Controllable Preparation of Polyacrylamide Hydrogel Microspheres in

Jun 12, 2012 - A thermal-initiated polymerization procedure is described for the controlled preparation of monodispersed polyacrylamide (PAM) hydrogel...
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Controllable Preparation of Polyacrylamide Hydrogel Microspheres in a Coaxial Microfluidic Device Bodong Yang, Yangcheng Lu,* and Guangsheng Luo The State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China ABSTRACT: A thermal-initiated polymerization procedure is described for the controlled preparation of monodispersed polyacrylamide (PAM) hydrogel microspheres. A coaxial microfluidic device was designed to disperse uniform drops (∼500 μm) of acrylamide monomer aqueous solution into n-octane. Using a delay loop immersed into a heat bath, the polymerization is initiated and carried out in separate droplets. Combining the improvement of heat transfer in the microfluidic device and the sufficient addition of n-octane, the controllable preparation can still be achieved at 95 °C, much higher than 20−60 °C as reported prevalently. Herein, the PAM microspheres can be prepared within 2 min or less, with the CV of diameter less than 4%. Furthermore, based on this controllable reaction platform, PAM microspheres were prepared at various reaction temperatures (higher than 90 °C) and monomer solution compositions to investigate the fundamental rules of controlling on their skeleton structure and absorbent capacity in deionized water.

1. INTRODUCTION Hydrogels are three-dimensional hydrophilic polymer networks with a moderately cross-linked structure. Their structure can dramatically shrink or swell by expelling or absorbing large amounts of water in response to external environmental changes with respect to ionic strength or pH,1−4 temperature,5−7 substrate concentration,8−12 electric signal, and light. Because of this versatile property, over the past three decades hydrogel materials have drawn more and more attention for their numerous potential applications for drug delivery systems,13−16 sensor,17−19 separation agents,20−24 lenses25,26 and scaffold materials in tissue engineering.27,28 Currently, conventional industrial hydrogel preparation methods involve solution polymerization, preferred for its simplicity and cheapness. However, many polymerization systems often involve highly exothermic reactions.29 The controllability of the reaction temperature thus poses a critical problem for hydrogel preparation via solution polymerization. For reactions conducted in batch reactors, the heat released is especially difficult to remove expeditiously, particularly when higher reaction rates (corresponding to higher temperatures or monomer concentrations) are involved. Consequently, the local temperature within the reaction system may exceed the desired range and get out of control; hotspots may produce undesirably poor quality of products or even explosive polymerization. Therefore, the reaction temperature for hydrogel preparation is usually controlled in the range from 20 to 60 °C.3,22,30 The polymerization time ranges from 10 min to hours. Nevertheless, product uniformity between batches is still poor, limiting the hydrogels’ functionalization and potential application. Thus, a method with better process controllability could greatly improve the application and performance of various hydrogel materials. Recently, microfluidic methods have provided a novel approach for the preparation of monodispersed droplets and microspheres. Small, uniform droplets can be obtained using hydrodynamic flow focusing, coaxial shear flow, or crossflow © 2012 American Chemical Society

shear in cross junction, coflow junction or T-junction microchannel geometries, respectively.31−35 These techniques allow for good control over droplet size and droplets of reactants surrounded by the continuous phase flow along the microchannel at the same velocity. These droplets form a system of many uniform microreactors undergoing the exact same process. In addition, due to the small size and high surfacearea-to-volume ratio, the heat within these droplets can be dissipated quickly to the surrounding environment.36−38 Based on these characteristics, hydrogel preparation processes may be better controlled in droplets generated in microfluidic systems and executed under a wider range of conditions. Won et al.39 attempted to utilize microfluidic techniques to produce hydrogel microparticles using UV-polymerizable monomer solution as the dispersed phase and mineral oil as the continuous phase. The hydrogel droplets were formed in pressure-driven flow in microchannels at high values of the Capillary number (Ca), and polymerized in situ by exposure to 365 nm UV light. Shah et al.40 used droplet-based microfluidic techniques to produce monodispersed poly(N-isopropyl acrylamide) gel particles ranging from 10 to 1000 μm in size. They demonstrated excellent control over both the outer dimensions and inner morphology of the particles. Microgels with embedded materials and voids, and gel microcapsules with single- and multiphase cores were successfully prepared in their experiments. In this work, we introduce a microfluidic method for fast preparation of hydrogel microspheres based on a thermal initiated polymerization at higher temperatures. An aqueous solution containing acrylamide (AMm) monomer, N,N′-methylene-bisacrylamide (BisAM) as a cross-linker, and ammonium persulfate Received: Revised: Accepted: Published: 9016

February 14, 2012 June 5, 2012 June 12, 2012 June 12, 2012 dx.doi.org/10.1021/ie3004013 | Ind. Eng. Chem. Res. 2012, 51, 9016−9022

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Figure 1. Schematic diagram of the experimental facility consisted of a coflowing microfluidic channel and a coiled telfon pipe for the in situ polymerization of AMm solution.

PTFE tube to achieve in situ solution polymerization within the droplets. The length of the PTFE tubing immersed in the heat bath was adjusted to provide about a 1.5 minresidence time at high temperature for each droplet to undergo polymerization. After polymerization, a vessel containing two separated phases of n-octane and deionized water was used to collect the products at the outlet of the PTFE tube. Hydrogel microspheres passed through the oil phase and into the water phase by sedimentation. The products were then removed for further characterization. Using this technique, PAM hydrogel microspheres were prepared continuously with a short residence time of less than 2 min. The upper limit of the polymerization temperature could be at least up to 90 °C - much higher than the 60 °C normally used for conventional solution polymerization. Thus this technique and system serves as a special high temperature platform for hydrogel material preparation. After sedimentation in the collection vessel, dehydrated PAM microspheres were obtained by first changing the hydrogel microsphere containing solvent from water to ethanol by flow substitution followed by solvent evaporation and vacuum drying to remove any residual ethanol impregnated in the hydrogel microspheres. 2.3. Characterization. Observation and Size Measurement. The observation of droplets inside the microchannel and hydrogel microspheres was carried out with a microscope at 50× to 200× magnification. A CCD camera was connected to the microscope and videos were recorded at 200 frames per second. Image analysis software was used to measure the diameter of droplets and hydrogel microspheres. Each point investigated below was determined by measuring more than 30 droplets (microspheres) randomly. The size distribution was illustrated by the coefficient of variation (CV) defined as CV = δ/daverage ×100%, where δ is standard deviation and daverage is the average diameter. Scanning electron microscope (SEMJSM 7401F JEOL, Japan) was used to characterize the microscopic structure of the hydrogel microspheres. The specimens were applied gold coating of about 100 A° for SEM. Analysis of Absorbent Capacity (AC). The absorbent capacity of hydrogel microspheres in deionized water was determined by TG analysis. First, approximately 100 mg of hydrogel microspheres were immersed into deionized water at 25 °C and ambient pressure for 12 hours to obtain saturated swelling. The swollen microspheres were periodically filtrated by filter paper to remove superficial water; about 20 mg of the microspheres were then used for TG analysis. The temperature was programmed to increase from 30 to 80 °C in 30 min and

(APS) as the initiator was dispersed as droplets into an oil phase (n-octane) by using the coflowing method in a coaxial microfluidic device. The polymerization was carried out within the droplets flowing in a delay loop to obtain PAM hydrogel microspheres, and the process was run continuously. The morphologies and sizes of the products were examined in reference environments and the absorbent capacity of water was measured to evaluate the controllability of the preparation method.

2. MATERIALS AND METHODS 2.1. Materials. Acrylamide (AMm, Beijing Biodee Biotech), N,N′-methylene-bis-acrylamide (BisAM, Beijing Biodee Biotech), ammonium peroxide (APS, Shanghai Chemical Reagent), ethanol (Sinopharm Group), n-octane (Sinopharm Group) were of analytical grade and used as received without any further purification. Fresh ultrapure water prepared using a milli-Q advantage A10 ultrapure water purification system was used throughout this work. 2.2. Microfluidic Device and Preparation Method.41 The schematic diagram of the experimental setup is shown in Figure.1. A coaxial microdevice, fabricated on two polymethyl methacrylate (PMMA) plates (40 × 20 × 3 mm and 40 × 20 × 1 mm) with an end mill and sealed by high pressure thermal sealing techniques was used to control the two-phase liquid flow. The main channel embedded in the PMMA scaffold was a PTFE tube 2.5 m in length with outer and inner diameters of approximately 600 μm and 1.0 mm, respectively. A quartz capillary tube (20 mm × ID 200 μm × OD 320 μm) for introducing the dispersed phase was inserted into the PTFE tube to form a coaxial geometry. Two stainless steel needles with outer diameters of 0.8 mm were fixed perpendicularly to the main channel to supply the continuous phase. The structure of the microchannel is illustrated in Figure 1. The dispersed phase was an aqueous solution containing acrylamide monomer. Acrylamide monomer was dissolved in deionized water in the presence of the cross-linker, BisAM, and the initiator, APS. The prepared solution was sparged with nitrogen gas for 20 min before polymerization. The continuous oil phase was n-octane. Two microsyringe pumps and three microsyringes were used to pump the two phases into the microfluidic device. The flow rates of the dispersed and continuous phase were 30 and 240 μL/min, respectively. The dispersed phase droplets were generated from the sheath shear flow in the coaxial geometry at room temperature and flowed forward along the length of the microchannel together with the continuous phase. A heat bath with an accuracy of 0.1 K was used to heat fluids flowing through the 9017

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then maintain the temperature at 100 °C for 90 min. The TG curves showed that microspheres dried to a constant final weight. The mass of the saturated microspheres (M) and dehydrated spheres (M0) were obtained from the online balance of the TG analysis and the absorbent capacity was calculated by using eq 1. AC =

M −1 M0

distributions are shown in Figure 4 (d). The CV values are 1.59%, 3.14%, and 3.45%, respectively. This preparation process showed good controllability over reaction parameters and product size. As mentioned above, good controllability will lead a uniform temperature distribution, while an inferior one will lead hot spots. The difference in temperature must result in discrepancies in network structure, making such properties as absorbent capacity different. Therefore, in order to prove our microfluidic method is outstanding in controllability, hydrogel microspheres of various size were obtained by adjusting the two phase flow rates thereby changing the prerequisite droplet size; The PTFE tube was lengthened to compensate the residence time to 1.5 min. Figure 5 shows the relationship between Vh (the volume of the water saturated hydrogel microsphere, calculated from the spheres’ diameter, measured by microscope) and the corresponding Vd (the volume of dehydrated hydrogel microsphere) it produces. The relationship follows the model Vh = 41.49 × Vd (R2 = 0.9990) well, showing the uniformity of temperature distribution for different droplet sizes from 450 to 600 μm. The hydrogel microspheres exhibited nearly the same swelling performance in water. We assumed the swelling performance was closely related to the hydrogel microspheres structure, which was generated gradually during the preparation process and closely related to the extent of chain propagation and cross-linking. The similar swelling performance is contributed to good controllability of polymerization within monodispersed and uniform-distributed droplets provided by microfluidic technology. 3.2. Surface and Internal Morphology of Hydrogel Microspheres. The SEM photographs of dehydrated PAM microspheres are shown in Figure 6. Figure 6(a) shows the exceptional sphericity of these microspheres after several steps of polymerization and dehydration. Figure 6(b) illustrates the three-dimensional network and submicrometer pore structure of the microsphere surface. Figure.6(c) shows a cross section of a microsphere and that a solid structure was obtained. Figure 6(d) gives a magnifying SEM photograph of this cross section. When compared with Figure 6(b) and Figure 6(d), we can see that internal structure of the microsphere is highly similar to that on the surface indicating that the obtained microspheres exhibit high microstructure homogeneity. 3.3. Absorbent Capacity: A Case of Properties to Be Controlled. The hydrogel swelling performance is crucial for hydrogel material application. Absorbent capacity is directly related to swelling performance. Herein, we examined the absorbent capacity to evaluate the connection between a controllable preparation process and the properties of the resulting hydrogel microspheres. Changing the amount of cross-linking agent (Figure 7(a)) shows that AC increases with the increase in cross-linking agent initially, and then decreases precipitously. Larger amounts of BisAM will lead to higher cross-linking density of the hydrogel, an increase in the elastic chain force, and lower solubility for parts of the polymer. The increasing elastic chain force resists the extension of the network, the swelling of the hydrogel, and decreases absorbent capacity. However, by decreasing the soluble part of the hydrogel, the polymer network can hold more water which leads to an increased absorbent capacity. The AC curve profile and maximum can be explained to some extent by these two contradictory effects of cross-linking. When the ratio of [BisAM] to [AM] is greater than 8 × 10−4, the AC will relieve its precipitous decreasing tendency with the

(1)

3. RESULTS AND DISCUSSION 3.1. Droplet Size and Hydrogel Microsphere Size. In this section, the reaction temperature was 90 °C; residence time was approximately 1.5 min. Under these conditions polymerization proceeded almost completely within the PTFE tube. Previous works on the mechanism of droplet formation in coaxial microfluidic devices have shown that a force balance between shear and interfacial forces controls droplet breakup and flow type. Moreover, the droplet size is mainly determined by the continuous phase flow rate.30,42 Figure 2 shows that

Figure 2. Effect of the two phase flow on AM droplet size. [AM] = 4.95 mol/L, without APS and BisAM.

droplet size decreases with the increasing of the continuous phase flow rate at a fixed dispersed phase flow rate, and slightly increases with the increasing of the dispersed phase flow at a fixed continuous phase flow rate. Therefore, droplets ranging from 400 to 600 μm can easily be obtained by varying the two phase flow rates. For an in situ polymerization process, changes to droplet size directly affect hydrogel microsphere size Thus, uniform hydrogel microspheres of varying size can also be generated simply by varying the two phase flow rate as shown in Figure 3.

Figure 3. Effect of the two phase flow on sizes of AM droplets, water saturated hydrogel and dehydrated micorspheres. [AM] = 4.95 mol/L, [BisAM]/[AM] = 1.48 × 10−4, [APS] = 0.0123 mol/L. The rate of dispersed flow is fixed at 30 μL/min.

Figure 4 (a−c) shows micrographs of droplets, water swollen microspheres, and dehydrated microspheres. Their size 9018

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Figure 4. Micrographs of monodispersed microspheres (droplets) and the size distributions. The continuous flow rate is 240 μL/min. The dispersed phase flow rate is 30 μL/min. [AM] = 4.95 mol/L, [BisAM]/[AM] = 1.48 × 10−4, [APS] = 0.0123 mol/L. (a) the average diameter of droplets is 566 μm with CV of 1.59%. (b) the average diameter of saturated hydrogel microspheres is 1141 μm with CV of 3.14%. (c) the average diameter of dehydrated microspheres is 357 μm with CV of 3.45%.

proportional to [AM]. Therefore, with the concentration increasing, thenetwork chain structure becomes relatively strongly cross-linked which is similar with the case of decreasing the amount of APS added. On the other hand, hydrogel prepared in concentrated conditions was reported to have some kinds of topological entanglements among the chains of the network.44 Increase in AMm concentration, which means a higher concentration of PAM chains, lead to a high level of entanglements. These entanglements works like crosslink joints enhance the elastic chain force of the network, inhibit the AC of hydrogel. Reaction temperature was also considered to have a critical effect on hydrogel preparation in conventional solution polymerization. Figure 7(d) shows that AC increases as the reaction temperature increases from 85 to 95 °C. We attribute this to two points. First, initiator decomposes much faster to active free radical with the increasing temperature, leading to a decrease in polymer chain length. As discussed above, this may lead to relatively weak cross-linking. Second, the increase of temperature raises the hydrolysis of amide group, bestowing the network more anion groups. This localization of charges increases the swelling force greatly,45 and hydrogel could absorb much more water. Because of these, AC increases with reaction temperature. It is widely recognized that theoretical research on relationship between network structure and AC is hard to realize since the deviation of reality network from ideal network, or defects of network in other words.43,44 However, as discussed above, the relationship of the initial chemical compositions and reaction temperature to AC can be explained based on the network structure and fundamental principles of polymer science. The controllable preparation method proposed here is highly suitable for fundamental research on hydrogel materials. As shown in Figure 7, highly controlled preparation leads to reproducible high absorbent capacities. This indicates that microfluidic platform for hydrogel preparation can be successfully

Figure 5. Relationship between Vd and Vh. [AM] = 4.95 mol/L, [BisAM]/[AM] = 1.48 × 10−4, [APS] = 0.0123 mol/L.

increasing amount of cross-linking agent. We attribute this to the regional high level of cross-linking during polymerization;43 this multiple cross-linking makes regions with high cross-linking level works like one junction point, decreasing the efficiency of cross-linker BisAM. Increasing the amount of initiator (Figure 7(b)) results in an increase in AC. This could be explained by the relative insufficient amount of cross-linking and the concept of kinetic chain length. It is known that the kinetic chain length is inversely proportional to the square root of the concentration of initiator, as shown in eq 1. So the initiation free radical centers increase with the increasing concentration of initiator while overall chain length decreases. For a network structure, the decreasing of the chain length leads to apparent ruptures of network chains, which can be seen as a cross-linking deficiency. Overall, hydrophilic network chains are less limited in increasing AC with the increase in the amount of initiator added. Increasing the concentration of AM monomer with the concentration of initiator fixed (Figure 7(c)) results in a decrease in AC. This can also be explained by the concept of kinetic chain length. When fixing the concentration of APS and the ratio of BisAM to AM monomer, the kinetic chain length is 9019

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Figure 6. Scanning electron microscopy (SEM) photographs of the dehydrated hydrogel microspheres. (a) appearance, (×100, 3.0 kV) (b) surface, (×10000, 3.0 kV) (c) and (d) cross-section morphology ( × 250, 3.0 kV) and ( × 10000, 3.0 kV). [AM] = 4.95 mol/L, [BisAM]/[AM] = 1.48 × 10−4, [APS] = 0.0123 mol/L.

Figure 7. The changing of AC with system or operating parameters. (a) effect of cross-linker (BisAM) concentration, [APS] = 0.0123 mol/L, [AM] = 4.95 mol/L, T = 90 °C. (b) effect of initiator (APS) concentration, [AM] = 4.95 mol/L, [BisAM]/ [AM] = 1.48 × 104, T = 90 °C. (c) effect of monomer (AMm) concentration, [APS] = 0.0123 mol/L, [BisAM]/[AM] = 1.48 × 10−4, T = 90 °C. (d) effect of reaction temperature, [APS] = 0.0123 mol/L, [BisAM]/[AM] = 1.48 × 10−4. The deviation bar is made based on 3 times of preparation and 3 times measurement of each batch at least. The bars are smaller than symbols themselves for those whose bars cannot be seen.

used to establish a credible relationship between preparation conditions and product properties, and the necessary conditions and

parameters for obtaining desirable hydrogel material properties can be identified. 9020

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4. CONCLUSION In this work, we describe a microfuidic method for the controllable preparation of monodispersed PAM hydrogel microspheres. A coaxial microfluidic device can be used to continuously produce droplets containing monomer, cross-linker and initiator. These droplets, having a highly uniform and controllable size in the range of 400−600 μm, can release heat efficiently in a short, consistent residence time for polymerization. In these droplets, reaction hot spots are still under control when the reaction temperature is increased to 95 °C, much higher than the traditional methods (20−60 °C). Correspondingly, the preparation time is significantly reduced to approximately 1.5 min. The resulting hydrogel microspheres show an entirely homogeneous porous structure on submicrometer scale, with a swelling performance almost exclusively determined by the reaction temperature and the initial composition. To demonstrate control over final product properties, changes to the adsorbent capacity of our prepared hydrogel microspheres under different conditions was studied and the results show good reproducibility. The relationships of the initial chemical compositions and reaction temperature with the adsorbent capacity are in accord with the network structure when considering fundamental principles of polymer science. Furthermore, we can use a microfluidic preparation platform to establish a credible relationship between preparation conditions and product properties, and the necessary conditions and parameters for obtaining desirable hydrogel material properties can be identified.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86 10 62773017. Fax: +86 10 62770304. E-mail: luyc@ tsinghua.edu.cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the support of the National Natural Science Foundation of China (20525622, 20876084, 21036002, 21176136) and National Basic Research Program of China (2007CB714302) on this work.

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REFERENCES

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