PROTOCOL TO BUILD 2D TRANSPARENT PDMS MICROMODELS FOR USE IN DRYING EXPERIMENTS
Luis A. Segura, Guillermo M. Badillo, Fabiola V. Villagra and Daniela L. Garrido Universidad del Bío-Bío, Food Engineering Department Avda Avda Andrés Bello S/N, Chillán, Chillán, Chile, email lseg!ra"!#io#io$%l
Keywords: flexogra!y, "ransaren" mi#romodels, orous ma"erials, olydime"!ylsiloxane
ABSTRACT
S!rin$ S!rin$age age during during drying drying affe#" affe#"ss "!e orou orouss s!ae s!ae and re!yd re!ydra"i ra"ion on #aabi #aabili"y li"y of any gi%en gi%en deformable deformable orous s"ru#"ure. s"ru#"ure. Moreo%er, Moreo%er, only a few s"udies !a%e been #arried ou" "o exlain "!e me#!anisms "!a" ro%o$e s!rin$age during drying. &e os"ula"e "!a" s!rin$age is mainly rodu#ed "!roug! #!anges in #aillary ressure during drying in a deformable orous s"ru#"ure. Subse'uen"ly, if #aillary ressure is mainly resonsible for s!rin$age in a ore s"ru#"ure, i" is "!en ne#essary "o $now "!e li'uid !ase dis"ribu"ion inside i" as drying o##urs. (ransaren" mi#romodels #an be defined as "ransaren" ne"wor$s of ores and #ons"ri#"ions "!a" simula"e some of "!e #omlexi"ies of na"ural orous media and are a useful "ool "o obser%e li'uid dis"ribu"ion a" ore s#ale. Mi#romodels !a%e "radi"ionally been buil" in glass and resin and !a%e been %ery useful "ools "o obser%e and 'uan"ify li'uid dis"ribu"ion and "ransor" me#!anisms in%ol%ed in drying. Sin#e "!ey are buil" in rigid ma"erials, s!rin$age #an only be indire#"ly s"udied. , A new me"!od based on "!e "radi"ional !o"oli"!ogra!i# "e#!ni'ue !as been de%eloed "o build deformable mi#romodels, w!i#! is an elas"omeri# ma"erial #alled olydime"!ylsiloxane )*DMS+. )*DMS+. (!is s"udy reor"s "!e ro"o#ol "o build D deformable deformable "ransaren" "ransaren" *DMS mi#romodels mi#romodels for fu"ure ali#a"ion ali#a"ionss in "!e drying drying ro#ess and w!i#! is ne#essary "o find e%iden#e abou" "ransor" roer"ies dire#"ly rela"ed "o s!rin$age
INTRODUCTION
Se%eral wa"er "ransor" me#!anisms are in%ol%ed during "!e drying of a ore s"ru#"ure. (!ese me#!anisms are resen" a" differen" s"ages of "!e ro#ess and deend on su#! fa#"ors as solid-li'uid-gas dis"ribu"ion, "oology and mor!ology of "!e ore s"ru#"ure, and #omosi"ion of "!e wall ores )Laurindo )Laurindo and *ra", //0, //0, San Mar"in e" al., 1+. 1+. (!e (!e "ransaren" "ransaren" mi#romodel mi#romodel is a useful "ool "o obser%e li'uid-gas dis"ribu"ion during drying a" ore s#ale. A##ording "o 2yar34n and Segura )11/+, 5"ransaren" mi#romodels #an be defined as "ransaren" ne"wor$s of ores and #ons"ri#"ions "!a" simula"e some of "!e #omlexi"ies of na"ural orous media )see Bu#$ley, // for a re%iew+. More generally, bead a#$s and single6ore models are mi#romodels7. 8" is ossible "o #larify #larify "!e wa"er wa"er "ranso "ransor" r" me#!an me#!anism ismss in%ol% in%ol%ed ed during during drying drying of a ore ore s"ru#"u s"ru#"ure re "!roug "!roug! ! mi#romodels be#ause "!ey s!ow #omlex li'uid-gas-solid in"era#"ions and "!e rela"ions!i of "!ese in"era#"ions wi"! "!e ore sa#e "oology during "!e ro#ess. Al"!oug! "!e use of mi#romodels !as rodu#ed ad%an#es in "!e #omre!ension of "!e !enomena asso#ia"ed wi"! drying ro#esses )Laurindo and *ra", //0, //9 (simanogiannis, e" al., /// 2yar34n and Segura, 11/ Dia3 e" al., 1 Segura and 2yar34n, 1 Vor!auer e" al.., 8n *ress+, "!ese mi#romodels !a%e been buil"
from rigid ma"erials su#! as glass or lexiglass )resin+ "!erefore, "!ey do no" #a"ure some imor"an" fea"ures of a biologi#al ma"rix su#! as flexibili"y and s!rin$age. (!ey also #anno" ro%ide e%iden#e of "!e "ransor" roer"ies dire#"ly rela"ed "o s!rin$age. As re%iously men"ioned, mi#romodels buil" of glass and resin !a%e been %ery useful "ools "o obser%e and 'uan"ify li'uid dis"ribu"ion and "ransor" me#!anisms in%ol%ed in drying !owe%er, sin#e "!ey are buil" of rigid ma"erials, s!rin$age #an only be indire#"ly s"udied. , A new me"!od based on "!e !o"oli"!ogra!i# "e#!ni'ue !as been de%eloed "o build deformable mi#romodels, w!i#! is an elas"omeri# ma"erial #alled olydime"!ylsiloxane )*DMS+. &i"! "!is $ind of ma"erials i" is ossible "o ob"ain mi#romodels wi"! a ore radius %alue grea"er "!an 1 µm o%er a s!or" eriod of "ime. Ano"!er imor"an" ad%an"age of "!is $ind of mi#romodel as #omared wi"! "!ose buil" of glass or resin is "!e ossibili"y of building ;D mi#romodels )Anderson e" al., 111+. (!e me"!od #onsis"s in modifying "!e #lassi#al !o"oli"!ogra!i# "e#!ni'ue. Duffy e" al. )//9+ were "!e firs" "o de%elo "!e "e#!ni'ue some aers !a%e been ublis!ed sin#e "!en )M#. Donald e" al., 111
V-Vis absorban#e and fluores#en#e+ )iii+ i" #ures a" low "emera"ure )i%+ i" is non"oxi# )%+ i" #an be deformed re%ersibly )%i+ be#ause i" is elas"omeri#, i" will #onform "o smoo"!, nonlanar surfa#es, and i" releases from deli#a"e fea"ures of a mold wi"!ou" damaging "!em or i"self=7. *olydime"!ylsiloxane mi#romodels !a%e been alied in differen" fields su#! as #!emis"ry )Mogensen e" al., 11;+, biomedi#ine )Ki6?um e" al., 110+, !ysi#s )S"uder e" al., 11@+, nano"e#!nology )yu e" al., 11@+, mi#romolds )*aras!ar e" al., 11 ia e" al., //0+, mi#ro%al%es )Coo e" al., 110 Ab!in$ar, 11+, mi#rofluidi# sys"ems )Anderson e" al, 111 M#donald and &!i"esides, 11+, #!emi#al sensors )Lowder e" al., 11 &essling e" al., 11+, bio#!i #ells )Baudoin e" al., 11 Mar'ue""e and Blue, 11@+, gas "ransor" sys"ems )S"roo#$ e" al., 11+, and #!emi#al and mi#ro#!emi#al analysis sys"ems )Bruin, 111+. 8n ea#! of "!ese ali#a"ions "!e *DMS manufa#"uring ro#ess s"ru#"ures %ary deending on "!e #on#en"ra"ion of "!e base and #uring agen" in "!e mix"ure )en e" al., 110+, or "!e #uring "ime modifi#a"ion roosed by "!e manufa#"urer )E!ronis e" al., 11;
Figure 1. (A) Rapid prototyping is the generation of a high-resolution transparency to be used as a photomask. (B) Fabrication of the master involves spin-coating photoresist on a silicon afer and developing the photoresist through a photomask. (!) Replica molding involves pouring a mi"ture of #$%& prepolymer and curing agent on the master' curing at *! for 1 h' and peeling the replica off the master. (Adapted from +essamine et al.' ,,).
MATERIALS AND METHODS
(!e following ma"erials were used "o #ons"ru#" "!e mi#romodels: !o"oolymer la"e )(2C2B2 model KM@;GS+, Sylgar 9@ Sili#one las"omer Base )Dow Eorning+, Sylgar 9@ Sili#one las"omer Euring Agen" )Dow Eorning+, Sili#a Gel, A#e"a"e s!ee"ing, #ommer#ial soda la"e glass )@ mm+, and glass and las"i# ma"erials. (!e following e'uimen" was used: a flexo s"a"ion #onsis"ing of an ele#"ri# o%en )model Hex (260+ ada"ed wi"! a fan and an ul"ra%iole" lig!" roIe#"ion #amera. A "emera"ure6#on"rolled o%en )S!el6lab ;J F+, Elose Fo#us Mi#ros#oe mono#ular @Jx )Model 1;1/9611+, ?arri#$ *lasma Eleaner )?arri#$ *lasma Eomany+, Flowme"er )Model 1;6J6H+, %a#uum um )(ri%a# D ,J 2erli$on Leybold Va#uum, SA.+, Analy"i#al Balan#e )*re#isa, model B ;1M+, and a Eanon rin"er )*ixma, 8*@/1+ were also used. 2"!er !andmade e'uimen" su#! as a #uring #!amber, a %a#uum #!amber, and a me"al ress were used. (!e firs" s"e "o manufa#"ure "!e *DMS mi#romodels was "o de%elo "!e a""ern design )ma"rix+ wi"! a ore si3e dis"ribu"ion of a sele#"ed ma"erial )e.g., ale, #arro", and o"a"o+. (!e nex"
s"e was "o ob"ain "!e !o"oolymer la"e mold. (!e summary of "!e s"e6by6s"e ro#edure is as follows: (!e !o"oolymer was #o%ered wi"! "!e a""ern )nega"i%e+ of a orous sys"em before being exosed "o ul"ra%iole" lig!" )>V+. (!e !o"oolymer w!i#! did no" rea#" wi"! "!e ul"ra%iole" lig!" was "!en remo%ed using wa"er. &a"er sele#"i%ely dissol%es "!e areas "!a" were no" exosed "o ul"ra%iole" lig!" and lea%es a orous s"ru#"ure rin"ed in "!e !o"oolymer ma"erial. Hex", "!e mold was filled wi"! Sylgard 9@ and #ured a" room "emera"ure "o ob"ain a layer of *MDS orous s"ru#"ure. Finally, a se#ond lane layer )wi"!ou" ores+ of Sylgard 9@ was de%eloed, Ioined, and bound "!roug! air lasma "o ob"ain "!e D *DMS mi#romodel. (!e manufa#"uring s"ages of "!e deformable *DMS mi#romodel, along wi"! modifi#a"ions "o "!e ro"o#ol de%eloed by Duffy e" al. )//9+ ia and &!i"esides, )//9+ and in e" al. )//9+, are des#ribed in de"ail below. 1) Pattern design
(o ob"ain mi#romodels "!a" #an #a"ure "!e main geome"ri#al and "oologi#al fea"ures of some deformable ma"erials )Kara"!anos e" al., //0 Da3 e" al., 1+, a Log6Hormal ore si3e dis"ribu"ion fun#"ion wi"! orous radii ranging be"ween 01 and ;01 µm was sele#"ed )Figure + A #ode in Visual Basi# )%ersion 0.1+ was used )2yar34n and Segura, 11/+ "o genera"e ore radii %alues "!a" obey a Log6Hormal dis"ribu"ion fun#"ion. (!e #ode was fed wi"! s"a"is"i#al %alues from "!e sele#"ed ma"erials, i.e., mean %alue of @1 µm, s"andard de%ia"ion of @1 µm, maximum radius of ;;1 µm, and minimum radius of 01 µm wi"! J1 x J1 ores in a##ordan#e wi"! "!e ro#edure des#ribed by 2yar34n )119+.
Figure ,. og-ormal pore si/e distribution function
2n#e "!e da"a from "!e ore si3e dis"ribu"ion fun#"ion were genera"ed, "!ey were drawn wi"! "!e Au"oEAD sof"ware )%ersion 0., Au"odes$ 8n#.+. A gradua"ed "!roa" wi"! %alues ranging from a minimum radius of 01 µm "o a maximum radius of ;01 µm was added and #onne#"ed "o "!e ne"wor$ a""ern "o measure "!e effe#" of s!rin$age as a fun#"ion of "!e #aillary ressure and !umidi"y sa"ura"ion of "!e ne"wor$ during a gi%en drying ro#ess exerimen". (!e !o"omas$ dimension was 9 x 0 #m, w!i#! is smaller "!an "!e !o"oolymer la"e, bu" w!i#! allowed #u""ing "!e edges of "!e *DMS layer "o ob"ain regular borders and s"raig!" edges. Figure ; s!ows a osi"i%e a""ern of J1
J1 ores wi"! ores "!roa" radii dis"ribu"ed a##ording "o a Log6Hormal dis"ribu"ion fun#"ion. *ore "!roa"s )#!annels+ are reresen"ed in w!i"e and "!e solid !ase in bla#$.
Figure 0. etork pattern generated in Auto!A$ (" pores' node-to node distance of 1 mm' and throat radius of log-ormal beteen 2 and 02 3m) ith a graduated throat ith values ranging beteen the loest and highest radius.
2) Developing photopolymer matrix
A ; x #m !o"oolymer la"e )(2C2B2 model KM@;GS+ was used "o build ma"rix suor" for "!e *DMS olymer. (!e !o"oolymer la"e was aligned wi"! "!e !o"omas$ s!own in Figure ; and la#ed in "!e >V #!amber for @.J minu"es a" a wa%eleng"! of ;0J nm. Hex", "!e !o"oolymer la"e was was!ed wi"! a songe and #lean wa"er a" room "emera"ure for ; min and "!e ma"rix was exosed min "o >V lig!" "o s"reng"!en "!e s"ru#"ure. Figure @a s!ows "!e de%eloed !o"oolymer ma"rix and Figure @b s!ows a glass frame )#u%e""e+ w!ere Sylgar 9@ mus" "!en be oured.
)a+
)b+
Figure 4.(a) %atri" developed in the photopolymer 5 b) %atri" developed ith the glass frame (cuvette)
3) Conditioning of glass plates
(!is ro#edure is similar "o "!e one des#ribed in a re%ious s"udy )Da3 e" al., 1+. 5Glass edges "o "!e #u%e""e were buffed wi"! sandaer )@11 gri" sandaer "o wa"er+ "o remo%e s!ar edges and elimina"e s#ra"#!es on "!e glass surfa#e. Hex", "!ey were immersed in a solu"ion #on"aining of de"ergen".
(!e glass la"es were #leared on bo"! sides wi"! a #o""on swab and rinsed wi"! abundan" running wa"er "o remo%e residual de"ergen". (!e la"es were "!en was!ed wi"! dis"illed wa"er "o remo%e residual sal"s resen" in drin$ing wa"er. (!e la"es were immedia"ely "ransferred "o an o%en, w!ere "!ey were dried for ;1 minu"es a" J1 NE. (o a%oid #on"amina"ion of "!e la"e surfa#es, "!ey were a""a#!ed wi"! me"al #lis, w!i#! were !eld "o "!e o%en ra#$.7
4) Fabrication of PDM Micromodel Device
(!e following ro#edure is des#ribed "o de%elo "!e *DMS de%i#e )deformable mi#romodel+: J ml of Sylgard 9@ )base and #uring agen"+ in a 1: ra"io, rese#"i%ely, was reared in a *e"ri dis!. Subse'uen"ly, "!e mix"ure was s!a$en "o ensure !omogenei"y be"ween "!e base and "!e #uring agen" and a %a#uum was alied for 1 min "o remo%e bubbles rodu#ed by agi"a"ing "!e mix"ure. 8" was "!en oured in"o "!e #u%e""e )Figure @b+ and #ured for @9 ! a" room "emera"ure. Finally, "!e mix"ure was #ured a" @J NE for J min in an o%en a" a #on"rolled "emera"ure.
)a+
)b+
Figure . (a) #$%& poured in the photopolymer matri"5 (b) #orous #$%& layer after curing as finished (46 hours).
(!e ro#edure s!own in Figure J was #arried ou" "o ob"ain "!e se#ond lane layer )wi"!ou" ores+ of *DMS, bu" ins"ead of using "!e !o"oolymer re%ealed ma"rix, a #ommer#ial soda glass was used wi"! "!e same dimensions of "!e !o"oolymer la"e );x #m+.
!) Micromodel "onding
(!e bonding ro#ess is des#ribed as follows: (!e "wo layers of *DMS )wi"! ores and wi"!ou" ores+ were immersed in e"!anol and "!en dried in a %a#uum a" room "emera"ure in a desi##a"or for !. Hex", bo"! *DMS layers were u" in "!e ?arri#$ *lasma Eleaner e'uimen" )Figures 0a and 0b+ for J min a" low ressure )116;11 m"orr+ wi"! a #on"rolled airflow of ml-min and a" a maximum ower of 0./ &a"". (!e lasma ro#ess #!anges "!e surfa#e roer"ies of elas"omer by adding free radi#als on "!e surfa#e of "!e *DMS layers "!eses are ni"rogen and oxygen in "!e #ase of air lasma. Af"er "!e lasma ro#ess, "!e "wo *DMS layers were Ioined and la#ed in a me"al ress )Figure 0#+ w!ere a ressure of ; Kgf was alied for @J min a" J NE. Alying "!ese #ondi"ions rodu#es an irre%ersible bonding of "!e "wo *DMS s!ee"s "o ob"ain a D "ransaren" *DMS mi#romodel.
)a+
)b+
)d+ Figure 2. (a) #lasma !leaner !hamber5 (b) #$%& layer inside the plasma chamber5 (c) 7rreversible sealing of #$%& sandich at a pressure of 0 8g f in an oven at *! for 4 min.
RESULTS AND DISCUSSION
From a subrou"ine in Visual Basi# )%ersion 0.1+ and a%ailable s"a"is"i#al da"a of a gi%en orous ma"erial )maximum and minimum "!roa" radius, mean, and s"andard de%ia"ion+, ore si3e dis"ribu"ions and ne"wor$ images were genera"ed using "!e Au"oEAD sof"ware. Figures a, b, and # s!ow a D *DMS mi#romodel buil" in "!e resen" s"udy. (!e mi#romodel was de%eloed wi"! a Log6Hormal ore si3e dis"ribu"ion fun#"ion wi"! orous %alues ranging be"ween 01 and ;01 Om. Figures d, e, f, and g s!ow images of mi#romodels buil" from differen" ma"erials su#! as glass and resin in order "o #omare "!e resul"s ob"ained in "!e resen" s"udy wi"! "!ose a%ailable in "!e se#iali3ed li"era"ure. Figure d s!ows a glass mi#romodel ob"ained by Pamorano )11+, Figures e, and f s!ow a resin mi#romodel ob"ained by Dia3 e" al., )1+, and Figure g s!ows a resin mi#romodel de%eloed by Bonne" and Lenormand, /. Eomaring "!e mi#romodel buil" in "!e resen" s"udy wi"! glass mi#romodels, "!e *DMS mi#romodel dislays #!annels )ore "!roa"s+ wi"! be""er ore s"raig!" #u"s and be""er resolu"ion "!an "!e o"!ers )2yar34n and Segura 11/+. By #omaring images from Figure i" is ossible "o see "!a" "!e glass mi#romodel )Figure d+ !as irregular #!annels rodu#ed by a#id a""a#$ "!is is #omared wi"! "!e !ig! resolu"ion a#!ie%ed in "!e #!annels rodu#ed wi"! *DMS )Figure #+. Figure f s!ows a resin mi#romodel de%eloed by Dia3 e" al., )1+ w!ere a good resolu"ion was also ob"ained. &!en #omaring "!e "!ree ma"erials, i" #an be seen "!a" "!e resolu"ion was imro%ed for *DMS mi#romodels be#ause "!e mi#romodel a""ern ob"ained in "!e resen" s"udy !as a s'uare
#onfigura"ion, w!ile "!ose ob"ained by Bonne" and Lenormand )/+ !ad an almos" #omle"ely #ir#ular s!ae. Ano"!er imor"an" a#!ie%emen" of "!e resen" s"udy is a #ons"an" "!roa" de"! #omared wi"! o"!er mi#romodel manufa#"uring "e#!ni'ues. (!e "!roa" de"! of "!e mi#romodels ob"ained in "!e resen" s"udy was 1.;; mm.
)a+
)b+
)#+
)d+
)e+
)f+
)g+
Figure a) ,$ #$%& micromodel5 b) deformability of the micromodel developed ith the protocol in this study5 c) /oom of the pore structure d) 9lass micromodel (:amorano' ,)5 e) and f) Resin micromodel developed by $ia/ et al''(,11)5 g) micromodel built by Bonnet and enormand (1;). .
(!e mos" imor"an" fea"ure of "!e *DMS mi#romodel buil" in "!e resen" s"udy as #omared wi"! "!e o"!ers s!own in Figure is i"s #aa#i"y "o be deformed w!en subIe#"ed "o a s"ress #ondi"ion, su#! as s!rin$age, during a drying ro#ess. (!e mi#romodel "!a" was buil" #on"ains a gradua"ed #!annel )Figure ;+ "!a" #an measure %aria"ions wi"! #aillary ressure as drying o##urs. Finally, "!ere is no e%iden#e in "!e se#iali3ed li"era"ure rela"ed "o "!e use of "!is $ind of *DMS mi#romodels "o s"udy "!e drying ro#ess, bu" "!ere is e%iden#e of "!e use of "!ese ma"erials in "!e de%elomen" of mi#rofluidi# sys"ems )see
Figure 6. $iagram of the techni
CONCLUSIONS
A ro"o#ol "o ob"ain D *DMS mi#romodels based on "!e modified ro"o#ol de%eloed by Dia3 e" al., )1+, Duffy e" al., )//9+, ia and &!i"esides, )//9+ and in e" al., )//9+ was imlemen"ed. (!e mi#romodels !a%e "!e following #!ara#"eris"i#s: s"raig!" #u"s of "!e solid ma"rix, "!roa" de"! of ; µm, and node "o node dis"an#e of mm. Addi"ionally, deformable D *DMS mi#romodel #a"ures "!e main fea"ure of "!e biologi#al ma"erial w!en subIe#"ed "o drying, w!i#! is #on"ra#"ion )s!rin$age effe#"+ . 8" is also ossible "o ex"end "!is use "o o"!er na"ural ma"erials. (!e mi#romodel "!a" was buil" #on"ains a gradua"ed #!annel "!a" #an measure %aria"ions wi"! #aillary ressure as drying o##urs. (!erefore, fu"ure resear#! will be fo#used on s"udying "!e effe#"s of #aillary ressure on s!rin$age during drying of a ore s"ru#"ure using "!is $ind of mi#romodel.
ACKNOWLEDGEMENTS
(!is s"udy was funded by F2HDEC( Gran" Q1;@.
REFERENCES
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