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ARTICLES Nanoengineering of a Biocompatible Organogel by Thermal Processing Jing-Liang Li,†,‡ Rong-Yao Wang,†,§ Xiang-Yang Liu,*,† and Hai-Hua Pan† Department of Physics, National UniVersity of Singapore, 2 Science DriVe 3, Singapore 117542, Centre for Micro-Photonics, Faculty of Engineering and Industrial Sciences, Swinburne UniVersity of Technology, Hawthorn, Victoria 3122, Australia, and Department of Applied Physics, Beijing Institute of Technology, Beijing 100081, People’s Republic of China ReceiVed: December 19, 2008; ReVised Manuscript ReceiVed: February 6, 2009
The formation of most organogels requires the compatibility of both the gelator and solvent. It is very desirable if the rheological properties of a gel can be manipulated to achieve the desired performance. In this paper, a novel organogel was developed and its rheological properties and fiber network were engineered by controlling the thermal processing conditions. The gel was formed by the gelation of 12-hydroxystearic acid as a gelator in benzyl benzoate. It was observed that the degree of supercooling for gel formation has a significant effect on the rheological properties and fiber network structure. By increasing supercooling, the elasticity of the gel was enhanced, and the correlation length of the fibers was shortened, leading to the formation of denser fiber networks. The good biocompatibility of both the gelator and solvent makes this gel a promising vehicle for a variety of bioapplications such as controlled transdermal drug release and in vivo tissue repair. Introduction The rheological properties and micro- and nanostructures are the most important parameters of an organogel that determine its performance in many applications such as controlled delivery of drugs.1 In recent years, significant efforts have been devoted to the identification of suitable combinations of gelator and solvent for a defined application. However, it is more important if the rheological properties of a gel can be manipulated to achieve a material with the desired performance without disturbing the gelator and solvent pair. Organogels are a class of soft functional materials that have attracted significant research interest in recent years.2-5 Organogels have a variety of important applications in cosmetics, drug delivery, scaffolding for tissue engineering and preparation of nanomaterials, bioseparation, and so forth.1,6-9 The most important properties of a gel for these applications are its pore size and viscoelasticity, which determines the efficiency (i.e., pore-size-dependent separation performance) and even applicability (i.e., suitable hardness for applications as scaffoldings) of a gel. A physical gel may occur when a hot solution of a gelator cools down to a lower temperature. It is generally believed that the fibrous network is formed through the molecular self-assembly of the gelator molecules driven by noncovalent forces, such as van der Waals interactions, hydrogen bonds, and π-π stacking.10 The sol-gel-sol process can be repeated by cooling and heating the system without changing the physical and chemical properties of the components involved. Furthermore, it was also generally agreed that gelation is initiated by the nucleation of the gelator and subsequent * To whom correspondence should be addressed. E-mail: phyliuxy@ nus.edu.sg. † National University of Singapore. ‡ Swinburne University of Technology. § Beijing Institute of Technology.
SCHEME 1: Thermal Engineering of the Micro- And Nanostructure of a Gel by Controlling the Degree of Supercooling on Gel Formationa
a The correlation length and pore of the network can be tailored in this way to get the desired performance, such as controlled drug release from the gel.
growth of the fibers.11-13 The crystallinity of gel fiber has been reported by many authors.11,14-16 On the basis of the nucleation mechanism, the microstructure of a gel netwok can be formed in a controlled manner by manipulating the thermodynamic driving force of the nucleation and crystallization process (Scheme 1). Supersaturation/supercooling, a parameter describing how far a system goes beyond an equilibrium state, is an important factor that determines the final microstructure of the network. Supercooling is defined as (T* - T)/T*, where T* is the equilibrium temperature of a gel at a certain gelator concentration and T is the temperature at which the gel is formed. The correlation length and thus pore size of the network of a gel can be finely controlled by controlling the thermal processing conditions of
10.1021/jp811215t CCC: $40.75 2009 American Chemical Society Published on Web 03/23/2009
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the gel or using suitable additives to enhance branching of growing fibrils.17-20 Precise control over pore size is very important since it determines the mechanical properties of a gel and drug release kinetics.21 The latter is especially significant in terms of controlled drug delivery. In this paper, a gel formed in a combination of 12hydroxystearic acid (HSA) with benzyl benzoate is examined. The influence of supercooling on the structure of the fibrous network of the gel and on the rheological properties will be examined. HSA is a structurally simple gelator and has been reported to gel a variety of solvents, such benzene, nitrobenzene and hexafluorobenzene,22 cyclohexane,23 toluene,24 vegetable oils,25,26 and so forth. Its gelling ability for benzyl benzoate (BB) has not been reported. Due to its high solubilization capacity for lipohilic drugs and biocompatibility, benzyl benzoate is commonly used for pharmaceutical purposes.27-30 Its gelation with poly(lactide-co-glycolide) (PLGA) and controlled drug delivery from the gels have been reported.31-33 Since both HSA and BB have good biocompatibility, this gel is very promising as a delivery vehicle for the controlled release of drugs. Materials and Methods Section Chemicals. 2-Hydroxystearic acid (>70%, with 20-30% stearic acid) (HSA) and benzyl benzoate (>99%) (BB) were obtained from Fluka and Sigma-Aldrich, respectively. Both of them were used as received. The chemical structures are as follows.
Measurement of Equilibrium Temperature. Gels at a series of concentrations were prepared in glass tubes and put in a water bath. The temperature at which the last tiny part of gel is completely dissolved is defined as the equilibrium temperature T*. To guarantee the accuracy of the measurements, in the neighborhood of the equilibrium temperature, the temperature was increased in steps of 0.2 °C and maintained for 30 min at each step. Real-Time Microscopic Observation of the Gel Microstructure. For the optical observation, gel samples were prepared by sealing hot HSA/BB solution in self-made glass cells with a spacer thickness of 0.1 mm. A conventional microscope (Olympus BX50) with a heating/cooling temperature controller (Linkam Scientific Instrument, THMS600) at the sample stage was used. The temperature ramp rate was set at 30 °C/min with an accuracy of (0.1 °C. The sol-to-gel transition was monitored by a microscope video system. The images from the microscope were converted to digital images through a JVC KY-F55B 3-CCD color video camera. A series of images were obtained during the gelation process and analyzed by image processing software (analySIS version 3.2). Rheological Study. The rheological properties of the organogel were measured by an advanced rheological expansion system (ARES-LS, Rheometric Scientific). Dynamic temperature ramp tests were carried out to obtain the storage modulus G′ (a measure of elasticity), loss modulus G (a measure of viscosity), and complex modulus G* (viscoelasticity, G* ) [(G′)2 +
Figure 1. Optical appearance (A) and equilibrium temperature (B) of the HSA/BB gel at different HSA concentrations. The concentrations of HSA in (A) are 0.5, 1.0, 1.5, 2.0, and 2.5 mol%, respectively (from left to right).
(G′′)2]1/2) as a function of time. The sol-gel process was carried out in situ between two plates with a gap of 0.85 mm. The samples were subjected to sinusoidal oscillation by moving both the upper (with a diameter 25 mm) and the lower circular plates. The amplitude of the oscillation was controlled to obtain a strain of 0.05% in the sample. The oscillation frequency was 0.1 Hz, and the temperature ramp rate was 30 °C/min. Scanning Electron Microscopy (SEM) Analysis. The microand nanostructures of the fibrous networks of the xerogel were examined using a field emission scanning electronic microscope (FESEM). The xerogels were obtained by extracting the solvent, captured in the network with CO2 supercritical fluid extraction equipment (Thar Design). The flow rate of CO2 was 20 g/min, and the extraction time was 1.5 h. The xerogel powder was used for SEM and XRD analysis. Each point of Figure 3D was taken as an average of about 30 meaurements from different SEM images of a same sample. Results and Discussions Figure 1A shows the optical appearance of the HSA/BB gels formed at different HSA concentrations at room temperature (∼20 °C). The gels are opaque, indicating the presence of structures with absorption at visible wavelengths. The opacity of the gel increases with an increase of the HSA concentration, which is due to the increase in fiber mass. The equilibrium temperatures, T*, of the gel at different HSA concentrations are plotted in Figure 1B. In the low concentration range (2.0 mol%), the equilibrium temperature does not show significant changes with an increase of the HSA concentration. Figure 2 gives the microstructure of HSA/BB gel formed at different temperatures. The pictures show that a denser network constructed from thinner fibers forms at a lower temperature (higher supercooling). Real-time observation (Figure 3) indicates that the formation of a HSA fiber network is a process initiated by the primary
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Figure 2. Fiber networks of HSA/BB gel formed at different temperatures. (A) 20, (B) 40, (C) 50, and (D) 55 °C. The concentration of HSA is 2.5 mol%. Scale bar: 100 µm. All of the pictures are on the same scale.
Figure 3. Real-time observation of the fiber network formation at 55 °C. (A) 3, (B) 4, (C) 5, and (D) 10 min. The circled areas show that branches grow from growing fibers. The concentration of HSA is 2.5 mol%. Scale bar: 100 µm. All of the pictures are on the same scale.
nucleation of HSA, followed by fiber growth and subsequent branching. It is generally believed that the fiber networks of small-molecule organogels are formed by molecular selfassembly through highly specific noncovalent interactions. Interestingly, our recent results have verified the fact that, in many small-molecule organogels, the formation of fibers and fiber networks is a process that consists of the primary nucleation of the gelators and the subsequent growth of the fibers (nucleation-growth process). On the basis of this mechanism, a permanent and strong 3-D fiber network can be formed when branching occurs at the tips or the side faces of growing fibers. This is called crystallographic mismatch branching (CMB). CMB occurs when the new layers on the tips of the growing fibrils undergo a certain degree of structural mismatch, which can be controlled by changing the supersaturation of the gelator or using suitable additives to influence the correlation between the fiber tips and the nucleating phase. The nucleation-growth-crystallographic mismatch branching (CMB-growth-CMB-...) mechanism is illustrated in Figure 4A. This implies that it is feasible to control the fiber growth, so as to obtain materials with the desired fiber network. A higher degree of supercooling/supersaturation can create a higher driving force for crystallization and result in a higher degree of mismatch, leading to the formation of a more densely branched network.
Figure 4. (A) Formation mechanism of the HSA/BB gel fiber network, nanostructure of the HSA/BB gel network formed at (B) 25 and (C) 50 °C; the scale bars are 500 nm. (D) Correlation length as a function of supercooling, and (E) XRD patterns of the HSA crystalline powder and HSA fibers (xerogel). The concentration of HSA is 2.5 mol%.
One parameter to measure the branching density is correlation length ξ, which is defined as the branching distance between two neighboring branch points along a fibril. The nanostructure of the HSA fiber network formed by 2.5 mol% HSA at 25 and 50 °C was shown in Figure 4B and C. A denser network was obtained at lower temperatures. (The equilibrium temperature for 2.5 mol% HSA in BB is 64 °C. Therefore, the supercooling is 0.61 and 0.22 at 25 and 50 °C, respectively.) The effect of supercooling on the correlation length is shown in Figure 4D. With an increase of supercooling from 0.22 to 0.61, the correlation length is reduced from around 1 µm to below 400 nm. At low supercooling rates, the correlation length decreases linearly with an increase of supercooling. In a recent work, the nucleation and crystallization of HSA in canola oil was studied.34 The fiber length on the microscale as a function of time and cooling rate was investigated. Similarly, it was found that by increasing the cooling rate, the fiber length could be reduced. To verify the crystalline nature of the HSA fibers formed in BB, the XRD spectra of HSA crystal powder and HSA fibers formed in BB were obtained (Figure 4E). It showed that the HSA fiber has the same XRD pattern as that of HSA crystal powder, indicating the crystalline nature of the fibers. It was observed that HSA can gel BB at concentrations as low as 0.5 mol% at room temperature (∼20 °C). The rheological properties of the gel were characterized. Typical curves for the evolution of the storage modulus G′ and loss modulus G′′ are
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Figure 6. Supercooling dependence of critical strain and elastic modulus (the HSA concentration is fixed at 2.5 mol%) (A) and concentration dependence of the elastic modulus of the HSA/BB gel formed at 40 °C (B).
Figure 5. Characterization of the rheological properties of HSA/BB gel. (A) Evolution of the storage modulus G′ and loss modulus G′′ during the gelation process at 50 °C. (B) Critical strain (γ0) analysis and (C) frequency dependence of the moduli. The HSA concentration is 2.5 mol%, and the temperature for gelation is 50 °C.
shown in Figure 5A. It shows that during the gelation process, G′ and G′′ increase steeply at the initial stage and level off gradually upon the completion of network construction. In addition, the maximum G′ is much higher than the corresponding G′′, which is a characteristic property of an elastic gel. Figure 5B gives the storage modulus G′ as a function of strain applied on the gel. It shows that at lower strains, the modulus is independent of the applied strain. With the increase in strain to a critical level (γ0), a sharp decrease of G′ was observed, which is attributable to the partial breakdown of the gel network structure (inset of Figure 5B). The strength of a gel depends on the nature of the bonds that form the network. We noticed that a network with permanent bonds (connections of fibers through mismatch branching) can give rise to a strong gel, while the one with temporary or transient bonds (physical entanglements of elongated fibers) led to a weak gel.19,20 The frequency dependence of dynamic moduli can give insight into the nature of the bonds forming a network. If the bonds are permanent, only a small frequency dependence is expected, and G′ is larger than G′′ at all frequencies. On the other hand, if the bonds have a temporary character, a significant frequency dependence can be observed, with G′ < G′′ at low frequencies and G′ > G′′ at
high frequencies.35 Figure 5C shows that the values of G′ and G′′ do not depend strongly on the oscillation frequency in the range of 0.01-25 Hz, and G′ > G′′ at all of the frequencies. Such characteristics are typical for a gel with permanent bonds. The effects of supercooling on the values of G′max are given in Figure 6A. The result displays that the elasticity of the gel increases with supercooling. This is attributable to the denser fiber networks formed at low temperatures. Figure 6A also shows that the critical strain γ0 of the gel is compromised at higher supercooling degrees. G′max as a function of HSA concentration at a fixed temperature of 40 °C is given in Figure 6B, which shows that G′max increases with the concentration of HSA and gradually levels off when the concentration reaches above 4.1 mol%. A comparison between Figure 6A and B indicates that the same level of elastic modulus can also be obtained using a lower HSA concentration of 2.5 mol% by controlling the supercooling of the system, saving almost half of the materials. This is important when it comes to mass production of a gel for commercialization purposes. The aforementioned results are particularly important in the engineering of biofunctional materials as, in such cases, the choice of gelators and solvents is not arbitrary. For a given pair of gelator and solvent, the performance might not be ideal. The approach illustrated in this paper opens a new avenue in acquiring the materials with excellent performance. As mentioned, 12-hydroxystearic acid and benzyl benzoate are biocompatible and can be used as a vehicle for drug delivery. The release of drug molecules trapped in the gel can be controlled by manipulating the correlation length and hence the pore size of the fiber network. Controlled transdermal release of drugs using this gel system will be studied. In summary, a biocompatible organogel formed by 12hydroxystearic acid in benzyl benzoate was prepared. The engineering of the micro-/nanostructure of the fiber network and rheological properties of the gel was carried out by thermal processing. The gels show a strong elasticity, weakly dependent on the oscillating frequency ranging from 0.01 to 40 HZ. The elastic modulus G′ can be enhanced by increasing supercooling
Nanoengineering of a Biocompatible Organogel during gel formation. This is attributed to the denser fiber networks achieved at both the micro- and nanoscales. Acknowledgment. The research was supported by Singapore ARC MOE funding (Project No: T206B1114). References and Notes (1) Lim, P. F. C.; Liu, X. Y.; Kang, L. F.; Ho, P. C. L.; Chan, Y. W.; Chan, S. Y. Int. J. Pharm. 2006, 311, 157. (2) Terech, P.; Weiss, R. G. Chem. ReV. 1997, 97, 3133. (3) Carretti, E.; Dei, L.; Baglioni, P.; Weiss, R. G. J. Am. Chem. Soc. 2003, 125, 5121. (4) Moreau, L.; Barthelemy, P.; El Maataoui, M.; Grinstaff, M. W. J. Am. Chem. Soc. 2004, 126, 7533. (5) Kishimura, A.; Yamashita, T.; Aida, T. J. Am. Chem. Soc. 2005, 127, 179. (6) Jung, J. H.; Kobayashi, H.; van Bommel, K. J. C.; Shinkai, S.; Shimizu, T. Chem. Mater. 2002, 14, 1445. (7) Corriu, R. J. P.; Leclercq, D. Angew. Chem., Int. Ed. Engl. 1996, 35, 1420. (8) Oya, T.; Enoki, T.; Grosberg, A. Y.; Masamune, S.; Sakiyama, T.; Takeoka, Y.; Tanaka, K.; Wang, G. Q.; Yilmaz, Y.; Feld, M. S.; Dasari, R.; Tanaka, T. Science 1999, 286, 1543. (9) van Esch, J. H.; Feringa, B. L. Angew. Chem., Int. Ed. 2000, 39, 2263. (10) Gronwald, O.; Snip, E.; Shinkai, S. Curr. Opin. Colloid Interface Sci. 2002, 7, 148. (11) Liu, X. Y.; Sawant, P. D.; Tan, W. B.; Noor, I. B. M.; Pramesti, C.; Chen, B. H. J. Am. Chem. Soc. 2002, 124, 15055. (12) Liu, X. Y.; Sawant, P. D. AdV. Mater. 2002, 14, 421. (13) Lescanne, M.; Colin, A.; Mondain-Monval, O.; Fages, F.; Pozzo, J. L. Langmuir 2003, 19, 2013. (14) Liu, X. Y.; Sawant, P. D. ChemPhysChem 2002, 3, 374. (15) Abdallah, D. J.; Sirchio, S. A.; Weiss, R. G. Langmuir 2000, 16, 7558.
J. Phys. Chem. B, Vol. 113, No. 15, 2009 5015 (16) George, M.; Weiss, R. G. J. Am. Chem. Soc. 2001, 123, 10393. (17) Li, J. L.; Liu, X. Y.; Strom, C. S.; Xiong, J. Y. AdV. Mater. 2006, 18, 2574. (18) Li, J. L.; Liu, X. Y.; Wang, R. Y.; Xiong, J. Y. J. Phys. Chem. B 2005, 109, 24231. (19) Wang, R. Y.; Liu, X. Y.; Narayanan, J.; Xiong, J. Y.; Li, J. L. J. Phy. Chem. B 2006, 110, 25797. (20) Wang, R. Y.; Liu, X. Y.; Xiong, J. Y.; Li, J. L. J. Phys. Chem. B 2006, 110, 7275. (21) Wallace, D. G.; Rosenblatt, J. AdV. Drug DeliVery ReV. 2003, 55, 1631. (22) Terech, P.; Rodriguez, V.; Barnes, J. D.; McKenna, G. B. Langmuir 1994, 10, 3406. (23) Tamura, T.; Ichikawa, M. J. Am. Oil Chem. Soc. 1997, 74, 491. (24) Terech, P.; Pasquier, D.; Bordas, V.; Rossat, C. Langmuir 2000, 16, 4485. (25) Tamura, T.; Suetake, T.; Ohkubo, T.; Ohbu, K. J. Am. Oil Chem. Soc. 1994, 71, 857. (26) Rogers, M. A.; Wright, A. J.; Marangoni, A. G. Soft Matter 2008, 4, 1483. (27) Simamora, P.; Dannenfelser, R. M.; Tabibi, S. E.; Yalkowsky, S. H. PDA J. Pharm. Sci. Technol. 1998, 52, 170. (28) Chhabra, S.; Sachdeva, V.; Singh, S. Int. J. Pharm. 2007, 342, 72. (29) Singh, S.; Singh, J. Int. J. Pharm. 2007, 328, 42. (30) Prabhu, S.; Tran, L. P.; Betageri, G. V. Drug DeliVery 2005, 12, 393. (31) Chen, S. B.; Singh, J. Int. J. Pharm. 2005, 295, 183. (32) Wang, L. W.; Venkatraman, S.; Gan, L. H.; Kleiner, L. J. Biomed. Mater. Res. B 2005, 72B, 215. (33) Wang, L. W.; Venkatraman, S.; Kleiner, L. J. Controlled Release 2004, 99, 207. (34) Rogers, M. A.; Marangoni, A. G. Cryst. Growth Des. 2008, 8, 4596. (35) Brinksma, J.; Feringa, B. L.; Kellogg, R. M.; Vreeker, R.; van Esch, J. Langmuir 2000, 16, 9249.
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