Preparation and Properties of Monodisperse pH-Responsive Microgels
7.2 Monodisperse pH-Responsive Chitosan Microgels
Up to now, a lot of researches have been carried out on anionic pH-responsive microgels based on poly(acrylic acid) and poly(methacrylic acid) [4, 5]. Such microgels are capable of swelling at high pH and shrinking at low pH because of the carboxyl groups being ionized at high pH and unionized at low pH. Very little research on cationic pH-responsive microgels has been reported. Whereas, the cationic pH-responsive microgels have pH-responsive swelling property in acidic condition due to the protonation, which is preferred in many circumstances [6,7].
Chitosan is an amino-polysaccharide obtained by alkaline deacetylation of chitin, a natural component of shrimp or crab shells. Due to its excellent biological activity, good biocompatibility and biodegradability, and antiulcer and antimicrobial properties, chitosan is receiving great interest for biomedical and pharmaceutical applications [8–11]. As a cationic polysaccharide with a pKa value around 6.27.0, chitosan has the ability to form hydrogels with a pH-responsive volume change [12].
When ambient pH is lower than the pKa value, the chitosan hydrogel swells due to the protonation of its free amino groups. As shown in Fig.7.1, the cross-linked chitosan hydrogels can be prepared using terephthalaldehyde as cross-linker via formation of a Schiff base bonding between amino groups of chitosan and aldehyde groups of terephthalaldehyde in neutral medium. The Schiff base bondings between chitosan and terephthalaldehyde also possess an interesting pH-responsive stability.
The microfluidic technique provides a facile method to produce monodisperse single or double emulsions which could be utilized as formation templates to prepare microspheres and microcapsules [13–17]. The authors’ group prepared monodisperse pH-responsive chitosan microspheres using uniform-sized water-in- oil (W/O) emulsions as the formation templates. As illustrated in Fig.7.2, the highly monodisperse W/O emulsion templates are fabricated by capillary microfluidic technique according to the published method [17], which guarantees good repro- ducibility and precise controllability of the emulsion size. In the W/O emulsion templates, chitosan is dissolved in the inner water phase, and the outer oil phase contains oil-soluble terephthalaldehyde as cross-linker. The chitosan microsphere is
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Fig. 7.1 Schematic illustration of chitosan hydrogel with terephthalaldehyde as cross-linker
Fig. 7.2 Schematic illustration of capillary microfluidic device for preparation of monodisperse W/O emulsions and the formation process of the chitosan microsphere
formed via the cross-linking reaction at the W/O interface as soon as the inner water fluid contacts the outer oil fluid in the collection tube of the microfluidic device. In the formation process, 2.0 wt % water-soluble chitosan is dissolved in water as the inner water phase using 1 mol/L NaOH to adjust its pH value to 6.7. A mixture of
Fig. 7.3 Optical photos of the W/O emulsions (a) and the prepared chitosan microspheres (b) dispersed in water. The scale bars are 100m
soybean oil and benzyl benzoate (1:1, v/v) containing 2.0 wt % terephthalaldehyde and 8.0 wt % polyglycerol polyricinoleate (PGPR) is used as the outer oil phase. The flow rates of the inner and outer fluids are Q1D600 L/h and Q2D2,000L/h, respectively. The production of W/O emulsions and subsequent interfacial cross- linking reaction are all performed at 20ıC, and the cross-linking reaction is carried out for 30 min.
Figure7.3shows the optical photos of the highly monodisperse W/O emulsions and the prepared chitosan microspheres. The cross-linking stability of prepared chitosan microspheres closely depends on environmental pH. In neutral medium, the microspheres maintain good spherical shape and structural integrity, as shown in Fig.7.3b. Whereas in acidic environment with low pH value, the amino groups of chitosan are protonated and positively charged, so the intramolecular electrostatic repulsion and enhanced hydrophilicity make the chitosan microspheres swell. With the protonation of the amino groups of chitosan going on, the Schiff base bondings become instable, and finally the cross-linked chitosan microspheres are decomposed in acidic environment as shown in Fig. 7.4. The green fluorescence of chitosan microspheres comes from the Schiff base bondings with autofluorescent properties [18,19].
The size of the chitosan microspheres mentioned above is almost above 100m in diameter, which is too big to apply as drug delivery system transporting in blood vessels. Chitosan microcapsules with small size (<50 m) are fabricated using W/O emulsions as templates by a simple microfluidic technique. The microfluidic device simply based on cover slips and microscope glass slides according to the authors’ reported method [20] is designed with an expanding nozzle to generate monodisperse picoliter-sized W/O emulsions. Figure7.5a illustrates the fabricating method of the simple microfluidic device, and the optical image and schematic of the shear focusing flow-focus device for O/W emulsions formation are shown in Fig.7.5b, c. The width of the expanding nozzle is about 60m and the surface wettability of microchannels in the devices is modified to be hydrophobic by coating self-assembled monolayer (SAM) with chlorotrimethylsilane on the channel
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Fig. 7.4 The confocal laser scanning microscopy (CLSM) images of the dissolving process of chitosan microspheres in an acidic buffer solution with pHD2.9 at 25ıC. The scale bars are 250m
Fig. 7.5 (a) Schematic illustration for fabricating the simple microfluidic device based on microscope glass slides and coverslips; (b) optical image and schematic illustration (c) of the shear focusing flow-focus device for W/O emulsions formation
surface. For the preparation of W/O emulsions, 2.0 wt % water-soluble chitosan and 0.5 wt % hydroxyethylcellulose are dissolved into deionized water as the inner water phase, soybean oil containing 0.051.0 wt % terephthalaldehyde as cross- linker and 4.0 wt % PGPR is used as the oil phase. Because terephthalaldehyde has
Fig. 7.6 CLSM images of the chitosan microgels with different cross-linking degrees:
(a) CPAD0.1 wt %, QIPD100L/h, QOPD1,000L/h; (b) CPAD0.5 wt %, QIPD100L/h, QOPD1,000L/h. (a1) and (b1): transmission channel, (a2) and (b2): green fluorescent channel, (a3) and (b3): overlay channel. All the bars are 25m
a finite solubility in inner water phase, the chitosan microcapsules could be formed via diffusion of terephthalaldehyde from outer oil phase to inner chitosan droplets in neutral medium to form the Schiff base bondings.
Different chitosan microgels are prepared by controlling the cross-linker con- centration (CPA) in the outer oil phase at same gelation time. The morphology of the chitosan microgel changes from hollow capsule to solid sphere as CPA
increases. Figure 7.6 shows the CLSM images of the chitosan microgels with different cross-linking degree after water evaporation. Obviously, the microgels with low cross-linking degree (CPAD0.1 wt %) have a hollow capsule structure (Fig.7.6a), while the microgels with high cross-linking degree (CPAD0.5 wt %) are solid spheres (Fig.7.6b). Furthermore, it can be clearly seen that these chitosan microgels have small sizes less than 50m. These monodisperse chitosan microgels with small size also have good pH-responsive properties in acidic medium like the chitosan microspheres mentioned above, which have a great potential in smart drug delivery systems and high-throughput screening of enzymes and cells.