ORIGINAL PAPER

Application of PDMS-based coating in drug delivery systemsusing PVP as channeling agent

Hanie Soroory • Arezou Mashak • Azam Rahimi

Received: 21 January 2013 / Accepted: 24 July 2013

� Iran Polymer and Petrochemical Institute 2013

Abstract PDMS derivatives have been extensively used

as coating in controlled drug delivery systems in pharma-

ceutical area. The major advantages of these materials are

facilitation of adjustment of desired drug release patterns,

improvement of film formation and storage stability. In this

study PDMS-based coating systems were designed and

applied to acetaminophen tablets and their release was

investigated from the PDMS-coated tablet dosage form as a

drug model. Thus, a water emulsion of PDMS containing

tetraethoxysilane as cross-linking agent and polyvinylpyr-

rolidone (PVP) as channeling agent was prepared. Then, a

uniform smooth thin coating of PDMS was applied on

acetaminophen tablets and in vitro acetaminophen release

from PDMS-coated tablets was carried out with a home-

made setup. The morphology of the coated surfaces was

studied by SEM technique. The effect of PVP on the

mechanical properties of PDMS film was investigated by

dynamic mechanical analysis. It was found that, PVP

increased the mechanical properties of PDMS. Com-

pounding of polyethylene glycol (PEG) with PVP (1:1) as

channeling agents was also studied and compared with

PVPs-containing samples. Despite more channeling agent

content in PEG–PVP samples, the drug release percentage

was decreased.

Keywords Polydimethylsiloxane � Polyvinylpyrrolidone �Coating � Drug delivery

Introduction

In the pharmaceutical industry, coating is often used to

change and enhance the performance of drug delivery

systems, especially solid dosage forms. Enteric coating

systems are designed to provide protection to oral tablets

delivered in the stomach. Literature search results indicate

that wide arrays of polymers have been utilized as coating

materials in oral dosage forms to achieve extended drug

release. The major advantages of these materials are:

facilitation of adjustment of the desired drug release pat-

terns, improvement of film formation and storage stability,

and providing the possibility to develop novel strategies for

site specific drug delivery [1]. Coating polymers such as

Eudragit, cellulose acetate, shellac and ethyl cellulose may

provide alternative materials for the development of these

systems.

One of the most well-known polymers used as coating

material is polydimethylsiloxane. The elastomeric poly-

siloxanes (silicones) with repeating unit [–SiRR’O–] are

the most important class of inorganic polymers in this

category. Of these, poly(dimethylsiloxanes) (PDMS) with

repeating unit [–Si (CH3)2O–] is commercially available

and used in many interesting applications [2–4]. PDMS

derivatives have been extensively used in the pharmaceu-

tical area, such as in controlled drug delivery systems due

to their biostability, noncarcinogenicity, nontoxicity, bio-

compatibility, and good mechanical properties [5, 6].

Poly(dimethylsiloxane) (PDMS) coatings were prepared

using end-hydroxylated poly(dimethylsiloxane) and dif-

ferent molecular weight polyethylene glycols (PEG) as

channeling agents to control drug release from pharma-

ceutical solid oral dosage forms. It is found that drug

release rate was controlled by the amount and molecular

weight of PEG. In other works, a pan-coating system was

H. Soroory � A. Mashak � A. Rahimi (&)

Iran Polymer and Petrochemical Institute,

P.O. Box: 14965-115, Tehran, Iran

e-mail: a.rahimi@ippi.ac.ir

Iran Polymer and

Petrochemical Institute 123

Iran Polym J

DOI 10.1007/s13726-013-0178-7

used to evaluate the effect of curing agent on drug release

from the tablets coated with the same silicone elastomer [7,

8]. PDMS has been used as a pharmaceutical tablet coating

for possible zero-order release (i.e., the highly desirable

delivery of a constant amount of drug per unit of time) and

a new method was developed to prepare stable PDMS

latexes suitable for spray coating on the drug tablets. The

effect of varying amounts of PEG (Mw of 8000 g/mol) in

PDMS lattices was also investigated on controlled drug

release from pharmaceutical solid oral dosage forms [9].

PVP possesses the following properties: solubility in

most conventional solvents, film formation ability, and

affinity to hydrophilic and hydrophobic surfaces. Also, it is

soluble in many standard pharmaceutical solvents, though

at high concentrations, making the solution highly viscous.

It was found that the aqueous solubility of many drugs such

as acetaminophen is increased in the presence of PVP. The

solubility of this drug at 25 �C is increased in the presence

of PVP. Dialysis studies indicated that the nature of

interaction between PVP and acetaminophen is physical

and reversible, and there is no strong binding between PVP

and acetaminophen in their solution [10]. Water-based

PDMS latex with polyethylene glycols (PEG) channeling

agent has been successfully used to control drug release

from pharmaceutical solid oral dosage forms [11–13].

In this study, water-based emulsion of hydroxy-termi-

nated PDMS with PVP as channeling agent was prepared

and coated on acetaminophen tablets to investigate the

effect of PVP on release behavior of this drug. As PDMS

has hydrophobic character, it is necessary to use a second

phase, e.g., a water-soluble polymer as channeling agent.

Polyvinylpyrrolidone (PVP) was used as a channeling

agent for improving the drug release process. The thermal

and mechanical properties of PDMS films containing PVP

were evaluated using TGA and DMTA techniques.

Experimental

Materials

Hydroxy-terminated polydimethylsiloxane (PDMS) with

viscosity 90–150 cst, tetraethoxysilane (TEOS) as cross-

linking agent, and sodium lauryl sulfate (SLS) as emulsifier

were purchased from Aldrich, USA. Polyvinylpyrrolidone

(PVP) Mw = 36000 g/mol and polyethylene glycol (PEG)

Mw = 6000 g/mol as channeling agents were obtained

from Merck (Germany). Acetaminophen tablets were

kindly provided by Aria Pharmaceutical Co., Iran. Solvents

such as toluene were purchased from Merck (Germany)

and used as received.

Instrumentation

In this study, ultrasonic processor (HD3200, Bandelin,

Germany), laser light scattering (LLS) (SEM-633, France),

infrared spectrometer (EQUINOX55, Bruker, Germany),

scanning electron microscopy (VEGA, TESCAN, The

Czech Republic), energy dispersive X-ray fluorescence

spectroscopy (INCA, Oxford Instrument, UK), differential

scanning calorimetry (DSC-PL, Polymer Laboratories,

UK), thermal gravimetry analysis (TGA-PL1500, Polymer

Laboratories, UK), dynamic mechanical thermal analysis

(DMTA-PL, Polymer Laboratories, UK), and ultra violet

spectroscopy (Shimadzu, Japan) were used.

Method

Preparing emulsion

The emulsions of PDMS in water with 1 % sodium lauryl

sulfate (SLS) as the surfactant were prepared by ultrasonic

processing for 10 min at room temperature. The emulsion

contained 30 (w/w %) PDMS, whereas its average particle

size was obtained 206 nm by LLS measurements. The

processing period affected emulsion properties particularly

its particle size. Particles with sizes greater than 400 nm

were agglomerated during curing and made coating pro-

cesses difficult.

Cross-linking of particles

To achieve good film formation, it is necessary to cross-

link PDMS. Different degrees of cross-linking affected the

characteristics of the resulting films and the specificities of

the pharmaceutical coating. The required cross-linking was

carried out by mixing the desired amount of TEOS (3.25

w/w %) into the PDMS emulsion and then stirring it for

15 min at room temperature. The molar ratio (R) of HCl/

TEOS was 0.5, giving a pH in the acidic range of 1–3. The

mixtures were then stirred at room temperature for 12 h.

Sol fraction

The samples were swollen in toluene and extracted after

4 days to remove any scissioned fragments and unreacted

materials. The networks were then slowly deswollen with

methanol, dried in air for 5 days, and reweighed. Values

for the soluble fraction of polymer, Sf, were then calculated

from the original (Wi) and final (Wf) weights using Eq. (1)

[3].

%Sf ¼Wi �Wf

Wi

� 100 ð1Þ

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Cross-link density

The samples were initially weighed, mi, and immersed in

10 mL toluene. Samples were placed in a linear laboratory

shaker (25 �C and speed 80 cycles/min-1) for 15 ± 1 h.

Then, swollen samples were removed and cautiously dried

to remove excess solvent before weighing, mg. During this

operation, samples were covered to avoid toluene evapo-

ration during weighing. Traces of solvent and other small

molecules were then eliminated by placing the sample in

an oven at 70 �C for 24 h. Finally, the samples were

weighed for the last time, ms, and volume fractions of

polymer in the samples at equilibrium swelling, m2m, were

determined from swelling ratio, G, and calculated as

follows:

G ¼ mg � ms

ms

� qe

qs

ð2Þ

and,

v2m ¼1

1þ Gð3Þ

where, qe and qs are the densities of elastomer and solvent,

respectively.

The samples cross-link density, m, was determined from

measurements in a solvent, using the Flory–Rehner rela-

tionship, given by Eq. (4):

m ¼ �Ln 1� m2mð Þ þ m2m þ v12m22m

V1 m1=32m � 2

u m2m

� � ð4Þ

where, V1 = 106.5 is the molar volume of the solvent

(toluene), m2m is the volume fraction of polymer in the

sample at equilibrium swelling, u = 4 is the cross-link

functionality and v12 = 0.47 is the interaction parameter

between polymer and solvent [8]. The densities of elasto-

mers were measured according to ASTM D-1505 at 23 �C

which changed from 1.088 to 1.144 g/cm3.

Film casting

The PDMS emulsion (20 mL) was cast into a polytetra-

fluoroethylene dish and dried for 8 days at room temper-

ature. The resulting film was placed in a vacuum oven and

dried at 60 �C for 24 h prior to any characterization

measurements.

Channeling agent

Different amounts of PVP (2.5, 5, and 7 w/w %) were added

to the PDMS emulsion and the free film was prepared from

them. PEG with Mw = 6000 (5 w/w % and at 1:1 mixture

with PVP) was used as another channeling agent.

EDXRF (Si-mapping)

PDMS films with and without PVP were prepared and then

placed in liquid nitrogen and after breaking coated with

gold. The samples were observed for Si-mapping.

TGA

A thermogravimetric analyzer was used to investigate the

thermal stabilities of the free-standing films under nitrogen

from 50 to 700 �C at a 10 �C/min heating rate.

DMTA

Samples of pure PDMS with different amounts of PVP in a

cubic form with thickness of 1–3 mm, width of 5–10 mm,

and length of 30–40 mm were tested to determine their

DMTA data. The mode of the tensile test was used (fre-

quency: 1 Hz and displacement: 0.5 mm).

Tablet coating

Tablets were covered with different polymers, e.g., Eu-

deragit, PDMS, and Shellac to control or modify the

release rate. In the pharmaceutical industry to apply these

polymeric coatings on tablets, aqueous or solvent-coating

processes are used [13]. There are different ways to coat

the tablets, e.g., spray and dip coating.

Dip coating is one of the most common techniques

used for coating tablets which were used in this research.

In this technique, drug tablets were immersed in the

PDMS emulsion for 5 min and ejected slowly at a con-

stant speed and dried. Acetaminophen tablets were coated

with PDMS emulsion. Then, they were dried for 3 days at

room temperature, placed in liquid nitrogen and cut to

cross sections. Their cross sections were observed by

SEM technique.

In vitro drug release study

In vitro acetaminophen release study from PDMS-coated

tablets was carried out with a homemade setup for the

release studies at 37 �C [14]. The tablets were tested for

24 h in 100 mL of phosphate buffer, pH 5.8, as release

solution with gentle stirring for maintaining sink conditions.

For the determination of acetaminophen release profile,

three coated tablets were placed separately in release

solution. At different time intervals of 1–24 h, 3 mL of

release media were taken and immediately replaced with a

fresh solution. The concentrations of released acetamino-

phen were measured with a double-beam UV–Vis spec-

trophotometer (Shimadzu UV-1650 PC) at the adsorption

maximum of 242 nm. Polydimethylsiloxane emulsions

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prepared with and without hydrophilic additives are listed in

Table 1.

Result and discussion

LLS analysis

The size of emulsion droplet is important. If it is more than

400 nm, during cross-linking reaction droplets will be

coagulated. Under a specific condition of time and power

of the apparatus, the average size of the droplets was

reduced to 206 nm.

Sol fraction and cross-link density

In this step the polymer chains were cross-linked with

TEOS (3.25 w/w %). To evaluate the progress of the

reaction network, sol fraction and cross-link density were

determined. Sol fraction and cross-link density of the

samples at 3.25 (w/w %) TEOS were 10.388 % and

0.0118 mol/cm3, respectively. Higher sol fraction means

that a lower percentage of chains participated in the cross-

linking reaction. In fact, sol fraction and cross-link density

are criteria for cross-linking reaction.

TGA analysis

Figure 1 shows TGA curves of pure PDMS coating and

PVP-loaded sample in a nitrogen atmosphere. The weight-

loss curves for pure PDMS and PVP-loaded coatings imply

a one-step degradation mechanism.

Partial oxidation of the silicone chain groups into silica

and the formation of species including carbon monoxide,

carbon dioxide, formaldehyde, hydrogen, and water occur

through methyl substitution oxidation. The starting decom-

position temperatures of PDMS and PVP-loaded coatings

are about 400 �C, but at higher temperatures degradation

of the blend occurs slowly. Thus, the thermal stability of

PVP-loaded coatings is somehow higher than that of the pure

PDMS coating. This behavior is due to the chemical inter-

action between carbonyl group of PVP and unreacted OH of

PDMS chains [15]. Therefore, it is demonstrated that the

stability of pure PDMS was improved with the formation of

hydrogen bonding between carboxyl groups of PVP, and the

hydroxyl groups remained in unreacted PDMS during cross-

linking reaction of PDMS.

EDXRF (Si-mapping)

The compatibility of PDMS and PVP greatly affects ther-

mal, mechanical, and release properties of the coating

system. The distribution of PVP phase in PDMS was elu-

cidated using the mapping technique. Figure 2 shows the

EDXRF Si mapping of F0/0 and F5/0. The red points in

these figures denote Si atoms. Figure 2a is related to pure

PDMS coating in which the Si atoms uniformly disperse

throughout the coating. By comparing these two maps in

Fig. 2a, b, it is observed that there is no Si atoms present in

some regions in the PDMS loaded with PVP. It is sug-

gested that this dark regions with no Si atoms belongs to

PVP that acts as channeling agent.

DMTA results

It is found that the mechanical properties of PDMS are

changed by incorporation of PVP. Figure 3a, b shows the

response of PDMS coating to a constant sinusoidal

mechanical stress of constant frequency as a function of

temperature as modulus and tan d, respectively. The values

of the storage modulus decreased as the temperature

increased. For pure PDMS, the a-relaxation attributed to the

glass transition occurred around -110 �C. Above -50 �C

the melting process of PDMS crystallites started and led to a

dramatic drop in the mechanical properties [16].

Viscoelastic properties of the PDMS/PVP films are

given in Fig. 3 for the temperature range of -150 to

150 �C. As the weight fraction of PVP increases in the

blends, an increase in the values of the dynamic modulus

Table 1 Formulation of the prepared samples

Formulation PDMSa (w/w %) PVPa (w/w %) PEGa (w/w %)

F0/0 30 – –

F100 – 100 –

F2.5/0 30 2.5 –

F5/0 30 5 –

F7.5/0 30 7.5 –

F0/5 30 – 5

F5/5 30 5 5

a Values of concentration (w/w %) are related to the solution of

PDMS in water Fig. 1 TGA thermographs of F0/0 and F5/0 film samples

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can be noticed in the temperature range above -30 �C.

Therefore, it can be concluded that the presence of the PVP

phase significantly improves the overall mechanical prop-

erties of the PDMS film [15]. It can be seen that for sam-

ples with more weight fractions of PVP (F5/0 and F7.5/0), an

increase in the storage modulus values occurred. However,

a drop of storage modulus was revaluated for the F2.5/0

sample probably due to the plasticizing effects of PVP at

lower concentration, which thereby caused more flexibility

of PDMS [16].

SEM results

Figure 4 shows the SEM micrographs of the coated tablets

with PVP/PDMS emulsion. The observed morphology

suggests that a uniform smooth film has been achieved by

coating method (Fig. 4a). No crack or porosity was

observed on the thin polymer coating on the tablets. It was

also found that the polymeric layer was uniform and,

throughout, its thickness was 204 lm (Fig. 4b).

Effect of PVP on release behavior of acetaminophen

The PDMS-based coating systems were designed for

acetaminophen tablets to prevent its release into the

stomach. PDMS cannot absorb water due to its hydro-

phobic characteristic. However to improve water intake

properties of PDMS coating, bulk modification is neces-

sary. It is found that addition of different amounts of pol-

yvinylpyrrolidone and polyethylene glycol as channeling

agents can improve its water absorption. These polymers

are soluble when immersed in the release media and drug

release from the polymeric coating is facilitated by diffu-

sion of water through the core and leaching out of the drug

[10].

To investigate the effect of PVP as channeling agent on

acetaminophen release profile, three formulations (F2.5/0,

F5/0, F7.5/0 samples) were designed and studied as presented

in Table 1. The percentage of drug released from coated

acetaminophen tablets with PDMS layer containing dif-

ferent amounts of PVP versus time is shown in Fig. 5.

All coated tablets showed small amounts of drug

releases (0–10 %) during the first 4 h in the release media.

The PVP contents and mechanical properties of the coat-

ings are two main parameters affecting the release behavior

Fig. 2 Si-mappings of a F0/0 and b F5/0 film samples

Fig. 3 DMTA results: modulus (a) and tand (b) for PDMS and

PDMS/PVP film samples

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of PDMS coating. For example, incorporation of higher

amounts of PVP (F7.5/0) resulted in faster release of acet-

aminophen through diffusional pathways due to dissolution

and leaching out of PVP in water, thereby increasing the

rate of drug release. On the other hand, an unusual release

behavior for F2.5/0 and F5/0 was observed, i.e., a higher

amount of drug was released from tablets coated by PDMS

layer with lower contents of PVP, which can be explained

on the basis of the mechanical properties of polymeric

coating, as the mechanical properties of a polymer are

important factors in its drug delivery behavior. As shown in

Fig. 3a, there is a dramatic drop in the mechanical prop-

erties of the F2.5/0 sample compared with F7.5/0 and F5/0

samples regarding DMTA results.

A gradual drop of storage modulus E0 at lower amount

of PVP (F2.5/0) and the resulting flexibility properties may

be due to the plasticizing effect of PVP in lower amounts,

which was confirmed by moving the Tg peak of F2.5/0

toward the Tg peak of PDMS (Fig. 3a). This is an indica-

tion of more miscibility of PDMS and PVP due to their

chemical structures [16]. Consequently, the amount of

acetaminophen released from F7.5/0 sample was more than

that of the F5/0 and F7.5/0 samples.

Effect of both PEG and PVP on release behavior

of acetaminophen

Controlled release formulations have been developed by

coating of acetaminophen tablets with PDMS containing

both PEG and PVP. PEG was also selected as channeling

agent, because it is known to be non-toxic and it has

hydrophilic characteristic to make it soluble in water. It has

been used as excipient for a number of pharmaceutical drug

formulations.

Both PVP and PEG are miscible and through combi-

nation, hydrogen bonding between carbonyl groups of

PVP-repeating units and complementary hydroxyl end

groups of PEG chains are formed. By forming two H bonds

through both terminal OH groups, PEG acts as a reversible

cross-linker for PVP chains [16]. However, when the tab-

lets were coated with PDMS coating containing both PVPFig. 4 SEM micrographs of: a coated tablet (a) and its 204 lm

coating thickness (b)

Fig. 5 Effect of different amounts of PVP (2.5, 5 and 7.5 w/w %) on

drug release rate of PDMS/PVP film samples

Fig. 6 Effect of PVP (F5/0), PEG (F0/5), and PEG ? PVP (F5/5) on

drug release rate of PDMS/PVP/PEG film samples

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and PEG, drug release occurred with lag time and was

reduced due to the slow solubilization of the two chan-

neling agents. The formation of channels took some time.

In this case, drug release took about 400 min. After

400 min, the absorption of water by the coating increased

and H bonds were broken; therefore, PVP and PEG were

swollen and dissolved in water separately. Finally, drug

release was followed by formation of channels.

Figure 6 shows the percentage release of acetaminophen

versus time for F0/5, F5/5 and F5/0 formulations. It is

observed that the highest amount of drug release occurred

for PEG. It can be explained by the fact that PEG is more

soluble in water than PVP; thus, formation of channels can

be easier.

From Fig. 6, it is also clear that drug release from

samples containing both PVP and PEG were unexpectedly

lower than PEG, whereas for coatings with more loading

levels (10 %) of PVP ? PEG as channeling agents, it was

expected that more channels were formed and thus more

drug was released. These phenomena can be explained by

considering molecular interactions between PVP and

PEG.

Conclusion

In this study, PVP and PEG were incorporated as chan-

neling agents in PDMS-based coating for acetaminophen

tablet. TGA and DMTA results showed the PVP effects on

thermal behavior and mechanical properties of the PDMS

polymer. The in vitro drug release profiles from coated

tablets were compared during 24 h. Different behaviors

were observed in drug release profiles on increasing the

amount of channeling agent. It was found that drug release

percentage increased for higher amount of PVP-loaded

sample (F7.5/0) due to enhancing the channel formation in

the release media. Higher drug release percentage was

observed for sample with lower PVP (F2.5/0) compared with

F5/0 due to reduction of mechanical properties. The results

also showed that drug release was increased by the addition

of PEG. It can be concluded that some molecular interac-

tions exist between PVP and PEG that affect their drug

release behavior.

Acknowledgments The financial support from Iran National Sci-

ence Foundation (Grant No. 87041557) is gratefully acknowledged.

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