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Research Papers

Effect of Laser-Induced Crystallinity Modification on Biodegradation Profile of Poly(L-Lactic Acid)

[+] Author and Article Information
Y. Lawrence Yao

Department of Mechanical Engineering,
Columbia University,
New York, NY 10027

Manuscript received September 3, 2012; final manuscript received July 2, 2013; published online November 5, 2013. Assoc. Editor: Yong Huang.

J. Manuf. Sci. Eng 136(1), 011005 (Nov 05, 2013) (9 pages) Paper No: MANU-12-1262; doi: 10.1115/1.4025394 History: Received September 03, 2012; Revised July 02, 2013

Poly(L-lactic acid) (PLLA) is of interest in drug delivery applications for its biodegradable and biocompatible properties. Polymer-controlled drug delivery relies on the release of embedded drug molecules from the polymer matrix during its degradation. PLLA degradation exhibits an induction period, during which an insignificant amount of degraded products and embedded drug can be released. Due to this induction period, drug release is initially nonlinear, a complication in drug delivery applications. PLLA degradation is a function of crystallinity, such that control over its crystallinity tailors drug release over time. In this study, the effect of laser-induced PLLA crystallinity reduction on degradation is investigated. Samples having lower surface crystallinity are shown to have higher rates of molecular weight reduction and earlier mass loss than nonlaser-treated samples, as observed from gel permeation chromatography and mass change. Wide-angle X-ray diffraction measurements show that crystallinity increases with degradation. A numerical model is implemented from hydrolysis and diffusion mechanisms to investigate the effect of laser irradiation on biodegradation. Controlled laser treatment of PLLA offers a method for constant drug release through the reduction of surface crystallinity.

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Figures

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Fig. 1

Schematic representation of (a) semicrystalline structure and (b) crystalline residue of polymer after hydrolysis, which preferentially occurs in amorphous region and on crystal fold surface, leading to mass loss [19]

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Fig. 2

Nondegraded PLLA sample (a) before and (b) after laser treatment with a fluence of 3 J/cm2. Laser-irradiated spots show less transparency due to increased surface roughness. The laser-treated sample degraded for 14 days is given in (c) and its high crystallinity reduces the transparency.

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Fig. 3

PLLA crystallinity and the ratio of O1s to C1s as a function of laser fluence. The error bar represents the standard deviation of 3 data points.

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Fig. 4

Surface morphology of the laser-treated sample (a) before and after degradation for (b) 3, (c) 8, and (d) 14 days under the stereomicroscope. The squares in (a) are laser spots. The edges of laser spots become less defined with degradation period, suggesting the erosion of laser melted layer. Degradation in the nonmelted bulk volume occurs in the later stage.

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Fig. 5

Cross section of the laser-treated sample degraded for 14 days. The bulk remains solid, suggesting that autocatalysis is not dominant.

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Fig. 6

WAXD profiles of the (a) nonlaser-treated and (b) laser-treated (F = 3 J/cm2) samples degraded for regular periods. Intensity of crystalline peaks increases with degradation period, suggesting a higher crystallinity. Profiles are shifted in y direction for viewing clarity.

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Fig. 7

Crystallinity as a function of degradation period determined from WAXD. Crystallinity increases with degradation period. A significant increase occurs on day 0.5 for both types of samples. Crystallinity also increases on day 8 and day 5 for the nonlaser-treated and laser-treated samples, respectively. The error bar represents the standard deviation of 3 data points.

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Fig. 10

Experimental results of sample mass with and without laser treatments. Mass decrease is observed after day 8 for nonlaser-treated sample and after day 3 for laser-treated sample. The error bar represents the standard deviation of 3 data points.

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Fig. 8

GPC profiles of the (a) nonlaser-treated and (b) laser-treated samples after regular degradation periods. For (a), the distribution becomes wider and shifts left as the degradation period increases to day 5, representing the random chain scission in the amorphous region. After day 8, a distinct new peak is developed due to selective chain scission of the fold surface of crystals. For (b), the MW distribution extends to the left before day 3, signifying the random chain scission of the laser melt layer. At day 5, two distinct peaks are developed due to the selective chain scission of the partially melted crystal fold surfaces. Profiles are shifted in y direction for viewing clarity.

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Fig. 9

Mw, Mn, and PDI of the (a) nonlaser-treated and (b) laser-treated samples after regular degradation periods. Mw and Mn decrease at a higher rate for the laser treated samples, as a result of fast degradation in the laser-melted layer. The nonhomogeneous degradation of the melted layer and bulk increases PDI. The error bar represents the standard deviation of 3 data points.

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Fig. 11

Simulated spatial distribution of monomer concentration in the (a) nonlaser-treated sample and (b) laser-treated sample degraded for 0.5 days

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Fig. 12

Simulated MW of the nonlaser-treated and laser-treated samples

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Fig. 13

Simulated mass change of the nonlaser-treated and laser-treated samples

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Fig. 14

Molar concentration of the simulated species as a function of degradation period

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