Hollow microspheres fabricated from instant adhesive

Materials Letters 65 (2011) 3415–3417 Contents lists available at ScienceDirect Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t Hollow microspheres fabricated from instant adhesive Toshinori Makuta ⁎, Yukina Tamakawa, Jun Endo Graduate School of Science and Engineering, Yamagata University, 4-3-16 Johnan, Yonezawa, Yamagata 992-8510, Japan a r t i c l e i n f o Article history: Received 27 May 2011 Accepted 29 July 2011 Available online 4 August 2011 Keywords: Hollow microsphere Microbubble Biodegradable polymer Ultrasound contrast agent Instant adhesive Cyanoacrylate a b s t r a c t Non-invasive surgery techniques and drug delivery system with acoustic characteristics of ultrasound contrast agent have been studied intensively in recent years. Many ultrasound contrast agents are commercially available, and they are usually composed of a microbubble coated by a surfactant or lipid bilayer, i.e., a hollow microsphere. We show that the hollow microsphere with polymer shell can be fabricated just blowing vapor of commonly-used instant adhesive into water as microbubbles. The hollow microspheres are composed of cyanoacrylate, a biocompatible material used for adhesion to human tissue such as skin, blood vessels, and organs. Moreover these are less than 10 μm in diameter, which is smaller than a blood capillary (approximately 10 μm). Therefore, with their stable polymer shells, the hollow microspheres are possible ultrasound contrast agents, and are expected to be generated at low cost on the medical frontline, and not in a pharmaceutical factory. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Ultrasonic diagnostics is widely used in medical settings because it is safe, low-cost, and enables real-time diagnosis. Non-invasive surgery techniques and drug delivery systems with the acoustic characteristics of ultrasound contrast agents (USCAs) have been studied intensively in recent years [1]. Many USCAs are commercially available [2], and they are usually composed of a microbubble coated by a surfactant or lipid bilayer, i.e., a hollow microsphere. The hollow microsphere is also used for various applications including the reduction of the material weight, the encapsulation and immobilization of bioactive and catalytically active substances, the modification of the impact strength of compounds, a better thermal and acoustical insulation [3]. In case of USCA, an aqueous solution containing hollow microspheres for USCA is prepared by shaking a mixture of low solubility gas, saline, and a bubble stabilizer such as a surfactant and lipid. The USCA collapses easily under blood circulation and ultrasound irradiation because they are merely stabilized bubbles without a solid shell created by surface adsorption of a surfactant or lipid [4]. Thus, hollow microspheres would find widespread applications if the solid shell of biodegradable polymers can be fabricated easily. A biodegradable polymer is a material that can be disintegrated by microbes or bacterium. Examples of such polymers include polycyanoacrylate, poly-lactic acid, poly-glycolic acid, and poly ecaprolactone [5]. Makuta et al. reported that the bubble template method fabricated hollow microspheres less than 3 μm covered with poly-lactic acid [6]. The PLA microsphere is hollow structure with a ⁎ Corresponding author. Tel./fax: + 81 238 26 3258. E-mail address: makuta@yz.yamagata-u.ac.jp (T. Makuta). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.07.115 single internal void and uniform in size, however, it takes more than 4 h for fabricating the microspheres. Here, we demonstrate that a hollow microsphere with a polymer shell can be fabricated by simply pumping the vapor of a commonly used instant adhesive into water to form microbubbles. The main ingredient of instant adhesive is generally a cyanoacrylate monomer, which is polymerized within seconds in the presence of water. Therefore, the cyanoacrylate vapors inside a microbubble initiate polymerization on the gas–liquid interface soon after microbubbles are generated in water. Consequently, thin film cyanoacrylate-coated hollow microspheres (CAHMs) are generated. CAHM is smaller than blood capillary, composes of biocompatible polymer, and has the imaging ability of harmonic ultrasound imaging. Therefore CAHM fabricated from instant adhesives has potential for not only USCA but also the carrier of drug delivery system by adhering the drug onto the hollow microsphere. 2. Material and methods CAHMs are fabricated by the following four steps. (1) 2 g of instant adhesive (Aron Alpha 201, Toagosei Co., Ltd., Japan) containing cyanoacrylate more than 95% is put into 34 mL glass bottle with air intake and connecting port, and it is vaporized by oil bath (EO-200, AS ONE Co., Ltd., Japan) of 180 °C. (2) Vaporized cyanoacrylate is supplied into microbubble generator at 750 mL/min by tubing pump (Masterflex 7554-80, Cole-Parmer Instrument Co. Ltd., USA). (3) An ultrasonic atomizer with hollow cylindrical step horn (UH-50F, SMT Co., Ltd., Japan) is used as microbubble generator, and 2.6 mm ejection orifice of the horn oscillates at 20 kHz with 40 μm peak to peak amplitude for generating microbubbles in distilled water (Direct-Q 3 UV, Millipore Co. Ltd., USA) including 0.02% deoxycholic acid for 3416 T. Makuta et al. / Materials Letters 65 (2011) 3415–3417 bubble stabilization. The vaporized cyanoacrylate broke up into microbubbles in water maintained at 12 °C by cool stirrer (CSB900N, AS ONE Co., Ltd., Japan) by means of microbubble generator as shown in Fig. 1. (4) Cyanoacrylate vapor in microbubbles was rapidly cooled and condensed on the interface and polymerized there in the presence of water. During 10 min bubbling into chilled water, most of microbubbles are covered by the film of cyanoacrylate. As for step (3), pumping instant adhesive vapor into water to form microbubbles is simple but extremely difficult. As the vapor forms a film once it comes into contact with water, gas released from a small opening [7] quickly halts microbubble generation by clogging, and the 2 fluid nozzles used to produce a swirl or jet flow [8] consumes the vapor on the undisturbed gas–liquid interface before a swirl or jet flow can be produced. Therefore, we used a large opening with rapid and high amplitude ultrasound oscillation that generates microbubbles of less than 10 μm diameter from instant adhesive vapor within milliseconds. With this method, capillary wave is formed on the gas– liquid interface formed on the opening under the weak ultrasonic irradiation and the head of capillary wave is detached and released as bubble by the fragmentation of the interface as the power of ultrasonic irradiation increased [9]. Furthermore, the collapse of generated bubbles oscillating harmonically with the ultrasonic irradiation generated many microbubbles under the 100 μm in diameter [10]. Fig. 2 shows the size distribution of air microbubbles without cyanoacrylate vapor obtained by laser size analyzer (Mastersizer 2000, Malvern Instruments Co. Ltd., UK) [11], and it indicates that most of microbubbles are less than 10 μm. 3. Results and discussion Fig. 2. Diameter distribution of air microbubbles generated from hollow ultrasonic horn. hundreds millisecond. Therefore, microbubble containing cyanoacrylate vapor can be released without the orifice clogging and the cyanoacrylate vapors initiate polymerization on the gas–liquid interface soon after microbubbles are generated in water. These results reveal that a cyanoacrylate polymer film instantaneously covers the microbubbles released into the water and hollow microspheres are fabricated. A few hollow microspheres of more than 10 μm are also fabricated as shown in Fig. 3A. However, they are relatively scarce and easily separable by floatation because they rise up and stay below water surface. Ultrafined nanoparticles as shown in Fig. 3B are also generated from cyanoacrylate contained in the uncapsulated bubbles less than 3 μm. However, they are thought to be Heat vaporizes the instant adhesive (Step (1)) and the vapor is introduced into water as microbubbles by the step horn (Step (2), (3)), then the water gradually becomes opaque (Step (4)). Fig. 3A shows the optical microscope image of the opaque water just below the water surface taken by optical microscope (VHX-900, Keyence Co. Ltd., Japan). Fig. 3A reveals that the water contains floated microparticles of less than 10 μm. Moreover, microparticles less than 3 μm follow Brownian motion and are homogeneously-dispersed in water. Fig. 3B shows scanning electron microscope images (JSM5410LV, JEOL Co. Ltd., Japan) of dispersed microparticles in water. Fig. 3B shows that microparticles are spherical and uniform in size (3 μm in diameter), and broken microparticle indicates that it has the hollow structure whose film thickness is less than 100 nm. One cycle of 20 kHz ultrasonic oscillation is 50 μs, and microbubble jet flow from the ultrasonic horn as show in Fig. 1 is fully developed within Cyanoacrylate Vapor Hollow Ultrasonic Horn Ultrasonic Oscillation Microbubble 1 mm WithoutUS oscillation 10 min HollowMicrosphere 1 mm With US oscillation Fig. 1. Illustration of fabricating hollow microspheres from microbubbles generated by hollow ultrasonic horn. Fig. 3. (A) Optical microscope image of water-dispersed hollow microspheres fabricated from instant adhesive. (B) Scanning electron microscope images of the hollow microspheres. (C) Harmonic echo image of a droplet containing the hollow microspheres falling into a water-filled beaker. T. Makuta et al. / Materials Letters 65 (2011) 3415–3417 decomposed faster than CAHMs by hydrolysis of cyanoacrylate polymer since mass and volume of the nanoparticles are quite smaller than those of CAHMs. The USCA uses the harmonic echo inherent in microbubbles to enhance ultrasound-imaging ability. Fig. 3 C shows the Harmonic ultrasound imaging (EUB-6500, Hitachi Medical Co., Ltd., Japan) of a droplet containing hollow microspheres in water (a bright area) with the harmonic mode of 2.5 MHz and it reveals that CAHMs also possess ultrasound-imaging ability. The essential requirements for practical USCAs, in addition to their acoustic property, are biocompatibility of the stabilizing material and a size that is less than that of a blood capillary to ensure smooth blood circulation. CAHMs are composed of cyanoacrylate, a biocompatible material used for adhesion to human tissue such as skin, blood vessels, and organs [12]. CAHMs are less than 10 μm in diameter, which is smaller than a blood capillary. Therefore, with their stable polymer shells, CAHMs are possible USCAs, and expected to be generated at low cost on the medical frontline, and not in a pharmaceutical factory. 4. Conclusions A hollow microsphere with a polymer shell was successfully fabricated by simply pumping the vapor of a commonly used instant adhesive into water with hollow ultrasonic horn. The main ingredient of instant adhesive is generally a cyanoacrylate monomer, which is polymerized within seconds in the presence of water. Microbubbles of cyanoacrylate vapors are generated within milliseconds. Therefore, the cyanoacrylate vapors inside a microbubble initiate polymerization on the gas–liquid interface soon after microbubbles are generated in water and thin film cyanoacrylate-coated hollow microspheres (CAHMs) are generated. CAHM is smaller than blood capillary, composes of biocompatible polymer, and has the imaging ability of harmonic ultrasound imaging. In addition, the potential advantage of CAHMs is, by just using instant adhesive, intensive drugs, including water-soluble or homogeneously dispersed water-insoluble drugs can adhere to the CAHM film upon its fabrication. When harmonic 3417 frequency ultrasound irradiation caused by the hollow structure is administered to the affected part of the body, CAHM-containing drugs are selectively oscillated, resulting in their activation by CAHM oscillation to produce heat or their release by collapse of the CAHM. If the drug delivery carrier that carries the necessary drug can be quickly and easily prepared with this method pro re nata on medical frontlines, further advancement of less-invasive and high-efficacy treatment would be expected. Acknowledgment This work was partially supported by the TEPCO Memorial Foundation. References [1] Lindner JR. 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