Polymer Replicas of Photonic Porous Silicon for Sensing and Drug Delivery Applications

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Science  28 Mar 2003:
Vol. 299, Issue 5615, pp. 2045-2047
DOI: 10.1126/science.1081298


Elaborate one-dimensional photonic crystals are constructed from a variety of organic and biopolymers, which can be dissolved or melted, by templating the solution-cast or injection-molded materials in porous silicon or porous silicon dioxide multilayer (rugate dielectric mirror) structures. After the removal of the template by chemical dissolution, the polymer castings replicate the photonic features and the nanostructure of the master. We demonstrate that these castings can be used as vapor sensors, as deformable and tunable optical filters, and as self-reporting, bioresorbable materials.

Synthesis of materials using nanostructured templates has emerged as a useful and versatile technique to generate ordered nanostructures (1). Templates consisting of microporous membranes (2, 3), zeolites (4), and crystalline colloidal arrays (5–7) have been used to construct elaborate electronic, mechanical, or optical structures. Porous Si is an attractive candidate for use as a template (8) because the porosity and average pore size can be tuned by adjusting the electrochemical preparation conditions that allow the construction of photonic crystals, dielectric mirrors, microcavities, and other optical structures (9). For many applications, porous Si is limited by its chemical and mechanical stability. The use of porous Si as a template eliminates these issues while providing the means for construction of complex optical structures from flexible materials that are compatible with biological systems or harsh environments.

Multilayered porous Si templates containing nanometer-scale pores are prepared (10) by an anodic electrochemical etch of crystalline silicon wafers with the use of a pseudosinusoidal current-time waveform, according to published procedures (9, 11–16). The thickness, pore size, and porosity of a given layer is controlled by the current density, duration of the etch cycle, and etchant solution composition (17). The multilayer templates possess a sinusoidally varying porosity gradient, providing sharp features in the optical reflectivity spectrum (Fig. 1) that approximate a rugate filter (18). The porous Si is converted to porous SiO2 by thermal oxidation, and the oxidized nanostructure (fig. S1) (10) is used as a template for solution-cast or injection-molded thermoplastic polymers.

Figure 1

Reflectivity spectra of an oxidized porous Si rugate film (top) and a polystyrene film cast from the porous Si template (bottom). The spectral peaks correspond to the second-order diffraction peak of the template and the second- and third-order diffraction peaks of the imprint. The porous Si template was etched using a sinusoidal current varying between 38.5 and 192.3 mA/cm2, with 70 repeats and a periodicity of 8 s. The total thickness of the porous Si film is 40 μm. The reflected light spectra were obtained using an Ocean Optics SD2000 charge-coupled device spectrometer using tungsten light illumination. Spectra are offset along the yaxis for clarity.

Removal of the porous SiO2 template from the polymer or biopolymer imprint by chemical dissolution provides a freestanding porous polymer film with the optical characteristics of the photonic crystal master (figs. S2 to S4). Reflection spectroscopy (Fig. 1) and scanning electron microscopy (SEM) (Fig. 2) confirm that the photonic structure of the porous Si master is retained in the polymer casting. The sharp optical reflectivity feature expected of a rugate filter is observed in both the template and the polymer casting (Fig. 1), confirming that the process replicates the microstructure. Cross-sectional SEM measurements (Fig. 2 and fig. S5) corroborate the optical data.

Figure 2

Cross-sectional scanning electron micrograph (secondary electron image) of the porous silicon template (top) and a polystyrene film cast from a similar template (bottom). Samples were prepared using identical procedures as those used for the samples represented in Fig. 1, except that the porous silicon film was etched using a sinusoidal current varying between 11.5 and 192.3 mA/cm2, with 70 repeats and a periodicity of 5 s. Scale bar, 1 μm.

Vapor dosing experiments confirm that the microporous nanostructure is retained in the castings. The position of the spectral feature for a rugate filter depends on the periodicity and refractive index gradient of the structure. When porous Si multilayers are exposed to condensable vapors such as ethanol or hexane, microcapillary condensation in the nanometer-scale pores produces an increase in the average refractive index of the matrix and a spectral red shift of the photonic feature (11, 19, 20). The shift of the spectral peak correlates with partial pressure of the analyte in the gas stream, following the Kelvin equation for condensible vapors (15, 19–21). Dose-response curves for ethanol vapor for the porous Si template and for the polystyrene casting both display a large relative response at partial pressures within a few percent of the saturation vapor pressure (Fig. 3), characteristic of capillary condensation in nanometer-scale pores (19, 20). Imprints prepared from melt-cast polyethylene display the optical spectrum characteristic of the rugate structure, but they show no spectral shift upon exposure to ethanol vapor. Presumably, the more viscous molten polyethylene does not impregnate the porous SiO2 sufficiently enough to replicate the nanometer-scale features that are required to generate microcapillary condensation effects, as occurs with the solution-cast material.

Figure 3

Dose-response curves for a porous Si rugate film template (solid circles, solid line) and its replicate polystyrene film (open circles, dashed line) upon exposure to ethanol vapor. The data are presented as the wavelength of the second-order peak maximum from the rugate structure as a function of P/Ps, where P is the partial pressure of ethanol and Ps is the saturation vapor pressure of ethanol (44 torr). The samples were prepared in a similar way to those whose spectra are presented in Fig. 1.

Biocompatible and bioresorbable polymers are of great interest for their use in prosthesis, medical suture, tissue engineering, and drug delivery systems. Biodegradable polyesters are the most widely studied and used polymers for application in the controlled release of drugs (22). In some cases, there is a desire to monitor the status of the biomaterial in vivo. Because the spectral reflectance peaks of the porous Si filters and their polymer castings can be tuned over a wide range (from at least 400 to 10,000 nm), the peaks can be placed at wavelengths corresponding to a region of relatively low absorption in human tissue. The spectrum of a porous Si photonic structure that exhibits two resonances, obtained through 1 mm of soft tissue of a human hand (Fig. 4), demonstrates that such measurements could be obtained in vivo. The measurement of the decay in intensity of the rugate peak could thus be used to monitor, for example, the release of drug from an implanted biocompatible polymer.

Figure 4

Absorption spectrum (transmission mode) of a porous Si rugate filter (bottom, rugate), 1 mm of human hand tissue (hand), and the porous Si rugate filter measured through 1 mm of human hand tissue (rugate + hand). The figure demonstrates the ability to monitor the spectral signature of the nanostructured materials in vivo. The spectra hand and rugate + hand are offset along they axis by +0.8 absorbance units for clarity.

To test the above hypothesis, we prepared a caffeine-impregnated poly(l-lactide) (PL) film, cast from a thermally oxidized porous silicon rugate template. Replication of the optical spectrum was observed in the biocompatible polymer upon removal of the porous silicon template. The photonic structure in the film degraded completely in about 5 days in a pH 10 aqueous buffer solution. The intensity of the rugate peak displays an approximately exponential decay over the first 3 days, reflecting the progressive hydrolysis of the biopolymer in the aqueous environment (fig. S6). Simultaneous measurement of the decay of the rugate peak and the appearance of caffeine in the solution (caffeine absorption occurs at 274 nm) confirms that the drug is released on a time scale comparable to polymer degradation (Fig. 5).

Figure 5

Intensity of reflected light from the polymer rugate structure [measured at 533 nm (open circles)] and UV absorbance of free caffeine in the solution [measured at 274 nm (solid circles)] as a function of time for a caffeine-impregnated PL casting immersed in aqueous solution (pH = 10).

Castings made from flexible polymers such as polydimethylsiloxane (solution cast) or polyethylene (melt cast) provide mechanically deformable filters (fig. S7). Deformation of the material by application of a moderate compressive stress produces a spectral blue shift (by a few nanometers) in the photonic feature, consistent with a decrease in the layer spacing of the rugate filter. The chemical and mechanical instability of porous Si has been identified as a considerable limitation for biological and environmental sensor applications. Because the castings possess the chemical and mechanical properties of the polymers used, the approach presented here provides a substantial improvement in the design of experiments and devices using nanostructured photonic materials but retains the simplicity of fabrication inherent in the electrochemical synthesis of porous Si.

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Materials and Methods

Figs. S1 to S7


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