Journal of Colloid and Interface Science 332 (2009) 336–344

Contents lists available at ScienceDirect

Journal of Colloid and Interface Science

www.elsevier.com/locate/jcis

New biodegradable materials produced by ring opening polymerisation ofpoly(l-lactide) on porous silicon substrates

Steven J.P. McInnes a, Helmut Thissen b, Namita Roy Choudhury c, Nicolas H. Voelcker a,∗a Flinders University, School of Chemistry, Physics and Earth Sciences, Bedford Park, South Australia, Australiab CSIRO Molecular and Health Technologies, Clayton, Victoria, Australiac Ian Wark Research Institute, University of South Australia, Mawson Lakes Campus, South Australia, Australia

a r t i c l e i n f o a b s t r a c t

Article history:Received 21 August 2008Accepted 27 December 2008Available online 10 January 2009

Keywords:Surface initiated ring openingpolymerisationPolylactidesPorous siliconBiodegradable

In this paper, we describe for the first time the preparation of biodegradable inorganic/organichybrid materials by grafting poly(l-lactide) (PLLA) from porous silicon (pSi) films and microparticles.To graft a PLLA layer from pSi, tin(II) 2-ethylhexanoate catalysed ring opening polymerisation wasperformed using pSi surface-bound hydroxyl groups as initiators. Chemical surface characterisationby means of diffuse reflectance infrared spectroscopy, X-ray photoelectron spectroscopy and watercontact angle measurements confirmed the presence of the PLLA film. Furthermore, atomic forcemicroscopy demonstrated the formation of PLLA nanobrushes on the pSi surface. We also ascertainedby interferometric reflectance spectroscopy that the PLLA layer successfully slowed down the corrosionof the porous silicon layer in aqueous medium. Finally, thermal gravimetric analysis showed weightloss transitions that closely resemble the expected decomposition peak for low molecular weight PLLA.We believe that biodegradable hybrid materials like the ones presented here will find uses in tissueengineering and drug delivery, for example in applications where complex degradation profiles arerequired that cannot be achieved with one type of material alone.

© 2009 Elsevier Inc. All rights reserved.

1. Introduction

The fabrication of porous silicon (pSi) from single crystallinesilicon is achieved by an electrochemical etch with a hydrofluo-ric acid (HF) and ethanol electrolyte. Wafer resistivity, electrolyteconcentrations and current densities determine the pore size andstructure. The structures used in this study typically have meso-pores in the range of 5–50 nm, however other pore sizes havebeen previously reported and can range from nanometers to a fewmicrons [1]. This wide range of pore sizes allows pSi to be gen-erated with high surface areas ranging from 400–1000 m2/g [1].The tunability of pore dimensions combined with the ability ofpSi to degrade into non-toxic silicic acid in aqueous solutions [2,3]has caused pSi to become the focus of many biomedical researchgroups.

pSi can be produced in a number of different formats depend-ing on the intended application [4–7]. Commonly electrochemicaletching produces a film of pSi typically ranging from a few μmto 100 μm in thickness. Once created, pSi can be used as filmsor can be converted into free-standing membranes by cleavagefrom the bulk silicon by the application of electropolishing cur-

* Corresponding author.E-mail address: nico.voelcker@flinders.edu.au (N.H. Voelcker).

0021-9797/$ – see front matter © 2009 Elsevier Inc. All rights reserved.doi:10.1016/j.jcis.2008.12.073

rent. Subsequently, membranes can be converted into micro- andnanoparticles [8–11].

A further advantageous characteristic of pSi is the wide rangeof possible surface modifications with readily available chemicals,such as silanes, and mild reaction conditions [4,12–14].

Polylactides are biodegradable polyesters, which display a widerange of mechanical properties and undergo degradation into bio-compatible monomer subunits. As thay degrade in vivo, they formlactic acid which is tolerated by the body as it is naturally pro-duced during glycolysis [15–20]. These properties have made poly-lactides and their copolymers the focus of many research groupsfor the production of biomaterials for drug delivery and tissueengineering [21–28]. Some more recent examples of the use ofpoly(l-lactide) PLLA includes the work of Chen et al. who func-tionalised multi-walled nanotubes with PLLA of various molecularweights using both a grafting-to [29] and a grafting-from [30] ap-proach. Hong et al. [31] have used a grafting-from approach withthe tin(II) 2-ethylhexanoate Sn(Oct)2 catalysed ring opening poly-merisation to coat PLLA onto hydroxyapatite nanoparticles.

The ring opening polymerisation of lactides has been car-ried out over a wide range of conditions [22,26,27,32–34]. Pre-vious work has shown that lower polymerisation temperaturesdecrease side reactions by minimising intermolecular and in-tramolecular trans-esterification of the polymer chains [16]. Tin(II)

S.J.P. McInnes et al. / Journal of Colloid and Interface Science 332 (2009) 336–344 337

2-ethylhexanoate (Sn(Oct)2) is the most common catalyst for thispolymer in research and industry and is approved for food and bio-materials applications by regulatory agencies [16]. It is importantto note that tin(II) 2-ethylhexanoate (Sn(Oct)2) may decomposeat around 100 ◦C to produce octanoic acid. This can cause alcoholesterification which in turn causes the formation of water and sub-sequently the formation of tin hydroxides and stannoxanes whichlead to uncontrolled initiation and side reactions [16,35]. The ringopening polymerisation mechanism is not yet fully understood; butseveral different mechanisms have been proposed. Penczek et al.have suggested a polymerisation mechanism involving the forma-tion of a tin alkoxide complex prior to the polymerisation [16]. Thegenerated tin alkoxide then initiates the polymerisation of l-lactidevia a coordination insertion mechanism. It is believed that the sur-face initiated polymerisation proceeds via this mechanism withthe alcohol on the surface acting as the co-initiating alcohol. Pre-viously Yoon et al. demonstrated that surface initiated ring openingpolymerisation is possible on silica (SiO2) surfaces functionalisedwith the hydroxyl group presenting N-(triethoxysilylpropyl)-O -polyethylene oxide urethane [26]. Choi and Langer have also shownthat the surface initiated polymerisation of l-lactide can be per-formed at 40 ◦C on hydroxylated gold surfaces and at 80 ◦C onaminosilane functionalised silica nanoparticles [27].

A combination of polymeric and inorganic biodegradable ma-terials might bring to light the advantages of both types ofmaterials. In fact, the literature does contain some examplesof pSi–polymer composite materials. For example, ring openingmetathesis polymerisation of norbornene has been used to cre-ate flexible 1-D photonic crystals of pSi [36]. pSi has also beenemployed as a template for polymer replicas, with various poly-mers including poly(styrene), PLLA and poly(methyl)methacrylate[37–39]. Electropolymerisation has given pSi–polyaniline com-posite media properties suitable for silicon-based light emittingdiodes [40]. Very recently, Whitehead et al. have described a pSi-polycaprolactone based scaffold for bone tissue engineering [41].However, to the best of our knowledge surface-initiated polymeri-sations have not been attempted on pSi before.

The biocompatibility of both the pSi and PLLA suggests thatcomposites of both materials should be safe for use as biomateri-als in the human body [17,42]. In addition, the clever combinationof biodegradable inorganic and organic materials may allow im-proved control over degradation profiles which could be beneficialto the fine tuning of erosion behaviour in drug delivery devicesand biodegradable implants or scaffolds. For this reason, we in-tended to investigate in detail the production and properties ofthin PLLA films grafted from pSi for the purpose of applying thesematerials to applications in areas such as tissue culture, biosensorsand biomaterials [2,43].

2. Experimental

2.1. pSi film preparation

P-type silicon (Si) wafers from Silicon Quest International(boron doped, resistivity = 3–6 � cm−1, 〈1-0-0〉) were etched in1:1 HF:ethanol electrolyte at a current density of 36.67 mA cm−2

using a etching cell with an exposed area of 1.8 cm2. WARNING:HF is highly corrosive and toxic and can be absorbed throughall exposure routes. The pSi films were then washed with copi-ous amounts of methanol, ethanol, acetone and finally CH2Cl2 anddried in a stream of nitrogen. Ozone oxidation was performed us-ing a Fischer OZON, Ozon-Generator 500. All oxidations were runfor 20 min at an ozone rate of 3.25 g h−1.

2.2. pSi microparticle preparation

Microparticles were fabricated from p-type Si wafers (borondoped, resistivity <0.001 � cm−1, 〈1-0-0〉) supplied by VirginiaSemiconductors. The wafer was anodised in 3:1 HF:ethanol solu-tion with a current density of 222 mA cm−2 for 2 min and thenelectropolished for 30 s at 500 mA cm−2. CH2Cl2 was then added,and the free-standing porous layer was manually fractured intomicroparticles for collection. The pSi microparticle suspension wasfiltered and washed with ethanol and CH2Cl2. Ozone oxidation wasperformed as described above.

2.3. Silanisation

N-(hydroxyethyl)-3-aminopropyl trimethoxysilane (HEAPS) andN-(triethoxysilylpropyl)-O -polyethylene oxide urethane (PEGS)were purchased from Gelest Inc., USA, and used as received. Sur-face treatments with HEAPS and PEGS silanes were carried outon oxidised pSi surfaces at room temperature for 2 min by sub-mersion in an anhydrous solution of silane in toluene (50 mM).Afterwards, the surfaces were rinsed with toluene, acetone andCH2Cl2 and dried under a stream of N2.

2.4. Surface polymerisation

Oxidised and silanised pSi films were soaked in 10 mL oftoluene containing 5 μmol Sn(Oct)2 catalyst for 1 h at 50 ◦C be-fore adding 10 mmol (1.44 g) of recrystallised l-lactide and leftfor polymerisation for 72 h at 110 ◦C (Scheme 1). Upon comple-tion of the polymerisation, the pSi film was removed and washedwith toluene, acetone and CH2Cl2 before being treated by Soxh-let extraction in anhydrous toluene for 30 min. All pretreatments,polymerisations and washing procedures were carried out under anitrogen atmosphere. The same procedure was also performed on

Scheme 1. Surface functionalisations and polymerisations.

338 S.J.P. McInnes et al. / Journal of Colloid and Interface Science 332 (2009) 336–344

Fig. 1. (A) AFM image of pSi–Ox (RMS = 0.131 nm) and (B) scanning electron microscopy cross-section of a pSi layer on a silicon substrate.

pSi microparticles, however after polymerisation washing was per-formed via multiple rounds of centrifugation and resuspension inclean toluene.

2.5. Infrared spectroscopy (IR)

All IR spectra were obtained using a Nicolet Avatar 370MCT(Thermo Electron Corporation, USA) equipped with a standardtransmission accessory. Spectra of the pSi films were recorded andanalysed using OMNIC version 7.0 software, in the range of 650–4000 cm−1 at a resolution of 1 cm−1, the background was takenusing an unetched Si wafer. IR of the polymerised pSi micropar-ticles was performed in transmission mode with crushed pow-ders in KBr discs, spectra of pSi microparticles were obtained inthe range of 400–4000 cm−1 at a resolution of 1 cm−1. IR datawas normalised to 100% absorption for the peak at approximately1100 cm−1.

2.6. X-ray photoelectron spectroscopy (XPS)

XPS analysis of surface modified samples was performed on anAXIS Hsi spectrometer (Kratos Analytical Ltd., UK), equipped witha monochromatic AlKα source. The pressure during analysis wasapproximately 5 × 10−8 mbar. All spectra were recorded at anemission angle normal to the surface. The elemental compositionof samples was obtained from survey spectra, collected at a passenergy of 320 eV. High-resolution spectra were collected at a passenergy of 40 eV. Binding energies were referenced to the aliphaticcarbon peak at 285.0 eV.

2.7. Contact angle measurements

Contact angles were measured by placing a 1 μL drop of wa-ter on the sample surface and capturing a digital image using aPanasonic Super Dynamic wv-BP550 Closed Circuit TV camera. Thecontact angle measurements were analysed by Scion Image forWindows Framegrabber software (Beta version 4.0.2). Three repli-cate measurements were performed for each sample.

2.8. Atomic force microscopy (AFM)

Tapping mode AFM was performed on a Multimode NanoscopeIV microscope (Veeco Corporation, USA). Commercial Si cantilevers(FESP, Veeco Corporation, USA) were used for all experiments. Theimages were processed and analysed using the Nanoscope 5.31r1software (Veeco Corporation, USA).

2.9. Scanning electron microscopy (SEM)

Scanning electron microscopy was performed on a Phillips XL30field emission scanning electron microscope operating at 10 keV ata working distance of 10 mm.

2.10. Degradation experiments

Interferometric reflectance spectroscopy was used to determinethe rate at which the various functionalised pSi surfaces degrade.The experiments were performed on a custom built interferometerwith an S2000 CCD Detector (Ocean Optics, USA). The interferom-eter is also fitted with a collimator to assist with better focus-ing of the light on the surface. All six surface functionalisationswere studied under phosphate buffered saline (PBS) (pH 7.4) flow(30 mL h−1) for 1 h intervals to study the rate of degradation.

2.11. Thermal gravimetric analysis (TGA)

TGA experiments were conducted under nitrogen at a flow rateof 50 mL/min on a SDT 2960 (TA Instruments, USA). The temper-ature was ramped from room temperature to 550 ◦C at 10 ◦C/min.The sample size ranged from 10–13 mg.

3. Results and discussion

3.1. pSi film fabrication and characterisation

AFM image analysis of oxidised pSi films (pSi–Ox) shows a sur-face roughness value (rms) of 0.131 nm and a pore size rangingfrom 10 to 30 nm (Fig. 1A) [44]. The pore depth was found to beapproximately 6.8 μm from cross-sectional SEM (Fig. 1B).

Freshly etched hydride terminated pSi typically displays multi-ple peaks at approximately 2100 cm−1 due the Si–H, Si–H2 and Si–H3 stretches (data not shown). [44] The ozone oxidation of freshlyetched pSi introduces stable Si–OH and SiO2 bonds on the surface.The IR of pSi–Ox (Fig. 2) shows a peak at 1100 cm−1 which cor-responds to the Si–O stretching [45] and a broad peak from 3100–3700 cm−1 which can be attributed to O–H stretching. The smallpeak at 1600 cm−1 is attributed to the deformation vibration inabsorbed H2O and is indicative of the presence of silanol bonds onpSi surfaces [46]. The absence of the Si–Hx stretches at 2100 cm−1

is conspicuous. The ozone oxidation of freshly etched pSi increasesthe hydrophilicity of the surfaces substantially, with water con-tact angles (sessile drop) of approximately 99 ± 3◦ [44] for thefreshly etched material decreasing to 14±2◦ after ozone oxidation.Silanisation of the pSi–Ox surface with HEAPS caused several newpeaks to appear in the IR spectrum at 1480 cm−1 correspondingto the C–H bending vibration and dual peaks at 2900 cm−1 and2980 cm−1 for C–H stretching modes. The broad peak at 3100–3700 cm−1 assigned to the O–H stretching vibration is also en-larged after functionalisation (Fig. 2). The PEGS functionalisationcan be confirmed by the presence of the C–H modes mentionedabove and, in addition, the presence of the peak at 1550 cm−1

representative of aliphatic amines (from the urethane) as well asthe peak at 1720 cm−1 for the C=O stretch (Fig. 2). HEAPS and

S.J.P. McInnes et al. / Journal of Colloid and Interface Science 332 (2009) 336–344 339

Fig. 2. Normalised transmission IR spectra of pSi–Ox, pSi–HEAPS and pSi–PEGS.

Table 1Summary of XPS atomic elemental % taken on all surface preparations.

Sample O (%) C (%) Si (%) Sn (%) O/C

pSi–Ox 56.39 1.80 41.80 – –pSi–HEAPS 44.77 30.74 19.79 – –pSi–PEGS 57.04 18.27 24.69 – –pSi–Ox–PLLA 42.88 49.84 1.27 5.4 0.86pSi–HEAPS–PLLA 39.24 51.67 4.65 2.09 0.76pSi–PEGS–PLLA 41.20 43.51 12.49 0.48 0.95

PEGS surfaces were also analysed by water contact angle mea-surements and they showed a slightly decreased wettability witha water contact angle increasing from 14◦ ± 2◦ to 29◦ ± 1◦ and29◦ ± 3◦ , respectively. The successful functionalisation of the pSi–Ox surface with HEAPS and PEGS silane were further confirmed bythe substantial increase in the percentage of carbon found via XPS(Table 1). The % of Si also decreased by about half indicating thatthe silane layer thickness is well below 10 nm. Here, 10 nm is theapproximate XPS information depth, taking into account that thisvalue is dependent on the atomic number of atoms detected on amaterial surface. These functionalisations provided us with threehydroxyl-terminated surfaces with different spacer lengths for ringopening polymerisations of l-lactide.

3.2. Ring opening polymerisation of l-lactide on pSi films

After the ring opening polymerisation of l-lactide onto thepSi with three different hydroxyl group presenting surfaces (Ox,HEAPS, PEGS), the IR spectra (Fig. 3) still show a strong char-acteristic peak at 1100 cm−1 due to the Si–O stretching on theoxidised surface, while the new peaks at 1380 cm−1 may be at-tributed to the methyl bending vibrations of the PLLA. The peakat 1600 cm−1 and the broad band at 3400 cm−1 are attributedto the free hydroxyl groups from the polymer termini and poten-tially non-reacted hydroxyl groups of the silane as well as surfacesilanol groups. The peak at 1480 cm−1 assigned to an asymmetric

Fig. 3. Normalised transmission IR scans of pSi-PLLA samples.

CH3 deformation mode has intensified and so has the dual peak at2900 and 2980 cm−1 corresponding to C–H stretching vibrations.Most notably perhaps is a strong new peak occurring at 1760 cm−1

attributed to the C=O stretching mode of the PLLA.A theoretical O/C ratio of 0.66 is expected for a perfectly uni-

form coverage of PLLA homopolymer. Our experimental values re-main slightly above that (Table 1) perhaps due to the XPS probingthe underlying pSi surface as well. This is consistent with ourobservation that for the surface with the highest Si content (pSi–PEGS–PLLA), the O/C ratio is also highest. The other two surfacesreach Si percentages of less than 5%. The low percentage of Sistill remaining in the spectra might also stem from defects in thecoating. Interestingly, the Sn and Si percentages in the samplesnegatively correlate, possibly due to the Sn remaining bound tothe PLLA layer. Considering this relationship between the Si and Snpercentages, it appears that the pSi–Ox–PLLA has been more effi-ciently polymerised as it has the lowest amount of Si and the high-est amount of Sn. Meanwhile the pSi–HEAPS–PLLA sample showsa higher Si signal and a lower Sn signal, indicating less polymeri-sation and this trend continues into the pSi–PEGS–PLLA samplewhere the Si is approximately 12% and the Sn is only 0.48%. Theseresults indicate that the longer spacer length might not be ben-eficial to obtain higher efficiency in ring opening polymerisation.Indeed, the silanol groups of the pSi–Ox seem to be a more activeinitiator.

The increase in the amount of carbon signal after polymerisa-tion is also informative. The pSi–Ox surface has increased from1.8% before to 49.8% carbon after polymerisation while the pSi–HEAPS has increased by approximately 21% to 51.7% and thepSi–PEGS shows the lowest carbon signal with just 43.5%. Thisagain suggests that the pSi–PEGS–PLLA has the thinnest PLLA filmamongst the three samples.

Another possible explanation for the high Sn content is thatthe Sn(Oct)2 catalyst remains adsorbed on the pSi surface even af-ter washing and Soxhlet extraction procedures. To investigate thispossibility, un-polymerised pSi samples and polymerised sampleswere incubated or polymerised for a 72 h period with 20 μmol ofSn catalyst, washed and analysed by XPS (Table 2). It can be seen

340 S.J.P. McInnes et al. / Journal of Colloid and Interface Science 332 (2009) 336–344

Table 2Summary of XPS atomic elemental % for Sn.

Sample Cleaning conditions Sn (%)

pSi–Ox Solvent wash only 12.3pSi–Ox Solvent wash + 1

2 h toluene Soxhlet 12.3pSi–Ox–PLLA Solvent wash only 4.4pSi–Ox–PLLA Solvent wash + 1

2 h toluene Soxhlet 5.2pSi–Ox–PLLA Solvent wash + 2 h toluene Soxhlet 3.8

that the Sn strongly adheres to the pSi surface, probably in theform of Sn(Oct)2 as the survey spectra showed a signal increasefor C1s from 4.3% to 28.3%, high resolution XPS was not performedfor confirmation. The Sn is not removed via Soxhlet extraction,however, the polymerisation removes the catalyst from the pSi sur-face and the Sn percentage is significantly reduced. The remainingSn appears to be strongly adsorbed, potentially as a Sn(Oct) cap-ping to the polymer layer, since even a 2 h Soxhlet extraction doesnot lower the percentage significantly. These results are consistentwith previous reports [47,48]. In order to remove the Sn from thepolymer, the PLLA is commonly treated with dilute chloric acid,which promotes the hydrolysis of the Sn–O bond. However, thisprocedure was not applied here since it would reduce the molecu-lar weight of the polymer by cleaving the polymer (C–O) backboneas well [47–49]. It is also possible that the silanisation of the pSilayer prevents catalyst adsorption to some extent.

The high-resolution C1s spectra revealed three componentpeaks (Table 3). The peak at 285.0 eV (component 1) indicatesthe presence of aliphatic carbon, while the peaks at 286.9 and289.0 eV (components 2 and 3) are attributed to the presenceof C–N/C–O and O–C=O/COOH, respectively. The HEAPS surfacefunctionalisation was confirmed by the presence of components 1and 2 indicating almost equivalent amounts of C–C and C–N/C–O bonds, while the PEGS coating was confirmed by the presenceof a large component 2 consistent with the expected contribu-tion from the poly(ethylene glycol) chain. Bulk PLLA gives a 3-peakstructure in the C1s spectrum consisting of components 1, 2 and 3in the ratio of roughly equal to the theoretical ratio of 40:30:30[50–52]. The PLLA coatings show some variation from this ra-tio. pSi–Ox–PLLA is the closest match with a ratio of 48:33:19.

Table 3Summary of high resolution C1s XPS spectra.

Sample Component 1(C–H)(%)

Component 2(C–O/C–N)(%)

Component 3(O–C=O/COOH)(%)

pSi–Ox 56.0 31.4 12.6pSi–HEAPS 45.6 48.4 6.0pSi–PEGS 13.9 75.0 11.0pSi–Ox–PLLA 47.8 33.1 19.0pSi–HEAPS–PLLA 33.8 46.2 20.0pSi–PEGS–PLLA 59.1 29.6 11.3

The pSi–Ox–PLLA and pSi–PEGS–PLLA samples show a reducedcomponent 3, which is offset by an increase in component 1. Allthree samples show a reduced component 3 which is offset by anincrease in component 1 for the pSi–Ox–PLLA and the pSi–PEGS–PLLA, possibly due to some of the Sn(Oct)2 catalyst still remainingeither endcapped or bound to exposed areas of pSi. For the pSi–HEAPS–PLLA sample, the reduction in component 3 is offset by anincreased component 2 probably caused by the 3◦ amine in thetop region of the underlying linker.

Tapping mode AFM images (Fig. 4) revealed an increase in sur-face roughness for all three samples after polymerisation. The oxi-dised surface showed the largest rms increase of 0.450 nm due tothe introduction of the PLLA layer (from 0.131 to 0.581 nm), whilstthe pSi–HEAPS–PLLA had an rms = 0.460 nm and was the flattestpolymerised sample. Still, it displayed the second largest increasein roughness of 0.243 nm. The pSi–PEGS–PLLA was the roughestsample and showed an increase in RMS from 0.438 nm after silani-sation to 0.575 nm after polymerisation (a change of 0.137 nm).This trend in roughness increase is consistent with the observa-tion from XPS that the pSi–Ox–PLLA sample has the thickest PLLAlayer and the pSi–PEGS–PLLA the thinnest. All samples showed ananobrush structure which has previously been demonstrated byChoi and Langer [27] on non-porous substrates. It is speculatedthat the PLLA forms nanobrushes due to the isotactic nature andhigh crystallinity of the PLLA. The nanobrush structure seen on ourpSi surfaces (ca. 5 nm high) are not as large as those seen by Choipossibly due the presence of the pSi layer affecting the outwardgrowth of the PLLA.

Fig. 4. Tapping mode AFM images of (A) pSi–HEAPS (RMS = 0.217 nm), (B) pSi–PEGS (RMS = 0.438 nm), (C) pSi–Ox–PLLA (RMS = 0.581 nm), (D) pSi–HEAPS–PLLA (RMS =0.460 nm) and (E) pSi–PEGS–PLLA (RMS = 0.575 nm).

S.J.P. McInnes et al. / Journal of Colloid and Interface Science 332 (2009) 336–344 341

Fig. 5. Degradation slopes of pSi–Ox, pSi–Ox–PLLA pSi–HEAPS, pSi–HEAPS–PLLA,pSi–PEGS and pSi–PEGS–PLLA.

Water contact angle measurements taken after the surface-initiated polymerisations qualitatively showed a significant in-crease for each of the three surface functionalities. While thesilanised surfaces had contact angles of around 29◦ , the poly-merised samples showed much higher contact angles due to theintroduction of the more hydrophobic PLLA layer; typically thickPLLA films have a contact angle of 79◦ ± 3◦ [53]. The contact an-gle change from pSi–Ox to pSi–Ox–PLLA was approximately 42◦(14◦ ± 2◦ to 56◦ ± 6◦) while the polymerisation of pSi–HEAPScaused a contact angle change of around 34◦ (29◦ ±1◦ to 63◦ ±1◦)and pSi–PEGS–PLLA showed the highest contact angle of the poly-merised samples at 72◦ ± 3◦ (a change of 43◦). These results con-firm qualitatively that a PLLA layer is present on the pSi surface.It must be kept in mind that porous substrates are non-ideal sur-faces for contact angle because the surface porosity is known toinfluence the surface energy of the water droplet by effects suchas lack of symmetry and loss of volume due to capillary actiondrawing in water. However, many groups have recently been de-veloping methods that allow the wettability of porous surfaces tobe calculated from the contact angle [54].

Interferometric reflectance spectroscopy was used to measurethe degradation rates of pSi protected by silanisation and poly-merisation. It has been previously shown that pSi corrosion can bestudied by monitoring the decrease of effective optical thickness(EOT) collected by interferometric reflectance spectroscopy [13,55].The degradation of all six surfaces was studied in PBS (pH 7.4). Thefastest degradation (0.8% EOT per hour) occurs on the ozone oxi-dised pSi (Fig. 5), this is due to the surface having relatively littleprotection from hydrolytic dissolution [13]. It is important to notehowever that this surface is much more stable than freshly etchedpSi surfaces which have been shown to degrade over 20× fasterthan oxidised pSi [44]. A coating, either in the form of a silanelayer or a polymer layer, increases this stability. Fig. 5 shows thatthe pSi–Ox–PLLA degrades at a rate of approximately 0.4% EOT perhour. The lack of a protecting silane layer exposes the pSi surfaceto hydrolytic attack [13,44,55,56], even in the presence of the PLLAnanobrushes. The low water contact angle of the pSi–Ox–PLLA(56◦) compared with 62◦ and 72◦ for the pSi–HEAPS–PLLA andpSi–PEGS–PLLA samples also suggest that the pSi–Ox–PLLA surface

Fig. 6. Typical scanning electron micrographs of (A) oxidised microparticles (100 μmscale bar) and (B) pSi microparticles with grafted PLLA (20 μm scale bar).

is the most wettable and that the pSi layer will subsequently bemore prone to hydrolytic degradation. The pSi–PEGS, pSi–HEAPS,pSi–HEAPS–PLLA and pSi–PEGS–PLLA all present very stable sur-faces which all degrade at a very similar rate (approximately 0.2%EOT per hour). These results indicate that the silane and the silane-grafted PLLA layers are both effective in slowing the degradation ofpSi in aqueous solution. Interestingly, the surface with the thickestPLLA layer degraded fastest in this experiment, showing that thePLLA layer itself is not dense enough to stop hydrolytic attack ofthe pSi. The above experiment only studied the degradation be-haviour of the pSi material used in this study. We did not expectsignificant degradation of the PLLA layer to occur within the time-frame of the interferometric reflectance experiment, however thedissolution of the pSi support will facilitate the release of the PLLAsegments, which will contribute to the decrease in the EOT.

3.3. pSi–PLLA microparticle preparation

Microparticles were prepared by anodisation, washed and sus-pended in ethanol before being ultrasonically fractured using asonicator (20 min) to create fine pSi microparticles. The micropar-ticles created in this manner are platelets of 10–11 μm thicknessand around 30–70 μm in width (Fig. 6A). Identical silanisations andpolymerisations to those performed on the pSi films were thenperformed on the pSi microparticles (Fig. 6B). Particle size afterpolymerisation was slightly lower (20–50 μm), due to mechanicalfractionation during the polymerisation or subsequent centrifuga-tion of the particles.

IR spectra of PLLA grafted pSi microparticles provide evidenceof successful surface-initiated polymerisations (Fig. 7). The peak at1600 cm−1 and the broad band at 3400 cm−1 are attributed tothe free hydroxyl groups from the polymer termini and potentiallynon-reacted hydroxyl groups of the silane as well as surface silanolgroups and are much stronger than in the films probably due tothe higher surface area of the microparticles. It is important tonote the presence of the strong peak at 1760 cm−1 from the C=Ostretch in the PLLA layer. The pSi–PEGS–PLLA layer shows a shoul-der to the lower wavenumbers due to the peak at 1720 cm−1 forthe urethane C=O stretching vibration from the PEGS linker. Thepeak at 1550 cm−1 in the pSi–PEGS–PLLA sample is attributed tothe C–N in the urethane bond, while the C–H stretching modesappear as a dual peak at around 2900–3000 cm−1. The peak at1460 cm−1 assigned to C–H bending vibrations in both the silanetether and the PLLA and the peaks at 1380 cm−1 are credited tomethyl bending vibrations.

3.4. Thermal analysis of pSi–PLLA microparticles

DSC and TGA experiments were performed on the PLLA graftedpSi films to obtain further confirmation of the presence of agrafted polymer layer. However, due to the high percentage of bulkSi compared to the very thin layer of pSi (ca. 7 μm) and polymerlayer (ca. 10 nm) it was not possible to obtain thermal transitions

342 S.J.P. McInnes et al. / Journal of Colloid and Interface Science 332 (2009) 336–344

Fig. 7. Normalised transmission IR scans of PLLA functionalised pSi microparticles.All spectra were obtained at 1 cm−1.

within the detection limits of the instruments. For this reason, pSi–PLLA microparticles were synthesised as microparticle preparationsafford a much higher active surface area and can be analysed with-out the contribution of the bulk Si to the sample mass. Besidesexperimental considerations for thermal analysis, such polymer-coated microparticles may also find uses in drug delivery or tissueengineering [16,19,21,28].

Thermal analysis on the PLLA grafted microparticles was per-formed by TGA to examine the effect of each surface attachedinitiator on the polymerisation behaviour (Fig. 8). Differences inmolecular weight are expected to result in variations of decompo-sition temperature in the TGA scans depending on the molecularweight and composition of the polymer chain. The decompositiontemperatures of each surface treatment can be seen in Table 4.pSi–Ox microparticles displayed no significant decomposition upto 500 ◦C. A PLLA sample prepared by solution polymerisation un-der the same conditions as the surface-initiated polymerisations(but in the absence of pSi microparticles) was found to have adecomposition temperature of 282 ◦C which is roughly 30–50 ◦Chigher than the observed decomposition temperatures of the PLLA

Table 4Decomposition temperatures for silanised and PLLA functionalised microparticles.

Sample Silanedecomp.temp. (◦C)

Weightloss(%)

Polymerdecomp.temp. (◦C)

Weightloss(%)

Adjusteddecomp.temp. (◦C)

PLLAa – – 282 94.7 –pSi–HEAPS 166 6.4 – – –pSi–PEGS 408 7.8 – – –pSi–Ox–PLLA – – 257 10.8 256pSi–HEAPS–PLLA 180 2 235 15.7 262pSi–PEGS–PLLA 433 4.9 250 12.7 272

a PLLA polymerised in solution under the same reaction conditions without anypSi present.

functionalised microparticles (Table 4), possibly due to the surfacebound polymer chains possessing a lower molecular weight [42].The literature has previously shown the decomposition tempera-ture of as polymerised unpurified PLLA to be significantly lower(ca. 287 ◦C [49,57]), than that of high molecular weight and highpurity PLLA (ca. 330 ◦C [47]). Hong et al. have also previouslyshown that when grafting PLLA from hydroxyapatite nanoparticles,the typical mass of PLLA grafted to the surface is 6% [31], whilethe grafting-from approach employed by Chen et al. on multi-walled carbon nanotubes produced a mass percentage (determinedby TGA) of up to 30% [30]. The derivative weight loss curve for thegrafted pSi-PLLA composites shows two peaks. The main peak isquite broad (150–350 ◦C) and relates to the decomposition of thePLLA. The weight loss corresponds to approximately 13% for eachof the three samples, in between the values obtained by Hong etal. and Chen et al. Below 200 ◦C, there is a small weight change,corresponding to the low molecular weight by-products and resid-ual solvents. On the high temperature side of the main peak, thereis a small peak attributed to silane linker decomposition at highertemperature.

The derivatives of the thermal degradation profiles of pSi–Ox–PLLA and pSi–HEAPS–PLLA are very similar and broad in naturewith two satellite shoulders (∼182 and ∼320 ◦C) to the main de-composition peak. In contrast, the pSi–PEGS–PLLA shows a verysharp peak with a small shoulder at 182 ◦C. This might indicatethat the binding site for the polymer chain in pSi–HEAPS–PLLA isclose to the surface and hence closely matches the thermal be-haviour of pSi–Ox–PLLA. On the contrary, for pSi–PEGS–PLLA, thelinker length is longer, imparting greater mobility of the functionalgroups, hence the surface grown PLLA can extend out.

The polymerised pSi microparticles show a small weight gainabove 350 ◦C possibly due to char formation or partial nitridationof the pSi [58,59].

Fig. 8. (Left) TGA weight loss curves and (right) the weight loss derivative for pSi–Ox–PLLA, pSi–HEAPS–PLLA and pSi–PEGS–PLLA functionalised pSi microparticles showingthe effect of the surface initiators on the thermal behaviour of the PLLA films.

S.J.P. McInnes et al. / Journal of Colloid and Interface Science 332 (2009) 336–344 343

As pointed out above, all PLLA grafted microparticle samplesshowed lower decomposition temperatures compared with thematerial polymerised in solution, However, the work of Cam andMarucci [47] offers another possible explanation for the lower de-composition temperatures for the grafted PLLA. They have studiedthe effects of residual monomers and metals on the thermal sta-bility of PLLA and found that the Sn content will lower the decom-position temperature according to:

Tdecomp = T0 − 6.8 ln(m), (1)

where T0 is the decomposition temperature of infinitely pure PLLA(330 ◦C), m is the amount of Sn impurity in ppm and 6.8 repre-sents the factor with which the Sn affects the PLLA decomposi-tion (found experimentally by Cam and Marucci). Using Eq. (1)combined with the XPS data for Sn impurities (Table 1), we cancalculate the expected decomposition temperatures for the PLLAlayer (see Table 4). The adjusted decomposition temperatures cor-respond quite well with the experimental decomposition temper-ature for all three different surface functionalisations. However, itis plausible that there is also an effect from the underlying silanelayer on the decomposition temperatures of the PLLA, as for bothsilanised samples the experimental values are approximately 20 ◦Clower than the values calculated according to Eq. (1).

The thermal analysis data confirms that a PLLA layer has beensuccessfully grafted from the pSi surface, however the weight loss% is likely to be different for the pSi film preparation becausethe microparticle fabrication procedure uses different Si wafersand etching conditions hence producing a different porosity andactive surface area. Since both pSi and PLLA rapidly degrade inbasic conditions, it was not possible to cleave the PLLA layerfrom the pSi and analyse the molecular weight via gel perme-ation chromatography. However, the work of Cam and Maruccisuggests that a decomposition temperature of 250 ◦C relates to amolecular weight of approximately 10,000 g mol−1. Other exam-ples of surface bound PLLA synthesised by means of a grafting-toapproach have produced maximum molecular weights of approxi-mately 4000 g mol−1 [60].

4. Conclusions

A new type of hybrid material composed of a porous siliconsubstrate covalently grafted with a biodegradable polymer is de-scribed here. PLLA was successfully grafted from the surface hy-droxyl groups present on oxidised and silanised pSi films and mi-croparticles. The presence of a PLLA coating was confirmed by thevibrational profile in the IR and elemental composition analysisby means of XPS. High-resolution carbon spectra showed a 3-peakprofile that is also indicative of PLLA homopolymers. AFM studiesof the PLLA functionalised surfaces showed that nanobrushes hadbeen formed upon surface-initiated polymerisation. PLLA graftedpSi microparticles were also prepared and characterised. Thesemicroparticles were used for thermal analysis showing a char-acteristic thermal decomposition of the PLLA and approximately13% (w/w) polymer. Thermal analysis showed that the PEGS linkerlength was sufficient to begin overcoming the effect of the surfaceconfinement; however the molecular weight of the PLLA remainedconstant over the three surface functionalisations. It is envisionedthat such hybrid biodegradable and biocompatible materials com-bining the properties of organic and inorganic matter will widenthe scope of applications for biomedical biodegradable devices andin particular will be useful in situations where drug delivery de-vices, implants and scaffolds for tissue engineering with complexbiodegradation behaviour are desired.

Acknowledgments

Support from Flinders University and the Australian ResearchCouncil is duly acknowledged. Our thanks go to Dr. Emily Anglinfor critical reading of the manuscript.

References

[1] L. Canham, Properties of Porous Silicon, Short Run Press, 1997.[2] L. Canham, R. Aston, Phys. World (2001) 27–31.[3] L. Canham, Adv. Mater. 7 (12) (1995) 1033–1037.[4] D. Xu, L. Sun, H. Li, L. Zhang, G. Guo, X. Zhao, L. Gui, New J. Chem. 27 (2003)

300–306.[5] L. Canham, C. Reeves, J. Newey, M. Houlton, T. Cox, J. Buriak, M. Stewart, Adv.

Mater. 11 (18) (1999) 1505–1507.[6] J. Wojtyk, K. Morin, R. Boukherroub, D. Wayner, Langmuir 18 (2002) 6081–

6087.[7] R. Bley, S. Kauzlarich, J. Davis, H. Lee, Chem. Mater. 8 (8) (1996) 1881–1888.[8] J. Dorvee, A. Derfus, S. Bhatia, M. Sailor, Nat. Mater. 3 (2004) 896–899.[9] J. Heinrich, C. Curtis, G. Credo, K. Kavanagh, M. Sailor, Science 255 (5040)

(1992) 66–68.[10] T. Schmedake, F. Cunin, J. Link, M. Sailor, Adv. Mater. 14 (18) (2002) 1270–1272.[11] J. Link, M. Sailor, Proc. Natl. Acad. Sci. USA 100 (19) (2003) 10607–10610.[12] Y.-L. Khung, S. Graney, N. Voelcker, Biotechnol. Prog. 22 (2006) 1388–1393.[13] A. Janshoff, K. Dancil, C. Steinem, D. Greiner, V. Lin, C. Gurtner, K. Motesharei,

M. Sailor, M. Ghadiri, J. Am. Chem. Soc. 120 (46) (1998) 12108–12116.[14] M. Stewart, J. Buriak, Adv. Mater. 12 (12) (2000) 859–869.[15] A. Lehninger, D. Nelson, M. Cox, Principles of Biochemistry, Worth Publishers,

New York, 1993.[16] A. Albertsson, I. Varma, Biomacromolecules 4 (2003) 1466–1486.[17] A. Andreopoulos, E. Hatzi, M. Doxastakis, J. Mater. Sci.-Mater. Med. 12 (2001)

233–239.[18] Y. Zhang, C. Chu, Biomed. Mater. Res. 59 (2002) 318–328.[19] M. Radwan, H. Aboul-Enein, Chirality 16 (2004) 119–125.[20] T. Mandal, L. Bostanian, R. Graves, S. Chapman, Pharm. Res. 19 (11) (2002)

1713–1719.[21] J. Panyam, V. Labhasetwar, Mol. Pharm. 1 (1) (2004) 77–84.[22] K. Yoon, Y. Koh, I. Choi, Y. Chi, K. Lee, J. Lee, D. Kim, S. Joo, W. Yun, J. Mater.

Chem. 13 (2003) 2910–2914.[23] W. Lee, J. Lee, C. Ha, Macromol. Res. 11 (6) (2003) 511–513.[24] F. Tasaka, Y. Ohya, T. Ouchi, Macromolecules 34 (2001) 5494–5500.[25] A. Kowalski, A. Duda, S. Penczek, Macromolecules 33 (2000) 7359–7370.[26] K. Yoon, Y. Koh, I. Choi, Macromol. Rapid. Commun. 24 (2003) 207–210.[27] I. Choi, R. Langer, Macromolecules 34 (2001) 5361–5363.[28] R. Langer, Acc. Chem. Res. 33 (2000) 94–101.[29] G. Chen, H. Kim, B. Park, J. Yoon, J. Phys. Chem. B 109 (47) (2005) 22237–22243.[30] G. Chen, H. Kim, B. Park, J. Yoon, Macromol. Chem. Phys. 208 (4) (2007) 389–

398.[31] Z. Hong, X. Qiu, J. Sun, M. Deng, X. Chen, X. Jing, Polymer 45 (19) (2004) 6699–

6706.[32] Z. Zhong, P. Dijkstra, J. Feijen, J. Am. Chem. Soc. 125 (2003) 11291–11298.[33] C. Williams, L. Breyfogle, S. Choi, W. Nam, V. Young, M. Hillmyer, W. Tolman,

J. Am. Chem. Soc. 125 (2003) 11350–11359.[34] H. Li, C. Wang, F. Bai, J. Yue, H. Woo, Organometallics 23 (2004) 1411–1415.[35] H. Kricheldorf, I. Kreiser-Saunders, A. Stricker, Macromolecules 33 (2000) 702–

709.[36] M. Yoon, K. Ahn, R. Cheung, H. Sohn, J. Link, F. Cunin, M. Sailor, Chem. Com-

mun. (2003) 680–681.[37] Y. Li, V. Kollengode, M. Sailor, Adv. Mater. 17 (10) (2005) 1249–1251.[38] Y. Li, F. Cunin, J. Link, T. Gao, R. Betts, S. Reiver, V. Chin, S. Bhatia, M. Sailor,

Science 299 (2003) 2045–2047.[39] J. Park, S. Meade, E. Segal, M. Sailor, Phys. Status Solidi A 204 (5) (2007) 1383–

1387.[40] B. Urbach, N. Korbakov, Y. Bar-David, S. Yitzchaik, A. Sa’ar, J. Phys. Chem.

C 111 (44) (2007) 16586–16592.[41] M. Whitehead, D. Fan, P. Mukherjee, G. Akkaraju, L. Canham, J. Coffer, Tissue

Eng. 14 (1) (2008) 195–206.[42] B. Ferreira, C. Zavaglia, E. Duek, Mater. Res. 4 (1) (2001) 34–42.[43] S. Anderson, H. Elliott, D. Walli, L. Canham, J. Powell, Phys. Status Solidi 197 (2)

(2003) 331–335.[44] S. Low, K. Williams, L. Canham, N. Voelcker, Biomaterials 27 (26) (2006) 4538–

4546.[45] F. Bessueille, V. Dugas, V. Vikulov, J. Cloarec, E. Souteyrand, J. Martin, Biosens.

Bioelectron. 21 (2005) 908–916.[46] F. Cunin, P.-E. Milhiet, E. Anglin, M. Sailor, C. Espenel, C. Le Grimellec, D. Brunel,

J.-M. Devoisselle, Ultramicroscopy 107 (2007) 1048–1052.[47] D. Cam, M. Marucci, Polymer 38 (8) (1997) 1879–1884.

344 S.J.P. McInnes et al. / Journal of Colloid and Interface Science 332 (2009) 336–344

[48] T. Mori, H. Nishida, Y. Shirai, T. Endo, Polym. Degrad. Stab. 84 (2004) 243–251.

[49] Y. Fan, H. Nishida, Y. Shirai, T. Endo, Polym. Degrad. Stab. 84 (2004) 143–149.[50] W.-K. Lee, M. Toselli, J. Gardella, Macromolecules 34 (2001) 3493–3496.[51] S. Zielhuis, J. Nijsen, R. Figueiredo, B. Feddes, A. Vredenberg, A. van het Schip,

W. Hennink, Biomaterials 26 (2005) 925–932.[52] Y. Jiao, Z. Liu, F. Cui, C. Zhou, J. Bioact. Compat. Pol. 22 (2007) 492–507.[53] F. Yang, C. Xu, M. Kotaki, S. Wang, S. Ramakrishna, J. Biomater. Sci. Polym. Ed.

15 (12) (2004) 1483–1497.[54] M. Rodriguez-Valverde, M. Cabrerizo-Vilchez, P. Rosales-Lopez, A. Paez-Duenas,

R. Hidalgo-Alvarez, Colloids Surf. A 206 (2002) 485–495.

[55] C. Steinem, A. Janshoff, V. Lin, N. Voelcker, M. Reza Ghadiri, Tetrahedron 60(2004) 11259–11267.

[56] V. Dubin, C. Vieillard, F. Ozanam, J. Chazalviel, Phys. Status Solidi 190 (47)(1995) 47–52.

[57] Q. Li, F. Li, L. Jia, Y. Li, Y. Liu, J. Yu, Q. Fang, A. Cao, Biomacromolecules 7 (8)(2006) 2377–2387.

[58] J. Hong, C. Hong, S. Choe, S. Shim, J. Polym. Sci. Polym. Chem. 45 (15) (2007)3477–3481.

[59] M. Björkqvist, J. Salonen, E. Laine, L. Niinistö, Phys. Status Solidi A 197 (2)(2003) 374–377.

[60] J. Tian, Y. Feng, Y. Xu, J. Polym. Res. 13 (2006) 343–347.