of the BuMA hydrophobic units within the polymer structure.
To confirm this hypothesis, we carried out DLS analysis on solutions of both Pluronic® F127 and PEGMAx-b-BuMAy-b-DEGMAz at 1 w/w% in PBS at 10°C and at room temperature (25°C), and the DLS histograms are shown as insets in Fig. 2. Concerning PEGMAx-b-BuMAy-b-DEGMAz, we observed micelles at both temperatures, as the histograms almost overlap. In contrast, unimers, i.e. free polymer chains, are present in solutions of Pluronic® F127 at 10°C, as this temperature is lower than its micellisation temperature. By increasing the temperature to 25°C, a peak corresponding to micelles appears, the size of which is in agreement with the literature,53, 57, 58 in addition to the peak which corresponds to the unimers. DLS at 25°C in deuterated phosphate buffered saline (D2O/PBS), which is the solvent used during SANS, as it will be discussed in the following paragraphs, agrees with the results in PBS. The hydrodynamic diameters and the polydispersity (PDI) values, resulted from DLS analysis at 10°C and 25°C are listed in Table S2, and the corresponding DLS histograms by intensity and by number are presented in Figs. S6 and S7, respectively.
Rheological Properties and Self-Assembly behaviour
As the two polymers under investigation present clear differences in their macroscopic properties (i.e. Tgel and Cgel), in their thermal properties by calorimetry, and in their self-assembly behaviour by DLS, we used a state-of-the-art technique to gain further insights into their nanoscale self-assembly behaviour. Thus, we implemented Rheo-SANS, which is a powerful and non-destructive technique that records the scattering profiles of the polymeric ensembles at the nanoscale and the macroscopic rheological properties simultaneously, at a range of temperatures. In this study, we report and discuss the Rheo-SANS analysis of the polymer solutions at 15 w/w%, as this is the Cgel of Pluronic® F127. The solutions were prepared in D2O/PBS to achieve good neutron contrast. The temperature ramp profiles of the PEGMAx-b-BuMAy-b-DEGMAz and Pluronic® F127 are presented in Fig.3 a) and b), respectively, while their SANS profiles at selected temperatures are shown in Fig.3 c) and d), respectively. The SANS profiles with the relative fit lines at all temperatures investigated can be found in Figs. S8 and S9, while the overlapped SANS profiles at selected temperatures are provided in Fig. S10, for direct comparison.
As corroborated by rheology (Fig. 3 a) and b)), both samples are in the liquid phase at room temperature, while they form gel as the temperature increases; Tgel is defined rheologically as the temperature at which the storage modulus G′ exceeds the loss modulus G′′ (i.e. G′>G′′).59 Interestingly, both samples are in the gel state at body temperature (Tgel, Pluronic® F127 = 28°C, Tgel, PEGMAx-b-BuMAy-b-DEGMAz = 32°C), while the gels are destabilised (G′′>G′, Tdegel) at 45°C, as expected by the visual tests (see Fig. 1 c) for PBS, and Fig. S11 for D2O/PBS). The transition temperatures of PEGMAx-b-BuMAy-b-DEGMAz are in a good agreement with the values determined visually, both in PBS and D2O/PBS, while the gelation area of Pluronic®
F127 in D2O/PBS is wider than in PBS, which is confirmed both visually and rheologically.
As can be seen in Fig. 3 c), and in more detail in Fig. S8 and S9, the in-house synthesised polymer, PEGMAx-b-BuMAy-b-DEGMAz, presents different SANS profiles as the temperature increases, indicating changes in the morphology of its self-assembled structures. We used an elliptical cylinder model with a hardsphere form factor to fit the data up to 43°C, while a broad peak model was used to fit the data from 45°C; this temperature coincides with the Tdegel by rheology.
From the SANS data, we inferred that the PEGMAx-b-BuMAy-b-DEGMAz formed micelles shaped as elliptical cylinders, whose best-fit values for the radius minor (blue dots) and the length (black dots) are shown in Fig. 4 (top panel, a)) as a function of the temperature. The best-fit radius axis ratio and volume fraction as a function of the temperature are shown in Fig. S12. As it is seen, the best-fit radius minor decreases from 47 Å to 44 Å, while the best-fit axis ratio increases from 1.3 to 1.5, as the temperature is increased from 23°C to 36°C. Interestingly, we observed a clear trend for the length of the cylinder, which increases significantly (from 126 Å to 460 Å), within the same temperature range; the fitting parameters of the elliptical cylinder above 36°C are not presented in Fig. 4, as the best-fit length was outside the limits of the SANS technique (2000 Å).
In order to fit the SANS data for PEGMAx-b-BuMAy-b-DEGMAz above the Tdegel, we used a BroadPeak model, which provided the best-fit position of the Bragg peak.60 This model is a combination of a Lorentzian-peak function and a power law decay and could suggest the presence of a bicontinuous structure61 above the Tdegel. As is seen in Fig. 4 (top panel, b)), the obtained Bragg peak position shifted from 0.044 Å–1 at 45°C to 0.059 Å–1 at 55°C. The d-spacing of this peak, calculated as d=2π/Q, is a characteristic distance between the scattering inhomogeneities,61 and it decreases from d=144.21 Å and d=106.19 Å as the temperature increases from 45°C to 55°C as the sample proceeds from gel syneresis (i.e. disturbance of the gels, attributed to the inhomogeneity in the gel which causes increased internal stress, leading to the exclusion of solvent, firstly reported by Graham in 186462, 63) to precipitation (i.e. clear separation into two phases – solid phase and liquid phase).
A proposed schematic of the elliptical cylinder structure adopted by PEGMAx-b-BuMAy-b-DEGMAz is also shown in Fig. 4 (top panel, c)), in which the hydrophilic PEGMA blocks, and the hydrophilic and thermoresponsive DEGMA blocks, shown in blue and green respectively, extend from the hydrophobic BuMA core, shown in orange, towards the aqueous environment.
The SANS profiles for Pluronic® F127, presented in Fig. 3 d), S9 and S10, are in a good agreement with the ones previously reported in the literature for concentrated solutions of the same polymer.50, 64-67 We used the IGOR software to fit the scattering, as it was found to capture the scattering features/peaks best. From the SANS analysis, we observed the presence of globular structures, as indicated by the fitted power low value (P~3).
Interestingly, the scattering profiles of the solution of Pluronic® F127 present a series of peaks/shoulders at low Q values, which could be attributed to interparticle interference.50, 65, 66 The first peak, which appears at Q ~ 0.035 Å–1, is well-distinguishable at all temperatures (see Fig. 3) and it is a characteristic peak of highly concentrated solutions of Pluronic® F127, widely reported in the literature. We observed a shoulder at Q ~ 0.061 Å–1 below 25 °C, at which the sample is in the solution state. At higher temperatures, a clear peak is detected at Q ~ 0.057 Å–1, as can be seen in Fig. 3 d) middle and right, followed by two shoulders at Q = 0.067 Å–1 and Q = 0.085 Å–1 / 0.094 Å–1 at higher temperatures. These features are also related to the interparticle interference and are associated with the formation of a polymer network.50, 65, 66
In addition to the series of peaks/shoulders at low Q values, a broad shoulder is present at Q ~ 0.12 Å–1, which is due to intraparticle interferences.65 We obtained a best-fit size core for the micellar core of approximately 4.4 nm, similarly to previously reported values.65 This shoulder remains unchanged over the temperature range tested and it is an indication that the size of the self-assembled structures of Pluronic® F127 is not affected by the temperature, as previously reported.66 This can also be seen in Fig. 4 (middle panel, a)), which presents the independence of the size of the core as a function of temperature. The suggested globular structure is shown in Fig. 4 (middle panel, b)), in which the well-hydrated PEG corona is shown in grey, and the compact hydrophobic core is illustrated as a black sphere.
It is worth noting that even though the position of the peaks, and thus the size of the globular structure is not affected by temperature, the scattering intensity clearly increases, indicating an increase in the volume fraction of the polymer. Thus, we confirm that the gelation of Pluronic® F127 is caused by an increase in the number of the globular structures, which has also been reported before in the literature.67
Our extensive Rheo-SANS analysis allowed us to reveal the differences in the nanoscale between PEGMAx-b-BuMAy-b-DEGMAz, and Pluronic® F127. Thus, we conclude that the formation of gel by PEGMAx-b-BuMAy-b-DEGMAz is caused by the growth of the micelles, as indicated by the significant increase in their length (Fig. 4). On the other hand, the gelation of Pluronic® F127 is not caused by the change in the micelle size, but by the concentration of the micelles and the close packing, as indicated by the increase in volume fraction. The proposed gelation mechanisms are shown schematically in Fig. 4 (bottom), where the tricomponent system (PEGMAx-b-BuMAy-b-DEGMAz) is shown in blue, orange, and green (top), while Pluronic® F127, which is a bicomponent system (EG66-b-PG99-b-EG66), is presented in grey and black (bottom).
In order to evaluate the applicability of novel polymer in drug delivery a series of ex vivo experiments were performed using 15 wt % solutions, containing 1 mg/mL sodium fluorescein. In these experiments 15 wt % solution of Pluronic® F127 was used as a positive control (capable of forming gel at physiological temperature) and 15 wt % of Pluronic® F68 was used as a negative control (not capable of forming gel at physiological temperature). Solutions of control polymers were also containing 1 mg/mL sodium fluorescein. All these solutions were injected intracamerally into the anterior chamber of freshly excised bovine eyes thermostated at physiological temperature and the spreading of sodium fluorescein was monitored visually using video recording (see exemplar videos in Supplementary) and analysed using image analysis (Fig.5). As it was expected, the negative control formulation based on Pluronic® F68 exhibited very quick spreading of sodium fluorescein in the anterior chamber due to the absence of gelation. Both PEGMAx-b-BuMAy-b-DEGMAz and Pluronic® F127 formulations exhibited significantly slower spreading of sodium fluorescein (p<0.05) compared to Pluronic® F68, which is related to their gelation in the anterior chamber. This reduced spreadability of sodium fluorescein indicates the applicability of PEGMAx-b-BuMAy-b-DEGMAz for the potential design of injectable delivery systems to the anterior chamber, where longer drug residence will be of great importance. It is interesting to note that the formulation based on PEGMAx-b-BuMAy-b-DEGMAz shows a significantly slower spreadability of sodium fluorescein compared to Pluronic® F127 (P<0.05). This potentially indicates a further advantage of this
polymer compared to Pluronic® F127.
In conclusion, we present a novel thermoresponsive terpolymer, namely PEGMAx-b-BuMAy-b-DEGMAz, that gels at body temperature at a concentration which is five times lower than the one of the commercially available Pluronic® F127. While PEGMAx-b-BuMAy-b-DEGMAz inherently forms micelles due to the incorporation of permanently hydrophobic block, the micellisation of Pluronic® F127 is temperature-dependent, driven by the thermoresponse of the PG units. We used state-of-the-art characterisation techniques, such as DSC and Rheo-SANS, to probe the self-assembled nanostructures and gain insights into the gelation mechanism of PEGMAx-b-BuMAy-b-DEGMAz. Thus, we conclude that PEGMAx-b-BuMAy-b-DEGMAz forms a gel due to the growth of the micelle structures to near cylindrical ensembles. On the other hand, the micelle size and shape adopted by Pluronic® F127 is independent of the temperature, but the volume fraction, and thus the population of globular structures increases as a function of temperature, thus leading to gel formation. The applicability of this novel methacrylate polymer for the use as an injectable intracameral formulation for ocular drug delivery is demonstrated.