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Technical Briefs

Flexure-Based Device for Cyclic Strain-Mediated Osteogenic Differentiation

[+] Author and Article Information
Kyung Shin Kang, Woon-Jae Yong, Dong-Woo Cho

Department of Mechanical Engineering,
POSTECH, Pohang 790-751, South Korea

Young Hun Jeong

Department of Mechanical Engineering,
Korea Polytechnic University,
Siheung 429-793, South Korea

Jung Min Hong

Department of Mechanical Engineering,
POSTECH, Pohang 790-751, South Korea

Jong-Won Rhie

Department of Plastic Surgery,
College of Medicine,
The Catholic University of Korea,
Seoul 137-701, South Korea

1These authors contributed equally to this work.

2Correspondence to: Dong-Woo Cho, Ph.D., Department of Mechanical Engineering, POSTECH, Center for Rapid Prototyping Based 3D Tissue/Organ Printing, POSTECH, San 31, Hyoja-dong, Nam-gu, Pohang, 790-751, Republic of Korea, e-mail: dwcho@postech.ac.kr

Contributed by the Bioengineering Division of ASME for publication in the Journal of Biomechanical Engineering. Manuscript received November 15, 2012; final manuscript received July 5, 2013; accepted manuscript posted July 29, 2013; published online September 23, 2013. Assoc. Editor: James C. Iatridis.

J Biomech Eng 135(11), 114501 (Sep 23, 2013) (8 pages) Paper No: BIO-12-1559; doi: 10.1115/1.4025103 History: Received November 15, 2012; Revised July 05, 2013; Accepted July 29, 2013

Application of low-magnitude strains to cells on small-thickness scaffolds, such as those for rodent calvarial defect models, is problematic, because general translation systems have limitations in terms of generating low-magnitude smooth signals. To overcome this limitation, we developed a cyclic strain generator using a customized, flexure-based, translational nanoactuator that enabled generation of low-magnitude smooth strains at the subnano- to micrometer scale to cells on small-thickness scaffolds. The cyclic strain generator we developed showed predictable operational characteristics by generating a sinusoidal signal of a few micrometers (4.5 μm) without any distortion. Three-dimensional scaffolds fitting the critical-size rat calvarial defect model were fabricated using poly(caprolactone), poly(lactic-co-glycolic acid), and tricalcium phosphate. Stimulation of human adipose–derived stem cells (ASCs) on these fabricated scaffolds using the cyclic strain generator we developed resulted in upregulated osteogenic marker expression compared to the nonstimulated group. These preliminary in vitro results suggest that the cyclic strain generator successfully provided mechanical stimulation to cells on small-thickness scaffolds, which influenced the osteogenic differentiation of ASCs.

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Figures

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Fig. 1

Kinematics of the nanoactuator based on the four-bar linkage and its right circular flexure hinge joint. The elongation of the piezoelectric actuator was amplified kinematically by the translation of coupler link BC. Design parameters: a = 70; b = 10; c = 10; h = 3; L = 20; R = 1; and t = 1 mm. The designed amplification factor was 7.0.

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Fig. 2

Flexure-based translational nanoactuator. (a) 3D geometry of the nanoactuator. The upper arrow indicates the direction of movement of the nanoactuator. The base is as in Fig. 1 and was connected to the cyclic strain generator. (b) The deformation and stress distribution were estimated at a driving force of 200 N, which is indicated by F in Fig. 1 (red arrows: driving force, green arrows: fixed points; color figure is shown in the online version). The deformation region was magnified by a factor of 10.

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Fig. 3

FEM calculation result of stress and displacement of the designed nanoactuator. (a) The maximum bending stress of the nanoactuator is described with respect to driving force. (b) The maximum elongation of the selected piezoelectric actuator in the designed nanoactuator structure.

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Fig. 4

The cyclic strain generator. (a) Two nanoactuators were combined in a single device. The elongations generated by the piezoelectric actuator were amplified by the nanoactuator. These amplified displacements were transferred through the push rods to the cells on the 3D scaffolds, which were in a 12-well plate positioned below the push rods. (b) Adjustable push rods are illustrated schematically in the circle at the lower right. (c) The concept of the cyclic strain generator was described. 1: nanoactuator, 2: piezoelectric actuator, 3: capacitive displacement sensor, 4: 12-well plate, 5: push rods, 6: scaffold.

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Fig. 5

Performance of the cyclic strain generator for sinusoidal signals (1 Hz). (a) Linearity of the performance of the cyclic strain generator. The displacement values (dots) are plotted according to the input DC voltage (R2 = 0.987) with their trend line. (b) Displacement was measured at a sampling frequency of 250 Hz when the input signal was sinusoidal (peak-to-peak displacement: 4.5 μm). (c) Peak-to-peak values (dots) of the sinusoidal displacement increased according to the input voltage (R2 = 0.985) with their trend line.

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Fig. 6

Morphological analysis of the 3D PCL/PLGA/TCP scaffolds. (a) The fabricated 3D PCL/PLGA/TCP scaffold was of a disk-type shape and of a size similar to that of the rat calvarial critical defect. Scale bar = 2 mm. (b) SEM image showing the rough surface of the 3D PCL/PLGA/TCP scaffold due to the embedded TCP. Scale bar = 400 μm. Original magnification, 40X. (c and d) Fluorescence microscopy image of actin-stained ASCs attached to the 3D PCL/PLGA/TCP scaffold day 1 after seeding. Original magnifications, 40X ((c): scale bar = 400 μm) and 100X ((d): scale bar = 250 μm).

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Fig. 7

Immunostaining of RUNX2 in ASCs on the PCL/PLGA/TCP scaffolds induced by low-magnitude strain in the cyclic strain generator. Frequency and magnitude were 1 Hz and 0.3%, respectively. Low-magnitude cyclic strain increased RUNX2 expression by ASCs on the 3D scaffolds. Original magnifications, 200X (scale bar = 250 μm) [(a–c), (e–g)] and 800X (scale bar = 500 μm) [(d), (h)].

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Fig. 8

Upregulation of the expression of osteogenesis-associated genes by ASCs on the PCL/PLGA/TCP scaffolds by the cyclic strain generator. The expression level of each group is shown as a fold change after normalization to that of the control group. The expression levels of all three genes were more than twofold higher than in the control group.

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