Chapter 4 Biodegradable Ferrogel (Gelatin)
4.3 Gelatin Ferrosponges
4.3.8 Magnetic-sensitive behavior of ferrosponges
Fig. 4.13 A plot of ln(Mt/M) versus ln (t) of (a)5G-5F, (b)5G-1F and (c)5G-3F showed a two-step relationship for calculating the values of k and n, and the corresponding schematic drawing of the shrinkage of the mesopores in the ferrosponges (d) while a magnetic field was applying
4.3.8 Magnetic-sensitive behavior of ferrosponges
The preliminary result demonstrated that the ferrosponges possessed magnetic-sensitive behaviors under an MF. The “Magnetic Sensitive Behavior (%)”
was defined as the difference in the amount of cumulative drug release under “MF off”
minus that under “MF on”. The higher value of the “Behavior” indicates the higher sensitivity of the ferrosponge to a given strength of magnetic field in terms of drug
3.0 3.5 4.0 4.5 5.0 5.5 6.0
release. Fig. 4.14 shows the magnetic-sensitive behaviors for the ferrosponges of 5G and 15G compositions. The magnetic-sensitive behavior showed about 4-5 times more pronounced for the 5G compositions than that for the 15G series. It is possible that ferrosponge with 5G compositions possess a better elastic property and at the same time, a stronger interparticle attraction to regulate the relaxation of the gelatin molecules than those of 15G compositions, resulting in the sharply enhanced magnetic-sensitive behaviors. More plausibly, according to a recent study on the concentration effect of the nano-magnets on magnetization of a PVA-based ferrogels [Liu, 2006], it is believed that under the same relative concentration of the nano-magnets, the ferrosponges with higher relative concentration of the gelatin showed lower magnetization, and is evidenced in Fig. 4.15, where the saturation magnetization is increased linearly with increasing nano-magnet concentration and decreased with increasing gelatin concentration. On this base, it realizes that the nano-magnet concentration in the ferrosponges causes a significant variation in magnetic-sensitive behavior. Fig. 4.15 further supports a stronger interaprticle attraction force and less rigidity (i.e., the molecular relaxation of the gelatin chains being less restricted) of the 5G ferrosponges. Furthermore, although 5G-1F and 15G-3F possessed the same gelatin-to-Fe3O4 ratio (i.e., G/F=3/1), 5G-1F exhibited more than twice the magnetic sensitive behaviors to that of the 15G-5F composition, because in 5G-1F, the magnetic nanoparticles binding on the looser polymer structure could be migrated easily.
Fig. 4.14 Comparison of the magnetic-sensitive behaviors of the ferrosponges for both 5G and 15G series compositions
0 1F 3F 5F
0 4 8 12 16
Magnetic Sensitive Behaviors (%)
Sample 5G series 15G series
Fig. 4.15 Inorganic/organic ratios of ferrosponges related to the magnetic sensitive behaviors (%) and saturation magnetization (Ms)
Upon a consecutive on-off operation of the magnetic field to the ferrosponges, an alternative change in drug release can be identified, as shown in Fig. 4.16. The cumulative drug-release amount decreased with applied magnetic field and increased again while the MF was turned off. This tunable release rate further substantiates the re-arrangement of the magnetic nanoparticles and this further indicates a potential use of this novel ferrosponges for drug delivery applications. With increasing concentration of the nano-magnets, i.e., from 1F to 5F composition, the ability for the ferrosponges to reaching a considerable drug release rate is increased significantly when the ferrosponges to reach a consecutive MF on-off operation. This suggests that the ferrosponges gain sufficient elasticity with the incorporation of critical amount of the nano-magnets, which can be elucidated in the ferrosponge with 5G-5F composition. This also indicates a considerable improvement of the anti-fatigue property of the ferrosponges, compared to neat gelatin matrix, further encouraging the use of the ferrosponges for drug delivery application.
10 20 30 40 50 60 70
Magnetic Sensitive Behavior (%) 5G-5F
0
Fig. 4.16 Relative drug release rates of the ferrosponges under repeated on-off MF operations, showing a fast degradation in the release rate with less MNP concentration, but much improved release behavior with respect to cyclic operation when MNP is increased and seems to be optimized in 5G-5F composition, i.e., indicating an improved anti-fatigue property
120 180 240 300
off
Relative Release Rate (%/min)
Time (min)
5G-5F 5G-3F 5G-1F
on
Magnetic Field
Chapter 5
Non-Biodegradable Ferrogel (PVA):
Effect of particle size and switching duration time 5.1 Introduction
Stimuli-response polymers represent one class of actuators that have the unique ability to change swelling behaviors, permeability and elasticity in a reversible manner.
Owing to these useful properties, stimuli-response polymers have numerous applications, particularly in medicine, pharmaceutics, drug-delivery, biosensors, enzyme and cell immobilization [Qiu, 2001; Miyata, 2002]. More recently, increasing interest has been devoted to the exploration of dual-responsive polymers, such as pH/thermo [Kim, 2002], thermal/magnetic [Furukawa, 2003; Deng, 2003; Pich, 2004], pH/ electric field [Fernandes, 2003], pH/magnetic [Chatterjee, 1999] sensitive hydrogels, which exhibit considerable sensitivity to external stimuli and can be used in extended fields.
Many kinds of such gels have been developed and studied with regard to their applications to several biomedical and industrial fields such as controlled drug delivery systems and muscle-like soft linear actuators. Saslawski et al. reported the gelatin microsphere that was cross-linked by polyethylenimine for the pulsed delivery of insulin by oscillating magnetic field [Saslawski, 1988]. The release rate of insulin from the alginate sphere with strontium ferrite microparticles (1 µm) dispersed can be much enhanced compared with that in the absence of magnetic field. Zrínyi et al.
reported that the magnetically-sensitive hydrogels can undergo quick, controllable changes in shape by introducing magnetic particles into the chemically cross-linked PVA that can be used as a new type of actuator to mimic muscular contraction [Mitsumata, 1999; Zrínyi, 1998 & 2000]. Furthermore, the magnetic-sensitive gels, or
"ferrogels", are typical representative of smart materials for mechanical actuators and have been the subjects of many studies in recent years [Hernández, 2004; Chatterlee, 2003].
Recently, it was reported that the polyelectrolyte microcapsules embedded with Co/Au nanoparticles could increase its permeability to macromolecules like
FITC-labeled dextran by alternating current (AC) magnetic switch [Lu, 2005]. However, the iron oxide nanoparticles have received wider attentions in diagnostic clinical practice as magnetic resonance imaging enhancers and currently in clinical phase IV, are the most successful application of nanotechnologies in medicine [Weissleder, 1995; Brigger, 2002]. So far, to our best knowledge, little investigation has been addressed on controlled delivery of therapeutic drugs under direct current (DC) magnetic field through the controlled deformation of the ferrogel based on iron oxide nanoparticles upon a simple “on” and “off” switch mode.
Furthermore, this magnetic-sensitive polymer is even superior to that traditional stimuli response polymer, such as pH or thermal sensitive polymer, because magnetic stimulation is an action-at-distance force (non-contact force) which is easier to adapting to biomedical devices. PVA hydrogel was used because it displays amphoteric characteristics and can be applied in aqueous environment as well as in organic solvent for the encapsulation of amphoteric drugs [Hatakeyema, 2005].
Moreover, PVA can be used as dispersing agents to uniformly disperse the Fe3O4
particles. In this study, we reported a magnetic-field-sensitive PVA-based ferrogel that can be used for controlled release of therapeutic drug by external magnetic stimulation. The responsivity and characteristics of the PVA-based ferrogel are systematically investigated in terms of iron oxide particles and swelling behaviors.
Furthermore, a mechanism of drug release via the on-off operation is also proposed.
5.2 Ferrogel preparation
The intermolecular interactions like hydrogen bond-bridges or polymer microcrystals are responsible for the formation of the three-dimensional network structure. A so-called freezing-thawing technique was used to prepare the ferrogel [Hatakeyema, 2005]. First, 5wt% polyvinyl alcohol (PVA, Fluka, M.W.: 72,000, degree of hydrolysation: 97.5-99.5 mol%) was dissolved in 10 ml dimethylsulfoxide (DMSO) at 80oC under stirring for 6 h, and then mixed with 17 wt% of magnetic particles at 60oC under ultrasonication for 6 h to ensure that the magnetic particles can be well dispersed. Three kinds of magnetic particles were used in this study: (1) larger magnetic particle (LM), diameter ca. 150-500 nm, Aldrich; (2) middle magnetic particle
(MM), diameter ca. 40-60 nm, Alfa Aesar; (3) smaller magnetic particle (SM), diameter ca. 5-10 nm, fabricated by in-situ co-precipitation process [Mak, 2005]. The resulting solution was then poured into plastic dish and kept frozen at -20oC for 16 h.
Subsequently, the gels were thawed at 25 oC for 5 h. This cyclic process including freezing and thawing was repeated for 5 times. Finally, prior to the release test, the ferrogels were washed five times and then immersed in the water for 24hr to completely remove DMSO. The physical gels prepared by this method were stored at 4oC until they were measured. The swelling ratio of the ferrogel [Yang, 2003] is defined as:
Swelling Ratio (SR) S% = Wt – Wdry
Wdry (5.1) where Wdry and Wt are the weight of the dry ferrogel and the ferrogel at time t under magnetic-field (MF, 400 Oe) switching, respectively. The free liquid on the surface of the swollen ferrogel was padded dry with filter papers before weighing.
5.3 Phisycal crosslinking (freezing and thawing process) of ferrogel The PVA ferrogel was fabricated by phisycal crosslinking method (freezing and thawing process) due to the hydroxyl groups of PVA molecules participated in hydrogen bonding. As show in Fig. 2.10, the number of crosslinking points and the cell walls of PVA hydrogels increases and become thicker with increasing freezing and thawing cycles. An in-situ experiement observed by DSC was used to evaluate the PVA crosslinking condition with freezing and thawing cycles. DSC measurement is based on the fabrication process of ferrogel. The control parameter in the DSC is (1) cooling from 25oC to -20oC (cooling rate: 1oC/min from); (2) kept frozen at -20oC for 16 hr; (3) heating -20oC to 25oC (heating rate: 1oC/min); (4) kept thawed at 25 oC for 5 h.
This cyclic process including freezing and thawing was repeated for 6 times. The result in Fig.5.1(a) & (b) show that crystal point (Tc) of PVA increased with the cycles of freezing and thawing increased (-5.3~1.1oC, cycle 1~6, c1~c6), indicated that the the number of crosslinking points increased to induce a solid network. Furthermore, the area of crystallization (peak area) increased with the cycles increase. It also demonstrated that the higher crystallization of PVA and the strongrt network would be found in the higher cycles of freezing and thawing process, but it seems to be
“saturation” when the cycles arrives 5-6 times, implying just slightly Tc and area of crystallization increase (PVA crystallization and .crosslinking have been stable) Therefore, five times of the freezing and thawing process would be used in this study.
Fig. 5.1 DSC analysis of PVA physical crosllinking by freezing and thawing cycles: (a) cooling curve; (b) heat flow and temperature change with different cycles
5.4 Characterization of magnetic-sensitive ferrogels
Fig. 5.2-(a) illustrates the photographs of magnetic-sensitive PVA-based (PVA5-LM17) ferrogels, where PVA5-LM17 represents the synthesis of the ferrogels with PVA concentration of 5 wt% and larger-sized magnetic particles (LM) of 17 wt% in this work. Moreover, it is observed that the magnetic-sensitive hydrogel exhibits excellent flexibility and elasticity. Furthermore, it was observed that the Fe3O4
nanoparticles were uniformly distributed in the PVA ferrogels as shown in cross-sectional SEM image as no magnetic field was applied. However, as the magnetic field (MF) was developed, a volume change in response to the on-off magnetization was observed for the PVA ferrogel. This phenomenon seems to reveal that the magnetic Fe3O4 nanoparticles are attracted between adjacent neighbors under magnetic field. Consequently, it implies that the reduced distance between the magnetic Fe3O4 nanoparticles as a result of attraction force induced by a given MF can be used to develop a close configuration of the ferrogel for controlled drug release.
The swelling ratio of the ferrogel is decreased (from 3.81 to 3.33) while MF is
-12 -9 -6 -3 0 3
c5c6 c4 c2 c3
Heat Flow Endo up (mW)
Temp. (oC)
switching “on” which is due to its contracting pores, but it is increased (from 3.33 to 3.72) while MF was turned off as shown in Fig 5.2-(b). Hence, the decreased swelling ratio of the ferrogel in an MF switching “on” mode may be used to explain the slow diffusion of drug. Furthermore, the calculated swelling rate is also shown in Fig 5.2-(b).
A transition of the swelling rate was clearly observed in response to the on-off magnetization. While MF switching from “off” to “on” mode, the swelling rate decreased, on the contrary, it increased. The magnetically sensitive swelling behaviors indicated that the ferrogel prepared in this investigation has an excellent magnetic-sensitive property.
Fig. 5.2 (a) Cross-sectional SEM image of magnetic particles disperse in PVA hydrogels and OM photos of PVA5-LM17 ferrogels; (b) Swelling ratio and swelling rate of PVA5-LM17 ferrogel in the magnetic fields switching “on”-“off” mode
Table 5.1 Permeability coefficient of the ferrogels in “on” or “off” mode of a given magnetic field
a SDT means switching duration time of the magnetic field.
b Average drug permeation amount (µg/min) and permeability coefficient (10-6 cm2/min) at magnetic fields (MF) switching "on" (n=3)
c Permeability coefficient (P) calculated by Eq.(2) (n=3)
d Average drug permeation amount (µg/min) and permeability coefficient (10-6 cm2/min)at MF switching
"off" (n=3)
e Average drug permeation amount (µg/min) and permeability coefficient (10-6 cm2/min) at MF OFF –cAverage drug permeation amount (µg/min) and permeability coefficient (10-6 cm2/min) at MF ON. (n=3)
f Average maximum drug bursting amounts (µg/min) of the drug bursting at the moment of switching MF from “on” to “off” mode. (n=3)
The magnetic-sensitive behaviors in the ferrogels can be further expressed by the difference in the permeated drug amount between the MF “off” mode and “on”
mode, as shown in Table 5.1. In addition, the calculated permeability coefficient and maximum amount of drug busting were also included. It was observed that the permeability coefficient of the pure PVA5 hydrogel was determined to be 105×10-6 cm2/min, which is lower than that of PVA5-LM17 ferrogel, i.e., 586×10-6 cm2/min while
Ferrogel SDT a
the MF is “off’. However, when MF switching to “on” mode, the permeability coefficient of the ferrogel decreased sharply (40×10-6 cm2/min) but that of the pure PVA hydrogels remains almost unchanged (107×10-6 cm2/min). This is essentially due to smaller pore size as a result of agglomeration of the nano-magnetic particles in the ferrogel. Furthermore, in Table 5.1, it is further indicated that the magnetic-sensitive behavior of the ferrogel (546×10-6 cm2/min) is much superior to that of the pure PVA gel (-2×10-6 cm2/min). Therefore, with an applied magnetic field, considerable differences in magnetic-sensitive permeability coefficient were detected in the ferrogels, as compared to that in pure PVA hydrogel.
5.5 Effects of switching duration time (SDT)
In Fig. 5.3-(a), it was found that the quantity and the release profile of the model drug from the ferrogels are strongly affected by the time duration between each on-to-off stage, and here we defined it as switching duration time (SDT) of the magnetic field. For a 5-min-period SDT, the drug release profiles demonstrate that the best “close” configuration, wherein the drug was effectively locked in the ferrogel and no sign of drug release can be detected for the SDT of 5-minute period.
However, with increasing SDT to 10 and 20 minutes, the “close” configuration of the ferrogel becomes less pronounced as time elapsed, wherein sign of drug release, can be clearly detected, as illustrated in Fig. 5.3-(a) and Table 5.1. In the case of 20-min SDT, an effective “close” configuration of the ferrogel that can effectively stop drug release can be kept up for 8-10 minutes; however, drug released, although in a relatively slow rate, from the ferrogel after the “effective” time period. Such an effective SDT can be repetitively observed without considerably changed for a number of repetitive on-off operations. Furthermore, irrespective of the SDT, a normal diffusion release profile can be detected right after the given MF switching from “on” to “off”.
Since the “close“ configuration of the ferrogel is an indication of particle agglomeration of the magnetic particles within the PVA matrix, on this base, a considerable reduction of the pore size and an increased tortuosity of the pore channels across the ferrogel membrane can be expected. Both factors will effectively hinder or block the diffusion of the drug solution from the other side (i.e.,
drug-containing donor side) of the ferrogel. The effective SDT of the ferrogels prepared in this study suggests an existing of an effective “closure” configuration of the ferrogel which seems to compromise with the diffusion potential of the drug solution from one compartment to the other. It is believed that such an effective “close”
configuration may be explained as a result of “fatigue” of the agglomerated magnetic particles under a given MF. The fatigue behavior can be possibly due to the relaxation of the polymer gel to relieve the stress that is induced by strain in the gel network when the magnetic particles move in response to the magnetic field. This process may be faster in the presence of smaller particles, which provides explanation on the rapid increase of permeability compared to the ferrogels with larger particle.
0 20 40 60 80 100 120
Time (mins)
Drugs premeation amounts (mg) Drugs bursting amounts (mg/min)
PVA5-LM17-20min SDT PVA5-LM17-10min SDT PVA5-LM17-5 min SDT
MF ON
MF OFF
(a)
Fig. 5.3 Rapid permeation properties and “close” configuration of the ferrogels dependent on (a) different switching duration time (SDT) and (b) various particle size of Fe3O4 of the ferrogels in the continuous switching “on-off” mode for a given magnetic fields; the drug permeation amount on switching “on-off” mode, and corresponding differential curve are shown in each figure in order to show the maximum drug bursting
0 10 20 30 40
Time (mins)
Drugs premeation amounts (mg) Drugs bursting amounts (mg/min)
PVA5-LM17-5 min SDT PVA5-MM17-5 min SDT PVA5-SM17-5 min SDT
MF ON
MF OFF
(b)
5.6 Effects of Fe3O4 particle size
The influence of Fe3O4 particle size on the effective “close” configuration is illustrated in Fig. 5.3-(b) and Table 5.1, where larger particles (LM) show effectively longer SDT. The results show the average permeability coefficient (40×10-6 cm2/min) of PVA5-LM17 ferrogel at MF switching “on” in 5-min-SDT is much lower than that (77×10-6 and 74×10-6 cm2/min) of PVA5-MM17 and PVA5-SM17 ferrogels. Moreover, the magnetic-sensitive behavior and average maximum drug bursting amount of the PVA5-LM17 ferrogel is much better than those of the PVA5-MM17 and PVA5-SM17 ferrogels.
In another comparative investigation, the effect of particle size on the magnetization using a vibrating sample magnetometer (VSM, Toei VSM-5, USA) is demonstrated in Fig. 5.4. Ferrogels with larger Fe3O4 particle (PVA5-LM17) encapsulated ferrogel displays a hysteresis loop with a larger saturation magnetization (Ms) of 15.28 emu/g, compared to those ferrogels with middle and smaller Fe3O4 nanoparticles (12.39 emu/g for PVA5-MM17 and 10.84 emu/g for PVA5-SM17), indicating that a strong magnetic field can be induced in the ferrogel.
However, the PVA5-MM17 ferrogel presents broader hysteresis loop and larger coercive force (Hc) than that of PVA5-LM17 (and PVA5-SM17 ferrogels), indicating that it is more difficult to reorient and move the magnetic particles in the ferrogel under magnetic fields. It is known that the hysteresis loss area and Hc are strongly dependent on the particle size and domain characteristics of magnetic particles. The PVA5-MM17 shows larger Hc because its particle size (60 nm) is near the critical size of single domain which was estimated about 100 nm for Fe3O4 particle [Klabunde, 2001]. Therefore, on this basis, the single-domain MM particles exhibit a greater hysteresis loss area and Hc (353.53 Oe) than the multi-domain LM particles (Hc:
159.63 Oe) in a given magnetic field; hence, the magnetic-sensitive behavior and
“close” configuration of PVA5-LM17 ferrogel are superior to those of PVA5-MM17 ferrogel, as shown in Fig. 5.3-(b) and Table 5.1. On the other hand, although the SM particles display a super-paramagnetic behavior with the lowest hysteresis loss area and Hc (17.55 Oe), the Ms of PVA5-SM17 is lower than that of PVA5-LM17 and the fine nanoparticles tend to aggregate together; hence, the observed
-6000 -4000 -2000 0 2000 4000 6000 -15
-10 -5 0 5 10
15 Ms
Hc PVA5-LM17
PVA5-MM17
PVA5-SM17
Magnetization (emu/g)
Fields (Oe)
magnetic-sensitive behavior and “close” configuration of PVA5-SM17 ferrogel are still less pronounced than that in the PVA5-LM17 ferrogel. The “magnetic-sensitive effects” in those ferrogels are in the order of PVA5-LM17> PVA5-SM17> PVA5-MM17 that is dependent on higher Ms and lower Hc, as illustrated in Fig. 5.3-(b) and Table 5.1. If the above argument is true, then, we believe that an “effective SDT” of the ferrogels from short duration period to long duration period can be well-designed with different drug release profiles.
Fig. 5.4 Hysteresis loop analysis of the ferrogels incorporated with various
Fig. 5.4 Hysteresis loop analysis of the ferrogels incorporated with various