Moisture diffusion and permeability characteristics ofhydroxypropylmethylcellulose and hard gelatin capsules

Ahmad S. Barhama,*, Frederic Tewes b,c, Anne Marie Healy b

aBasic Sciences Department, College of Engineering and Information Technology, University of Business and Technology, Jeddah, Saudi Arabiab School of Pharmacy and Pharmaceutical Sciences, Trinity College Dublin, Dublin 2, Irelandc INSERM U 1070, Pôle Biologie-Santé, Faculté de Médecine & Pharmacie, Université de Poitiers, Poitiers, France

A R T I C L E I N F O

Article history:Received 10 November 2014Accepted 13 December 2014Available online 16 December 2014

Keywords:Hard capsulesGelatinHPMCDVSSorption–desorptionDiffusionPermeability

A B S T R A C T

The primary objective of this paper is to compare the sorption characteristics of hydroxypropylme-thylcellulose (HPMC) and hard gelatin (HG) capsules and their ability to protect capsule contents.Moisture sorption and desorption isotherms for empty HPMC and HG capsules have been investigatedusing dynamic vapour sorption (DVS) at 25 �C. All sorption studies were analysed using the Young–Nelson model equations which distinguishes three moisture sorption types: monolayer adsorptionmoisture, condensation and absorption. Water vapour diffusion coefficients (D), solubility (S) andpermeability (P) parameters of the capsule shells were calculated. ANOVAwas performedwith the Tukeycomparison test to analyse the effect of %RH and capsule type on S,P, and D parameters. The moistureuptake of HG capsules were higher than HPMC capsules at all %RH conditions studied. It was found thatvalues ofD and P across HPMC capsules were greater than for HG capsules at 0–40 %RH; whereas over thesame %RH range S values were higher for HG than for HPMC capsules. S values decreased gradually as the%RH was increased up to 60% RH. To probe the effect of moisture ingress, spray dried lactose was loadedinto capsules. Phase evolution was characterised by scanning electron microscopy (SEM), X-ray powderdiffraction (XRD), and differential scanning calorimetry (DSC). The capsules under investigation are notcapable of protecting spray dried lactose from induced solid state changes as a result of moisture uptake.For somewhat less moisture sensitive formulations, HPMCwould appear to be a better choice than HG interms of protection of moisture induced deterioration.

ã 2014 Elsevier B.V. All rights reserved.

1. Introduction

In the pharmaceutical field, hard capsules are used as a storagemedium for finely divided blends or formulations containing activepharmaceutical ingredients (APIs) that are to be delivered orally orby inhalation (Hosny et al., 2002; Steckel et al., 2004). Capsulescontaining drugs are usually made of hard gelatin (HG) orhydroxypropylmethylcellulose (HPMC) (Bae et al., 2008; Berntssonet al., 1997).

Gelatin is a naturally occurring protein of animal collagen thathas notable hygroscopic properties and is used to manufacture HG

capsules (Chang et al., 1998). It is a good film-forming materialsuitable for preparing capsule shells that dissolve readily inbiological fluids at body temperature (Pennings et al., 2006).Gelatin has characteristics which make it suitable for the capsulemanufacturing processes, including gels, film-forming and surfaceactive properties (Sherry Ku et al., 2010). However, HG capsulesundergo shell brittleness after exposure to low humidityconditions, are incompatible with hygroscopic materials, suscep-tible to hydrolysis, and inherently reactive toward many sub-stances, including reducing sugars, plasticizers and preservatives(Missaghi and Fassihi, 2006). HPMC capsules proved to be asuitable alternative to gelatin, with many patents granted for themanufacturing process, including thermal gelation and a gellingsystem with additives (Ogura et al., 1998). Moreover, HPMCcapsules have several distinct advantages over HG. Besides the factthat it has no animal-derived raw materials risk, HPMC is a non-ionic polymer and the capsule has fewer compatibility issues withmost drugs and excipients (Ogura et al., 1998). HPMC capsules are

* Corresponding author at: Department Head of Basic Sciences, College ofEngineering and Information Technology, University of Business and Technology, P.O. Box 11020, Jeddah 21361, Saudi Arabia. Tel.: +966 12 2159372;fax: +966 12 2159010.

E-mail address: ahmad.s@ubt.edu.sa (A.S. Barham).

http://dx.doi.org/10.1016/j.ijpharm.2014.12.0290378-5173/ã 2014 Elsevier B.V. All rights reserved.

International Journal of Pharmaceutics 478 (2015) 796–803

Contents lists available at ScienceDirect

International Journal of Pharmaceutics

journa l homepage: www.e lsevier .com/ locate / i jpharm

made from a cellulose–like polymer consisting of glucose unitslinked together byb-1,4 glycosidic linkages and considered to be ahydrophilicmaterial, as characterised by its highmoisture sorptioncharacteristics (Laksmana et al., 2009; Siroka et al., 2008).

A main limitation to the use of hard capsules resulted from anexchange of moisture between the capsule shell and the fill(Strickland and Moss, 1962). The usefulness of such capsules isstrongly dependent on their capacity to protect the contents in thepresence of moisture. The typical moisture content of HG capsulesgenerally may vary between 13 and 16% byweight of water (Changet al., 1998) compared to 2 and 6% for HPMC capsules (Sherry Kuet al., 2010) when received from the suppliers. Sherry et al. (2010)concluded that the water content of the polymeric material of thecapsules is a function of the relative humidity (RH) of thesurroundings and temperature. When the capsules are filled andstored in a vapour tight container, the moisture will redistributebetween the various components until a uniform relative humidityis attained in the capsule shell, fill and surrounding (Sherry Kuet al., 2010).

Lactose is themost widely used excipient in the pharmaceuticalindustries due to its low toxicity, ready availability and compati-bility with the majority of low molecular weight drugs (Guenetteet al., 2009). It is well known that the solid state of lactose can beeither amorphous or crystalline and it exists in two isomeric forms,namely, a-lactose monohydrate and b-lactose (Larhrib et al.,1999). Amorphous lactose can be prepared by spray drying orfreeze drying. Spray dried lactose is thermodynamically unstableand hygroscopic. It has a tendency to gain moisture from itssurroundings with ease and subsequently plasticize or cake(Barham and Hodnett, 2005). Several researchers have investigat-ed the crystallisation kinetics of lactose at different relativehumidities at room temperature. They found that the amorphouslactose will initially sorb moisture from its surroundings and thenrelease the moisture when it crystallizes. This process will occurspontaneously above 50% RH at 25 �C (Barham and Hodnett, 2005;Islam et al., 2010; Jouppila et al., 1997; Shrestha et al., 2007).

In general, the overall aim of the current studywas to determinethe effectiveness of the capsules at protecting a moisture sensitivecompoundand identifyingwhich isbetter in this regard.Amorphouslactose was chosen as a moisture sensitive model compound toinvestigate the impact of encapsulation methods such as hardcapsules on lactose stability upon exposure to controlled humidityenvironments. Evolution of lactose phases obtained upon crystal-lisation and their interactions with water vapour were evaluated.Sorption–desorption isotherms, water permeability, solubility, anddiffusion coefficients of empty HPMC and HG capsules weredetermined at various relative humidity values at 25 �C.

2. Materials and methods

2.1. Materials

2.1.1. Hard capsulesHard gelatin (HG) capsules of size no. 3 were purchased from

Farillon Ltd. (Essex, UK). Hydroxypropylmethylcellulose (HPMC)capsules of size no. 3 were received as a gift from Capsugel1,France. Specifications of HPMC capsules were the same for bodyand cap, i.e. Coni-snap (V43.700), Vcaps1 Capsules (Natural TR.V900). Hypromellose (E464) was 100% of the total HPMC capsulecomposition.

2.1.2. Preparation of spray-dried lactoseAnhydrousspray-dried lactosewasproducedbyspraydryinga5%

(w/v) a-lactose monohydrate (Sigma–Aldrich, Ireland) solution indeionised water with a Büchi 290 mini spray dryer (Büchi

Labortechnik GmbH, Germany), using a standard 2-fluid nozzlewith a 0.7mm tip and 1.5mm cap. The spray drying process wascarriedout in theopenmodeat8ml/min solution feed rate. The inlettemperature was adjusted to 160 �C and the resultant outlettemperature was 95–97 �C. Aspirator setting and the atomising airflowratewere setat40m3/hand473 l/h, respectively.After thespraydrying process, anhydrous lactose was collected in air tight glasscontainers and kept in desiccators containing silica gel to protect itfrom environmental humidity. Amorphicity of the spray-driedlactosewas verified byX-ray diffraction as described in Section 2.4.3DeionisedwaterusedinthisworkwasHPLCgradeandobtainedfroma Purite Prestige Analyst HP water purification system.

2.2. Methods

2.2.1. Dynamic vapour sorption (DVS)Moisture sorption and desorption characteristics of empty

HPMC and HG capsules was determined at a constant tempera-ture of 25�0.1 �C using a DVS Advantage-1 automated gravi-metric vapour sorption analyser (Surface Measurement Systems,London, UK). The DVS-1 measures the ingress and loss of watervapour gravimetrically with a mass resolution of �0.1mg. Prior tobeing exposed to any vapour, capsules were equilibrated at 0% RHto establish a dry reference mass. After drying, all empty capsuleshells in the DVS were exposed to a stepwise increase of %RH (0%;20%; 30%; 40%; 50%; 60%; 70%). The same %RH profile wasemployed for desorption. At each stage, the equilibriumbehaviour was defined when the mass variation versus timedm/dt was �0.002mg/min for at least 10min before the partialpressure was increased or decreased. An isotherm was thencalculated from the completed sorption and desorption profilesusing the DVS-1 analysis software, Surface Measurement Sys-tems1, 2003. The amount of water taken up by the capsules wasexpressed as a percentage of the dry capsule mass (equilibrated at0% RH). All DVS measurements reported in this work wereconducted in triplicate.

2.2.2. Mathematical modelling: moisture distribution analysis usingthe Young–Nelson equations

The Young–Nelsonmodel equationswerefitted to the sorption–desorption data of the isotherms. The model can differentiatebetween bound monolayer, normally condensed, externallyadsorbed moisture and internally absorbed water and is basedon equations of the form (Bravo-Osuna et al., 2005; Kachrimaniset al., 2006; Tewes et al., 2010):

Ms ¼ Aðbþ uÞ þ BuRH (1)

Md ¼ Aðbþ uÞ þ BuRHmax (2)

where Ms and Md are, respectively the mass percentage of watersorbed and desorbed on the polymers at the equilibrium for each %RH. A and B are constants characteristic of each system. In thismodel, u is the fraction of the surface covered by at least one layerof water molecules Eq. (3), where E is an equilibrium constantbetween monolayer water and the normally condensed wateradsorbed externally to the monolayer (Bravo-Osuna et al., 2005;Kachrimanis et al., 2006), and b is defined by Eq. (4).

u ¼ RHRH þ ð1� RHÞE (3)

b ¼ � E� RHE� ðE� 1ÞRH þ E2

E� 1ln

E� ðE� 1ÞRHE

� �

� Eþ 1ð Þlnð1� RHÞ (4)

A.S. Barham et al. / International Journal of Pharmaceutics 478 (2015) 796–803 797

Au is the mass of water in a complete adsorbed monolayerexpressed as a percentage of the dry mass of each system. A(b + u)is the total amount of adsorbed water, and Ab is the mass of waterwhich is adsorbed beyond the mass of the monolayer (i.e. inmultilayer or cluster adsorption). B is themass of absorbedwater at100% of RH, and, hence, BuRH is the mass of absorbed water whenthe water coverage is u for a given %RH. The experimental datawere fitted to Eqs. (1) and (2) by means of an iterative multiplelinear regression using, as fitting criteria, the sum of the squares ofthe residuals between the experimental and the calculated values.The degree of adjustment was expressed by the multiplecorrelation coefficients (Microsoft1 Excel 2007). According tothe model characteristics, from the estimated values of A, B, and E,the corresponding profiles of water adsorbed in monolayer (Au),multilayer (Ab) and absorbed (BuRH) were obtained.

2.2.3. Determination of diffusion coefficientsWater sorption–desorption kinetics obtained for different %RH

were analysed in order to determine the diffusion coefficient (D) ofwater molecules in the capsule walls using the Crank's solution toFick's 2nd law for gaseous diffusion in a planar sheet [15–16](Eq. (5)):

Mt

Meq¼ 4

l

ffiffiffiffiffiffiDtp

r(5)

whereMt is the amount ofmoisture sorbed by the capsule at a timet,Meq is the corresponding mass sorbed at equilibrium, and l is thethickness of the capsule wall. This relationship is linear at theinitial condition, that is for 0.1�Mt/Meq�0.5, and was used tocalculate D. The wall thickness (l) of HPMC and HG capsules wasaccurately determined using a Zeiss AxioVision optical microscope(Carl Zeiss Microimaging, Göttingen, Germany). Cross sectionalimages of the capsules were collected after exposing the capsulesindividually to a series of constant %RH environments of 0 %RH, 40%RH, and 70 %RH at 25 �C in the DVS apparatus. Equilibrium wasdefined for each %RHwhen the mass variation versus time (dm/dt)was�0.002mg/min for at least 10min. All the optical images wereexamined using AxioVs V 4.7.0.0 software in order to determine thecross section thickness (l) of each capsule studied. The l value wascalculated as an average of three capsules of each type in a series ofapproximately 10 measurements at magnification levels of�200 and �400.

2.2.4. Calculation of permeability and solubility coefficientsWater permeation coefficients P [(Kg moisture/m3 capsules

Pa)� (m2/s)] across HPMC and HG capsules were calculated fromthe relationship: P = SD,where S (Kg moisture/m3 capsules Pa) andD (m2/s) are the solubility and diffusion coefficients respectively ofwatermolecules at a given RH condition. The solubility coefficientswere calculated from the equilibrium moisture content data using(Gouanve’ et al., 2007; Mwesigwa et al., 2008):

S ¼ cp

(6)

This relationship defines the solubility coefficient in terms ofthe vapour pressure (p, Pa) exerted by the water above the capsule.The term c is the equilibrium concentration of water in the capsuleshell and was calculated using Eq. (7), the volume of the capsulewall (Vp,m3) and the difference between the final mass (Meq, kg) tothe initial mass (M0, kg) of the capsule during water ingress(Gouanve’ et al., 2007; Mwesigwa et al., 2008).

c ¼ Meq �M0

Vp(7)

True density measurements of HPMC and HG capsules and Vp

were determined by an AccuPyc 1330 Pycnometer (Micromer-iticsTM) using helium gas (99.995% purity). All capsules were driedin the DVS apparatus at 0% RH (25 �C) prior to density analysis. Thepycnometer was calibrated immediately before performing theanalysis at room temperature. A 1 cm3 sample cup was used.During each analysis the evacuation rate was 0.034 kPa/min, thenumber of purges and runs was 5. Measurements were carried outin triplicate on each empty capsule and the averaged results wererecorded.

2.3. Statistical analysis

Analysis of variance (ANOVA) was performed using a generallinearmodelwith the Tukey comparison test usingMinitab Release16.2.3. For all tests, p�0.05 was used as the criterion to assessstatistical significance.

2.4. Characterisation of physicochemical properties of lactose

2.4.1. Capsules filled with spray-dried lactosePrior to sorption–desorption experiments being conducted, all

capsules were filled manually with approximately 10–12mg ofanhydrous spray dried lactose and were immediately transferredto the dynamic vapour sorption (DVS) apparatus, held at 25 �C.

2.4.2. Scanning electron microscopyScanning electron micrographs of anhydrous spray dried

lactose and lactose, which had been loaded into HPMC or gelatincapsules and following the DVS experiments, were captured usinga Tescan Mira XMU (U.S.A) variable pressure scanning electronmicroscope. All samples were fixed on an aluminium stubs withdouble-sided adhesive tabs and a 10nm thick gold film was thensputter coated on the samples before visualisation.

2.4.3. X-ray diffractionX-ray powder diffraction measurements (XRD) were conducted

on samples in low background silicon mounts, using a RigakuMiniflex II, desktop X-ray diffractometer (Japan) with the Ilaskriscooling unit. The samples were scanned over a range of 5–40� 2uusing a step size of 0.05� 2u per second. The X-ray sourcewas Cu Karadiation (l =1.542Å) with Ni-filter suppressing Kb radiation. TheCu tube was run under a voltage of 30kV and a current of 15mA.

2.4.4. Differential scanning calorimetryThe thermal behaviour of spray dried lactose samples was

studied using a PerkinElmer Pyris Diamond differential scanningcalorimeter. The instrument was calibrated using indium (mp156.6 �C; DH =28.45 J/g). Approximately 2–4mg of sample wasaccurately weighed into a sealed aluminium pan. An emptyaluminium sample pan was placed in the reference holder andboth holders were covered with platinum lids. Sample andreference pans were heated up to 240 �C at 20 �C/min using N2

as a purge gas (40ml/min), and the heat flow (mW) was measuredas a function of temperature.

2.4.5. Particle sizingParticle size measurements of anhydrous spray dried lactose

were determined as previously described using a MalvernMastersizer 2000 (Nolan et al., 2009).

798 A.S. Barham et al. / International Journal of Pharmaceutics 478 (2015) 796–803

3. Results and discussion

3.1. Moisture sorption and desorption isotherms of the capsules

Sorption and desorption isotherms for the capsules aredisplayed in Fig. 1A. This figure was constructed from the averageequilibrium values of the moisture contents of the capsulesobtained at each %RH interval. Data indicated that the maximumsorption capacity of water vapour is significantly higher for HGthan for HPMC capsules. The maximum mass gain was14.97%�0.32 for HG capsules, which was significantly differentto the HPMC capsules (10.60%�0.05). When the %RH wasgradually increased from 0 to 20%, the amount of water vapoursorbed for HPMC capsule was equal to 2.15%�0.03, which wassignificantly lower than the amount sorbed by the HG capsule4.37%�0.17. This step is generally attributed to the surfaceadsorption process that is typically limited to only a few percentincrease in mass (Burnett et al., 2006). The isotherm obtained forHG could be related to the Type IV isotherm of the IUPACclassification, obtained with mesoporous adsorbent. This isothermrepresents unrestricted monolayer–multilayer adsorption. Theisotherm obtained for HMPC can be related to the Type V isotherm,as observed by Villalobos et al. (2006). In such an isotherm, the

adsorbent-adsorbate interaction is weak as compared with theadsorbate-adsorbate interactions and the material are mesopo-rous.

Fig. 1B presents the extent of hysteresis between desorptionand sorption processes for HPMC and HG capsules isotherms.Hysteresis was calculated from the difference between the netmass equilibrium values of the capsules revealed for desorptionand sorption processes at certain %RH values. The degree of thehysteresis was then calculated according to Eq. (8) as described by(Okubayashi et al., 2004; Siroka et al., 2008), where:Mdesorption andMsorption were the equilibrium moisture gains in desorption andsorption phases, respectively, at the same %RH. At 20% RH, thehysteresis value of HG capsules was the highest among all thestudied capsules, at 66%, andwas then gradually decreased to 8% at60% RH. At 40% RH, the hysteresis valuewas 30% for HPMC capsulesand this decreased to 11% at 60% RH. This trend could be attributedto the effects ofmoisture contents and themoisture holding abilityof the capsule walls (Okubayashi et al., 2004).

Hysteresisð%Þ ¼ Mdesorption �Msorption

Msorption� 100% (8)

There can be a variety of reasons for the occurrence of isothermhysteresis. For example, hysteresis appearing in the multilayerrange of physisorption isotherms is usually associated withcapillary condensation in mesopore structures. However, foramorphous or partially amorphous polymers, hysteresis is oftendue to bulk absorption of water, which may also result in swellingeffects (Hill et al., 2009). The higher the hysteresis value, the morewater molecules are retained within the capsule shells. Thepresence of hysteresis between the sorption and desorption

[(Fig._1)TD$FIG]

0 10 20 30 40 50 60 70 800

5

10

15

20

Ne

t C

ha

ng

e i

n M

as

s (

%)

Relative Humidity (%)

(A)

20 40 600

25

50

75

(B)

Hys

tere

sis

(%

)

Relative Humidity (%)

Fig. 1. Comparisons of the HPMC and HG capsules measured using DVS at 25 �C(n =3) of: (A) Moisture sorption and desorption isotherms, and (B) Hysteresis (%)values between the desorption and sorption processes calculated from sorption–desorption isotherms.

[(Fig._2)TD$FIG]

0.0%

1.0%

2.0%

3.0%

4.0%

5.0%

6.0%

7.0%

8.0%

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Mo

istu

re C

on

ten

t (%

)

Relative Humidity

HPMC CapsulesMonolayer

Multilayer

Absorbed

A = 3.78B = 5.91E = 0.57

0.0%

1.0%

2.0%

3.0%

4.0%

5.0%

6.0%

7.0%

8.0%

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Mo

istu

re C

on

tent

(%)

Relative Humidity

HG CapsulesMonolayer

Multilayer

Absorbed

A = 3.94B = 10.25E = 0.13

Fig. 2. The predicted water distributions according to the Young–Nelson model forHPMC andHG capsules presented asmonolayer, multilayer, and absorbedmoisture.

A.S. Barham et al. / International Journal of Pharmaceutics 478 (2015) 796–803 799

isotherms of the capsules indicated that the diffusion of watermolecules from the bulk to the surface was slower than surface tothe bulk. Hysteresis of HG capsules was significantly decreasedwhen the %RH increased. HG capsules retainedmoremoisture thanHPMC capsules at lower %RH, as observed from Fig. 1B.

The most well-known approach for modelling hysteresis inorganic polymer isotherms is that of Young and Nelson (Fig. 2). ForHG capsules, absorption represents themainway inwhichwater istaken up, as can be confirmed by the low values of the A parameter,compared with those of the B parameter of the Young–Nelsonequations (Fig. 2). Moreover, HG capsules were saturated by awatermonolayer from20% RH (3.0wt%). In contrast themonolayerdevelopment in HPMC capsules increased slowly up to 70% RH(2.5wt%), showing a weak water-HMPC interaction as comparedwith the water-water interactions in HG capsules. During theformation of the water monolayer on the surface of HPMCcapsules, water molecules were adsorbed as multilayers and aswell as being absorbed in the same proportions. Whereas, for HGcapsules, water molecules were, interestingly, absorbed between20% RH (approximately 2.0wt%) up to 70% RH (approximately8.0wt%).

Fig. 3 demonstrates the swelling phenomenon which occurredin HPMC and HG capsule shells in a manner such as to change theirthickness dimensions during the moisture ingress experiments. InHPMC capsules, when the %RH was changed from 0 to 40% thethickness of the capsule wall (l) increased. Surprisingly a furtherincrease in %RHup to 70% lead to a notable decrease in the l value ofHPMC. From 0 to 40% RH for HG, l values were increased and werenot further affected when the RH was increased up to 70% RH. Thisswelling behaviour of the capsules can be attributed to the waterabsorbed, as predicted by Young and Nelson equations.

3.2. Diffusion, solubility and permeability coefficients of the capsules

Fig. 4A presents a comparison of the solubility coefficients (S)calculated from the sorption and desorption isotherm character-istics of all HPMC and HG capsules studied in this work. In general,all S values obtained for HPMC and HG capsules decreasedprogressively when the %RH increased up to 30% and levelled offfrom 40% RH up to 70% RH.

Mwesigwa et al. (2008) claimed that the S parameter identifiedthe amount of water distributed in the polymer films underequilibrium conditions in relation to the amount present in thevapour phase above the film. It can be interpreted as a partitioncoefficient of watermolecules between the two phases (Mwesigwaet al., 2008).

Between 0 and 40% RH in the sorption process, D values acrossHPMC capsules were as high as 69.1�10�14m2/s and decreased by2-fold at 70% RH (Fig. 4B). In HG capsules, D values increasedgradually up to 60% RH to 32.7�10�14m2/s and then decreased to24.9�10�14m2/s at 70% RH. In the desorptionprocess,maximumDvalues were obtained for both HPMC and HG capsules at 50% RH.Lower water D values obtained across HG capsules could result inhigher interaction of water molecules with HG than with HPMC.

[(Fig._3)TD$FIG]

0

40

80

120

2 62 92

Cap

sule

thic

knes

s la

yer (

l,µ

m)

Relative Humidity (%)

HPMC Capsules

HG Capsules

Fig. 3. Effect of % Relative Humidity on HPMC and HG capsules thickness layers (l)measured after DVS experiments at 25 �C (n =3).

[(Fig._4)TD$FIG]

0-20 20-30 30-40 40-50 50-60 60-700.00

0.06

0.12

0.18

(A)

S(K

g m

ois

ture

/ (m

3 C

ap

su

les

Pa

))Relative Humidity (%)

0-20 20-30 30-40 40-50 50-60 60-700.0

4.0x10-13

8.0x10-13

1.2x10-12

1.6x10-12

D(m

2 /s)

Relative Humidity (%)

(B)

0-20 20-30 30-40 40-50 50-60 60-700.0

1.0x10-14

2.0x10-14

3.0x10-14

(C)

P [

(Kg

mo

istu

re/m

3ca

ps

ules

Pa)

x (m

2/s

)]

Relative Humidity (%)

Fig. 4. Water vapour parameters determined for HPMC and HG capsule shells at25 �C (n =3) of: (A) water solubility coefficients S, (B) water diffusion coefficients D,and (C) water permeability coefficients P.

800 A.S. Barham et al. / International Journal of Pharmaceutics 478 (2015) 796–803

In a recent study, the water diffusion coefficient throughamorphous HPMC films stored in a wide range of %RH andtemperatures was predicted. The authors found that D valuesranged from 600�10�14 to 2.4�10�14m2/s (Laksmana et al.,2009). When the glassy HPMC films take up moisture from theenvironment, water molecules induce both swelling of the filmsand the reduction of the glass transition temperature. The diffusionof the small water molecules across the polymer films wasassumed to occur through the free volume in the polymer film(Laksmana et al., 2009).

Permeability parameters (P) calculated for HPMC and HGcapsules are shown in Fig. 4C. In HPMC capsules during thesorption process, the maximum P value was observed at 20% RHand it decreased gradually up to 70% RH. A similar trend wasobserved for HG capsules. The P value for HPMC was greater thanHG by 5-fold at 20% RH and levelled off for both capsules at 60% RH.Interestingly, P values for HPMC and HG capsules exhibited asimilar trend in the desorption process when moving from 70% RHup to 30% RH. At 20% RH, the P value for HPMC was 2-fold greaterthan that of HG.

Between 50 and 70% RH, the overall water permeabilitymeasured across HPMC capsules was smaller than for HG capsules.

At this range the fluxofwatermolecules acrossHPMC capsuleswaslower compared to HG capsules. This could be explained by theswelling behaviour and the free volume space of the HPMCcapsule.

The significance of the capsule type and %RH on the response ofS, D, and P parameters was assessed using ANOVA. All the P-valuesobtained from the statistical analyses were less than 0.05 in thiswork. It can be concluded that the S, D, and P parameters aresignificantly different for the two capsule types at the different %RH values.

3.3. Effect of capsules as a moisture buffer upon moisture ingress bylactose using DVS

Fig. 5A presents the SEMmicrograph of free flowing spray driedlactose recorded after the spray drying process. The generalappearance of the anhydrous lactose particles consisted ofspherical shaped particles with smooth surfaces. These particleshave a median particle size less than 5mm, as determined by thelaser diffraction technique. Anhydrous lactose was then loadedinto HPMC and HG capsules to examine their effectiveness inprotecting the contents from ambient conditions. These loaded

[(Fig._5)TD$FIG]

Fig. 5. SEM micrographs of: (A) anhydrous spray-dried lactose (directly recorded after the spray-drying process), (B) free flowing powder of spray dried lactose afterrecrystallisation in DVS, (C) spray dried lactose stored in HPMC capsule after recrystallisation in DVS, and (D) spray dried lactose stored in HG capsule after recrystallisation inDVS. Samples of B, C, and D were recorded immediately without grinding following the DVS experiments between 0%–70%–0% RH for consecutive 12 steps at 25 �C(magnification of 5K).

A.S. Barham et al. / International Journal of Pharmaceutics 478 (2015) 796–803 801

capsules were exposed to increasing RH from 50% to 70% RH. AfterDVS experiments, SEM micrographs revealed that the crystallisa-tion of lactose from the amorphous state (Fig. 5A) led to theformation of a plate-like crystalline habit (Fig. 5B–D). This resultwas in a good agreement with previous findings relating to thechange in habit of freely powder form of spray dried lactose uponcrystallisation in humid air (Barham and Hodnett, 2005). Further-more, the kinetics of water ingress observed during the change of %RH from 50 to 70% for of the free flowing powder the free as well asthe encapsulated lactose within HPMC and HG capsules showed adecrease in mass ingress during the equilibrium stage (Fig. 6). Thedecrease in mass could be explained by the crystallisation of theamorphous spray dried lactose, which expels moisture oncrystallisation (Ambarkhane et al., 2005).

The PXRD pattern of spray dried lactose, recorded directly afterthe spray-drying process was typical of X-ray amorphous material,showing an amorphous “halo” in the diffraction pattern (Fig. 7,trace A). FollowingDVS experiments, lactose examined byXRDwascrystalline and resulted in a mixture of two anomers of lactose,namely a-lactose monohydrate characterized by 2 Bragg peaks ofdiffraction at 2u =12.5�, 16.4� and anhydrous b-lactose character-ized a Bragg peak at 2u =10.5� (Fig. 7, traces B–D) (Barham andHodnett, 2005).

The DSC thermogram of lactose recorded after the spray dryingprocess (Fig. 8A) exhibited a change in the heat capacity of 0.54 J/g �C with an onset value at 118 �C, indicative of a glass transition.This event was followed by a single exothermic peak with onset at180 �C, peaking at 194 �C (DH =�26 J/g) and characteristic of acrystallization step. An endothermic melting feature of anhydrousa-lactose occurred at 209 �C (onset), peaking at 215 �C (DH =29 J/g). This phase decomposed and no further thermal behaviour wasobserved up to 240 �C (Garnier et al., 2008).

After DVS experiments, evolution of lactose phase’s uponexposure to moist air resulted in a mixture of a-lactosemonohydrate and anhydrous b-lactose. This was confirmed bythe DSC thermal behaviour of lactose as presented in Fig. 8B.Hence, a-lactose monohydrate exhibited an endothermic peakassociated with the dehydration, and of water molecules beingremoved from the crystal lattice. The dehydration process wascharacterised byan onset step observed at 139 �C, peaking at 145 �C(DH =111 J/g) (Lehto et al., 2006). However, two successiveendothermic peaks were consequently observed as a result ofa-lactose melting at 212 �C, peaking at 219 �C (DH =68 J/g) andb-lactose melting onset at 228 �C, peaking at 233 �C (DH =65 J/g),respectively (Islam and Langrish, 2010).

In Fig. 8, traces C and D present the thermal behaviour of lactoseloaded into HPMC and HG capsules, respectively. As observed inthese traces, the thermal events such as dehydration and meltingof lactose phaseswere similar to those observed in Fig. 8B, showingthat HPMC and HG capsules were not able to protect amorphouslactose from crystallisation.

4. Conclusions

In this study moisture sorption and desorption isotherms weredetermined for HPMC andHGhard capsules that arewidely used inthe pharmaceutical industry. It was observed that values of S, D, Pparameters were significantly affected by the factors analysed inthis study i.e. capsule types and %RH. Thus, different moisturecharacteristic behaviour as well as differentwater fluxoccurred forthe same %RH range studied in HPMC and HG capsules. Moisturesorption resulted in crystallisation of the loaded lactose intoa-lactose monohydrate and anhydrous b-lactose. Therefore,neither capsule type adequately protected the contained

[(Fig._7)TD$FIG]

Pos. [°2 Th.]10 20 30

Inte

nsity

(a.

u.)

500(a.u.)

1000(a.u.)

-Lactose

monohydrate

2=12.5º

-Lactose

2=10.5º

Fig. 7. Typical XRD patterns of: (A) anhydrous spray-dried lactose (directlyrecorded after the spray-drying process), (B) free flowing powder of spray driedlactose after recrystallisation in DVS, (C) spray dried lactose stored in HPMC capsuleafter recrystallisation in DVS, and (D) spray dried lactose stored in HG capsule afterrecrystallisation in DVS. Samples of B, C, and D were recorded immediately withoutgrinding following the DVS experiments between 0%–70%–0% RH for consecutive12 steps at 25 �C.

[(Fig._6)TD$FIG]

Fig. 6. Kinetic profiles generated by exposure to increasing humidity in DVS from50% to 70% RH of: (A) anhydrous spray dried lactose, (B) anhydrous spray driedlactose stored in HG capsule, and (C) anhydrous spray dried lactose stored in HPMCcapsule.

[(Fig._8)TD$FIG]

Fig. 8. Comparative DSC thermograms of: (A) anhydrous spray-dried lactose(directly recorded after the spray-drying process), (B) free flowing powder of spraydried lactose after recrystallisation in DVS, (C) spray dried lactose stored in HPMCcapsule after recrystallisation in DVS, and (D) spray dried lactose stored in HGcapsule after recrystallisation in DVS. Samples of B, C, and D were recordedimmediatelywithout grinding following the DVS experiments between 0%–70%–0%RH for consecutive 12 steps at 25 �C.

802 A.S. Barham et al. / International Journal of Pharmaceutics 478 (2015) 796–803

hygroscopic amorphous lactose from crystallisation or deteriora-tion which was induced by moisture ingress, possibly impairingthe formulation stability. Overall, HPMC capsules would be moreappropriate to use than HG for a formulation that was not quite asmoisture sensitive.

Acknowledgements

This work was funded by Science Foundation Ireland (Grants07/SRC/B1154 and 12/RC/2275) & Enterprise Ireland (Grant CFTD/06/119) under the National Development Plan.

References

Ambarkhane, A.V., Pincott, K., Buckton, G., 2005. The use of inverse gaschromatography and gravimetric vapour sorption to study transitions inamorphous lactose. Int. J. Pharm. 294, 129–135.

Bae, H.J., Cha, D.S., Whiteside, W.S., Park, H.J., 2008. Film and pharmaceutical hardcapsule formation properties of mungbean waterchestnut, and sweet potatostarches. Food Chem. 106, 96–105.

Barham, A.S., Hodnett, B.K., 2005. In situ X-ray diffraction study of thecrystallization of spray-dried lactose. Cryst. Growth Des. 5, 1965–1970.

Berntsson, O., Zackrisson, G., Ã-stling, G., 1997. Determination of moisture in hardgelatin capsules using near-infrared spectroscopy: applications to at-lineprocess control of pharmaceutics. J. Pharm. Biomed. Anal. 15, 895–900.

Bravo-Osuna, I., Ferrero, C., Jiménez-Castellanos, M.R., 2005. Water sorption–desorption behaviour of methyl methacrylate-starch copolymers: effect ofhydrophobic graft and drying method. Eur. J. Pharm. Biopharm. 59, 537–548.

Burnett, D.J., Garcia, A.R., Thielmann, F., 2006. Measuring moisture sorption anddiffusion kinetics on proton exchange membranes using a gravimetric vaporsorption apparatus. J. Power Sources 160, 426–430.

Chang, R.K., Raghavan, K.S., Hussain, M.A., 1998. A study on gelatin capsulebrittleness: moisture transfer between the capsule shell and its content. J.Pharm. Sci. 87, 556–558.

Garnier, S., Petit, S., Mallet, F., Petit, M.N., Lemarchand, D., Coste, S., Lefebvre, J.,Coquerel, G., 2008. Influence of ageing: grinding and preheating on the thermalbehaviour of a-lactose monohydrate. Int. J. Pharm. 361, 131–140.

Gouanve’, F., Marais, S., Bessadok, A., Langevin, D., Me’tayer, M., 2007. Kinetics ofwater sorption in flax and PET fibers. Eur. Polym. J. 43, 586–598.

Guenette, E., Barrett, A., Kraus, D., Brody, R., Harding, L., Magee, G., 2009.Understanding the effect of lactose particle size on the properties of DPIformulations using experimental design. Int. J. Pharm. 380, 80–88.

Hill, C.A.S., Norton, A., Newman, G., 2009. The water vapor sorption behavior ofnatural fibers. J. Appl. Polym. Sci. 112, 1524–1537.

Hosny, E.A., Al-Shora, H.I., Elmazar, M.M.A., 2002. Oral delivery of insulin fromenteric-coated capsules containing sodium salicylate: effect on relativehypoglycemia of diabetic beagle dogs. Int. J. Pharm. 237, 71–76.

Islam,M.I.U., Langrish, T.A.G., 2010. An investigation into lactose crystallization underhigh temperature conditions during spray drying. Food Res. Int. 43, 46–56.

Islam, M.I.U., Langrish, T.A.G., Chiou, D., 2010. Particle crystallization during spraydrying in humid air. J. Food Eng. 99, 55–62.

Jouppila, K., Kansikas, J., Roos, Y.H., 1997. Glass transition, water plasticization, andlactose crystallization in skim milk powder. J. Dairy Sci. 80, 3152–3160.

Kachrimanis, K., Noisternig, M.F., Griesser, U.J., Malamataris, S., 2006. Dynamicmoisture sorption and desorption of standard and silicified microcrystallinecellulose. Eur. J. Pharm. Biopharm. 64, 307–315.

Laksmana, F.L., Hartman Kok, P.J.A., Vromans, H., Van der Voort Maarschalk, K.,2009. Predicting the diffusion coefficient of water vapor through glassy HPMCfilms at different environmental conditions using the free volume additivityapproach. Eur. J. Pharm. Sci. 37, 545–554.

Larhrib, H., Zeng, X.M., Martin, G.P., Marriott, C., Pritchard, J., 1999. The use ofdifferent grades of lactose as a carrier for aerosolised salbutamol sulphate. Int. J.Pharm. 191, 1–14.

Lehto, V.-P., Tenho, M., Vähä-Heikkilä, K., Harjunen, P., Päällysaho, M., Välisaari, J.,Niemelä, P., Järvinen, K., 2006. The comparison of seven different methods toquantify the amorphous content of spray dried lactose. Powder Tech. 167,85–93.

Missaghi, S., Fassihi, R., 2006. Evaluation and comparison of physicomechanicalcharacteristics of gelatin and hypromellose capsules. Drug Dev. Ind. Pharm. 32,829–838.

Mwesigwa, E., Basit, A.W., Buckton, G., 2008. Moisture sorption and permeabilitycharacteristics of polymer films: Implications for their use as barrier coatingsfor solid dosage forms containing hydrolyzable drug substances. J. Pharm. Sci.97, 4433–4445.

Nolan, L.M., Tajber, L., McDonald, B.F., Barham, A.S., Corrigan, O.I., Healy, A.M., 2009.Excipient-free nanoporous microparticles of budesonide for pulmonarydelivery. Eur. J. Pharm. Sci. 37, 593–602.

Ogura, T., Furuya, Y., Matsuura, S., 1998. HPMC capsules–an alternative to gelatin.Pharm. Tech. Eur. 10, 32–42.

Okubayashi, S., Griesser, U.J., Bechtold, T., 2004. A kinetic study of moisture sorptionand desorption on lyocell fibers. Carbohydr. Polym. 58, 293–299.

Pennings, F.H., Kwee, B.L.S., Vromans, H., 2006. Influence of enzymes andsurfactants on the disintegration behavior of cross-linked hard gelatin capsulesduring dissolution. Drug Dev. Ind. Pharm. 32, 33–37.

Sherry Ku, M., Li, W., Dulin, W., Donahue, F., Cade, D., Benameur, H., Hutchison, K.,2010. Performance qualification of a new hypromellose capsule: part Icomparative evaluation of physical, mechanical and processability qualityattributes of Vcaps Plus1, Quali-V1 and gelatin capsules. Int. J. Pharm. 386,30–41.

Shrestha, A.K., Howes, T., Adhikari, B.P., Bhandari, B.R., 2007. Water sorption andglass transition properties of spray dried lactose hydrolysed skim milk powder.LWT-Food Sci. Tech. 40, 1593–1600.

Siroka, B., Noisternig, M., Griesser, U.J., Bechtold, T., 2008. Characterization ofcellulosic fibers and fabrics by sorption/desorption. Carbohydr. Res. 343,2194–2199.

Steckel, H., Borowski,M., Eskandar, F., Villax, P., 2004. Selecting lactose for a capsule-based dry powder inhaler. Pharm. Tech. Eur. 16, 23–35.

Strickland, W.A., Moss, M., 1962. Water vapor sorption and diffusion through hardgelatin capsules. J. Pharm. Sci. 51, 1002–1005.

Tewes, F., Tajber, L., Corrigan, O.I., Ehrhardt, C., Healy, A.M., 2010. Development andcharacterisation of soluble polymeric particles for pulmonary peptide delivery.Eur. J. Pharm. Sci. 41, 337–352.

Villalobos, R., Hernández-Muñoz, P., Chiralt, A., 2006. Effect of surfactants on watersorption and barrier properties of hydroxypropyl methylcellulose films. FoodHydrocolloid 20, 502–509.

A.S. Barham et al. / International Journal of Pharmaceutics 478 (2015) 796–803 803