MINIATURE OSMOTIC ACTUATORS FOR CONTROLLED MAXILLOFACIAL DISTRACTION OSTEOGENESIS

Y.H. Li, C.C. Wang, C.W. Chang, and Y.C. Su* Department of Engineering and System Science

National Tsing Hua University, TAIWAN ABSTRACT

We have demonstrated miniature actuators that can convert chemical potential directly into steady mechanical move-ments for distraction osteogenesis. Pistons and diaphragms powered by osmosis are employed to provide the desired displacements for bone distraction and the release of morphogenetic proteins, respectively. In the prototype demonstra-tion, vapor-permeable thermoplastic polyurethane is employed as the semi-permeable material. The maximum distrac-tion force from the piston-type actuator is found to be 6 N, while the piston travels at a constant rate of 32 μm/hr for about one week. Meanwhile, the release rate from the diaphragm-type actuator is 0.15 μl/hr throughout the experiment. KEYWORDS: Osmotic Pump, Distraction Osteogenesis, Drug Delivery System, Miniature Actuator

INTRODUCTION

Osmosis is a passive transport mechanism widely found in a variety of biological systems [1] and drug delivery ap-plications [2]. When applied to microsystems, osmotic actuation is attractive because it is constant-rate and requires no electricity for operation [3]. Distraction osteogenesis is a surgical process that fractures a bone into two segments, gradually moves them apart, and facilitates bone growth in the gap to reconstruct skeletal deformities [4]. The rate and rhythm of distraction were found to have significant influences on the osteogenic activity within the distraction gap. A distraction rate of about 1 mm/day is ideal, while a rate below 0.5 mm/day might lead to premature union and a rate above 1.5 mm/day might result in delayed or non-union. Furthermore, a greater distraction frequency is expected to produce an improved outcome. Meanwhile, the force required to move bone segments apart is estimated to be around the order of 1 N. Traditionally, the distraction is facilitated by an internally mounted mechanical device which requires repeated adjustment throughout the process. Even though the slow distraction is relatively painless, the need for fre-quently manual adjustment could be tedious and distressing. Meanwhile, an electrically-powered auto-distractor is usu-ally bulky and heavy, which is very likely to discomfit the patient and affect the compliance. To address the need for autonomous and controlled distraction osteogenesis, this paper presents micro-actuators that can convert chemical poten-tial directly into steady mechanical movements. As such, the demonstrated miniature osmotic actuators could potentially serve as versatile apparatuses for distraction osteogenesis, and fulfill the needs of a variety of implantable applications.

Fig1: Schematic illustration of the miniature actuators Fig2: Cross-sectional illustration of the actuators

OPERATING PRINCIPLE

Figure 1 illustrates the working scheme of the osmotic actuators for maxillofacial distraction osteogenesis. There are two cylindrical-shaped actuators in an integrated system. The piston-type actuator is employed to produce the desired constant-rate distraction, while the diaphragm-type one is used to release bone morphogenetic proteins locally to facili-tate osteogenesis. Once the osmotic actuators are implanted, they acquire water from the surrounding environment to power the desired bone distraction and protein release. The details of the proposed miniature osmotic actuators are illus-trated in Figure 2. By filling up the closed chamber with osmotic agent, the solution inside the chamber remains satu-rated throughout the actuation period. Meanwhile, water molecules diffuse across the breathable/semi-permeable plug, which is mounted on one end of the actuator, and raise the pressure inside the chamber. Since the osmotic pressure in the surrounding environment is most likely to be constant and negligible, the driving osmotic-pressure gradient and therefore the resulting actuator outputs (i.e., bone distraction and protein release) are expected to be constant throughout the process.

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An elastomeric piston-ring (as shown in Figure 2) is placed between the relatively rigid piston and casing to achieve the desired sealing. The sealing relies mainly on the elasticity and incompressibility of the piston-ring, and the existence of an initial interference or preload. To secure the sealing between the piston-ring and casing, the contact pressure be-tween them needs to be always higher than the fluid pressure. With an initial squeeze and exposed to fluid pressure, the flexible and incompressible (i.e., with a Poisson’s ratio close to 0.5) piston-ring behaves like a liquid of very high sur-face tension, maintaining preload and transferring fluid pressure to the sealing contact. To investigate the contact pres-sure quantitatively, simulation employing ANSYS (ANSYS, Inc.) is performed. Figure 3 shows the simulation results of contact pressure resulted from varying initial interferences (I) and fluid pressure (P). The outer radius of the flexible pis-ton-ring is chosen to be 10% larger than the inner radius of the rigid casing (i.e., an interference of 100 μm), which re-sults in an initial contact pressure up to 0.28 MPa. Under this condition, the piston assembly can fit tightly while slide smoothly (with corn oil functioning as a lubricant) along the casing. Meanwhile, friction and assembly difficulty can be significantly reduced by replacing the piston with a diaphragm. The induced water flow penetrates the breathable/semi-permeable plug, and steadily squeezes and peels the diaphragm from the casing. In addition to a tiny release hole located on the release end of the diaphragm-type actuator, an elongated release path is created between the outer surface of the casing and the inner surface of an internally threaded cap to minimize potential diffusive flows.

Fig3: Simulated contact pressure distribution Fig4: Photograph of a fabricated actuator assembly

RESULTS AND DISCUSSION Figure 4 shows the photograph of a fabricated, integrated actuator assembly for distraction osteogenesis. The breath-

able/semi-permeable material employed is a certain type of vapor-permeable, thermoplastic polyurethane (DINTEX FT-1880, Ding Zing Chemical Products). The material is received in a form of 15-μm-thick sheets, and then shaped into cy-lindrical plugs with desired thicknesses by a thermal forming process, which heats the sheets up to around 220 °C with the application of a pressure up to 5 MPa. Figure 5 illustrates the relationship between the measured driving, output, and friction forces. Ideally, the driving force will be the sum of the output and friction forces. Under a driving pressure ranging from 1 to 6 kg/cm2, it is found that the output force varies from 0 to 1.8 N, while the friction force remains around 0.4 N. The contact between piston assembly and Teflon casing was lubricated by corn oil, which is believed to contribute to the dramatic reduction in the friction force. Meanwhile, the sealing between the two components was veri-fied by the fact that there is no bubble observed when the pressurized-air experiment was operated under water.

Driving forcey = 0.308x

Output forcey = 0.299x - 0.3721

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Fig5: Relationship between measured forces Fig6: Force output and piston velocity of piston-type actuators In the prototype demonstration, piston-type actuators with an inner radius of 1 mm, a breathable/semi-permeable

plug thickness of 2 mm, and a Teflon casing length of 3 cm were fabricated and characterized. The trials lasted for more than 6 days, and the measured displacements and distraction forces are shown in Figure 6, in which the data collected from 5 tested actuators are presented. At the beginning of the trails, there is a delay of roughly 2 hours (not shown in the figure), during which water is expected to fill the voids inside the actuators first and then build up a pressure gradient eventually overcoming the friction. Once starting to move, the linear velocity of the piston is found to be constant at 32 μm/hr (or 0.77 mm/day) during this one-week period. Meanwhile, the distraction force increases as the linkage com-

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pressing the spring-type force gauge further. The maximum distraction force is found to be around 6 N, which was re-corded at the end of the trails. Based on this result, it is concluded that as long as the resistant force being less than 6 N, the osmotic actuator should be able to maintain a constant-rate distraction even if the resistance is varying. In addition to the piston-type ones, diaphragm-type actuators with an inner radius of 500 μm, a breathable/semi-permeable plug thick-ness of 0.8 mm, and a Teflon-casing length of 3 cm were also fabricated and characterized. A delay of roughly 1.2 hour, which is probably due to the void inside the actuators, was observed at the beginning of the trails. After that, the volu-metric displacement of the diaphragm is found to be linearly proportional to time, with a measured constant volumetric release-rate of 0.15 μl/hr (or 3.6 μl/day). The trails lasted for more than 100 hours, until dye solution were mostly drained from the reservoirs. The measured volumetric displacement outputs, which are the averaged values of 3 tested actuators and recorded every 10 hours, are shown in Figure 7. In addition, the outputs of actuators with a breath-able/semi-permeable plug thickness of 1.2 mm are also illustrated in the figure. It is found that the release rate is in-versely proportional to the thickness. Figure 8 illustrates a captured squeezing and peeling sequence of a dye-loaded diaphragm, which is shaped by surface tension effects to lower the required start-up pressure. Meanwhile, a sequence of dye flowing through an elongated release path is shown in Figure 9. The effective length of the release path is estimated to be 2.2 cm and its cross section is observed to be an isosceles triangle with its longest side equal to 0.33 mm and a height of 0.084 mm. The ratio of diffusive flow rate to convective flow rate is estimated to be 0.0033, which indicates a good consistency by the osmosis effect. Meanwhile, the pressure drop throughout the release path is estimated to be 117 Pa, which is negligible compared to the driving pressure provided by the osmosis effect. In addition to the distraction and release characterization, currently the release of bone morphogenetic proteins and animal trails of the proposed os-motic miniature actuators are performed to investigate in detail their application in maxillofacial distraction osteogenesis.

Fig8: Squeezing and peeling sequence of a diaphragm-type actuator

Fig7: Displacement of diaphragm actuators Fig9: Sequence of dye flowing through an elongated release path

CONCLUSION This paper presents miniature actuators that are capable of converting chemical potential directly into steady me-

chanical movements for maxillofacial distraction osteogenesis. Pistons and diaphragms powered by osmosis are em-ployed to provide the desired linear and volumetric displacements for bone distraction and potentially the release of bone morphogenetic proteins, respectively. The cylindrical-shaped miniature actuators are composed of polymeric materials and fabricated by molding and assembly processes. In the prototype demonstration, vapor-permeable, thermoplastic polyurethane was employed as the semi-permeable material. 3-cm-long actuators with piston and diaphragm radii of 1 mm and 500 μm, respectively, were fabricated and characterized. The maximum distraction force from the piston-type actuator is found to be 6 N, while the piston travels at a constant velocity of 0.77 mm/day for about one week. Mean-while, the release rate from the diaphragm-type actuator is measured to be constant, 3.6 μl/day, throughout the experi-ment. The sizes and output characteristics of the self-regulating actuators could be readily tailored to realize optimal dis-traction rate, rhythm, and osteogenic activity. As such, the demonstrated miniature osmotic actuators could potentially serve as versatile apparatuses for distraction osteogenesis, and fulfill the needs of a variety of implantable applications. ACKNOWLEDGEMENTS

This work was supported in part by the National Science Council of Taiwan under Contract No. NSC 96-2221-E-007-116-MY3. The demonstrated systems were fabricated in the ESS Microfabrication Laboratory at National Tsing Hua University, Taiwan. REFERENCES [1] Vogel S 1998 Cats’ Paws and Catapults (New York: Norton) [2] Theeuwes F 1975 Elementary osmotic pump J PHARM SCI 64 1987 [3] Su Y, Lin L, and Pisano A 2002 A water-powered osmotic microactuator J MICROELECTROMECH S 11 736 [4] Davies J, Turner S, and Sandy J 1998 Distraction osteogenesis - a review BRIT DENT J 185 462 CONTACT *Y.C. Su, tel: +886-3-5742374; ycsu@ess.nthu.edu.tw

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