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Journal of Chromatography A, 1162 (2007) 167–174

Organic solvent nanofiltration for microfluidic purificationof poly(amidoamine) dendrimers

Jack T. Rundel a, Brian K. Paul b, Vincent T. Remcho a,∗a Department of Chemistry, Oregon State University, Corvallis, OR 97331, USA

b Department of Industrial and Manufacturing Engineering, Oregon State University, Corvallis, OR 97331, USA

Available online 27 June 2007

bstract

A nanofiltration method has been developed in a microfluidic format for the continuous-flow pressure-driven purification of half-generationoly(amidoamine) (PAMAM) dendrimers, a family of macromolecules characterized by highly branching structures radiating from a central core,ithout additional solvents or buffers. An organic solvent resistant nanofiltration membrane, STARMEM 122, has been fully integrated into a hardolymer microfluidic module by transmission laser welding. The membrane was initially characterized in a bench-top test fixture to determine

he solvent permeance and percent rejection of a surrogate molecule, Rhodamine B, at lower than typical operating pressures (P < 7 bar). The

icrofluidic module then underwent similar testing at 1.4 bar with the surrogate and with the generation-0.5 PAMAM dendrimer. This approacho nanofiltration will readily interface to upstream microreactors.

2007 Elsevier B.V. All rights reserved.

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eywords: Nanofiltration; Microfluidic; PAMAM; Dendrimer; Transmission la

. Introduction

Over the past few decades, nanofiltration methods haveeen developed on the macroscale for applications such asastewater purification and food production. Organic solvent

esistant polymeric membranes with low molecular weightutoffs (MWCO < 1000) have recently become commerciallyvailable and applied in areas such as the recycling of lubricat-ng oils [1]. Similar techniques, implemented on the microscale,ffer tremendous potential for application in pharmaceuticalnd bioanalytical industries where product purification meth-ds are needed with minimization of solvent volume in bothqueous and non-aqueous environments. Of particular interests the development of microreactors for on-demand synthesis ofanoparticles and macromolecules in which reduced residenceimes can be realized for diffusion-limited parameters such as

ixing of reagent streams and heat exchange.However, product purification in a microfluidic format

resents significant challenges. Traditional methods, such asolvent vaporization, are impractical, environmentally unsoundnd energy intensive. Most popular preparative chromatographic

∗ Corresponding author. Tel.: +1 541 7378181; fax: +1 541 7372062.

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021-9673/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.chroma.2007.06.042

elding

ethods, such as solid-phase extraction, are limited to batchode and do not address higher throughput needs. Continuous

eparations are possible via simulated moving bed chromatog-aphy (SMB) and centrifugal counter-current chromatographyCCC). However, these methods will not scale easily to theicrofluidic format.In this study, an organic solvent resistant polyimide nanofil-

ration membrane, STARMEM 122 (Membrane Extractionechnologies, London, UK), has been integrated into a microflu-

dic module and characterized for product purification fol-owing on-chip synthesis of half-generation poly(amidoamine)PAMAM) dendrimers. The method allows continuous-flowressure-driven product purification in a microfluidic formatithout additional solvents or buffers. Initial characterizationf the membrane was achieved in a bench-top test fixture toetermine the permeance of solvents and percent rejection ofsurrogate molecule at lower than typical operating pressures

P ≤ 7 bar).Dendrimers, with a high degree of monodispersity and spher-

cal symmetry, have been used previously to characterize pore

ize and Donnan exclusion effects in polymeric membranes [2].rganic solvent resistant nanofiltration membranes have beensed on the macroscale in solvent exchange and homogeneous-atalyst recycling following organic syntheses [3]. Polyimide

1 atogr. A 1162 (2007) 167–174

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Table 1Molecular masses of the solvent, reagents and products of PAMAM synthesisup to G2

Substance Symbol MW

Methanol MeOH 32Ethylene diamine EDA 60Methyl acrylate MA 86Poly(amidoamine) G−0.5 404Poly(amidoamine) G0 517Poly(amidoamine) G0.5 1205Poly(amidoamine) G1 1430PP

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68 J.T. Rundel et al. / J. Chrom

ltration membranes have been fabricated in situ for use inicrofluidic systems [4]. To the authors’ knowledge this is therst report of incorporating a polymeric filtration membrane

nto a hard polymer microfluidic device by transmission laserelding.

. Theory

.1. Nanofiltration

In filtration processes, the fraction of the feed stock whichasses through the filter is the permeate whereas the fractionhich is rejected by the filter is known as the retentate. A fil-

ration process is typically characterized by a molecular weightutoff (MWCO) at which 90% rejection occurs for species ofhat molecular weight Eq. (1). Nanofiltration has been defined as“pressure-driven membrane-based separation process in whicharticles and dissolved macromolecules smaller than 2 nm areejected” [5] and also as having a MWCO range of 200–10006].

ejection, R = 1 − Cp

Cr(1)

n nanofiltration processes percent rejection has been observedo decrease at lower pressures [7]. Percent rejection and per-eance are also highly dependent upon the solvent and solute

onditions [8,9]. This is thought to be due to solute/solvent,olute/membrane and solvent/membrane interactions that areot easily predictable. As a result, nanofiltration membranesust be evaluated empirically for differing solvent systems and

ressure ranges.

.2. PAMAM dendrimers

Poly(amidoamine) dendrimers are a family of macro-olecules characterized by highly branching structures

adiating from a central core. A brief synopsis of the diver-ent synthesis of PAMAM dendrimers with an ethylene diamineEDA) core is provided here in order to provide context for theanofiltration application at hand. The details of the synthesisre provided in previous works [10,11].

Divergent synthesis of PAMAM dendrimers consists of theteration of two steps in which half-generations (Gx + 0.5) andull-generations (Gx), x = {−1, 0, 1, . . .}, are built upon a core ofhe preceding generation. The first synthetic step is the reactionf EDA, also known as G−1, with an excess of methyl acry-ate (MA) resulting in G−0.5 with four methyl ester terminatedranches. In the second synthetic step, G−0.5 reacts with excessDA to produce G0 with four amine-terminated branches. Theycle is repeated to achieve higher generations of ever increasingass and size (Table 1).Excess reagent must be removed at the end of each syn-

hetic step. Any remaining EDA from full-generation syntheses

r MA from half-generation syntheses will react in the next syn-hetic step to produce G−0.5. Solvent removal is traditionallyone by evaporation which is time consuming, labor intensive,nergy intensive and environmentally unsound unless the vapor

is

oly(amidoamine) G1.5 2807oly(amidoamine) G2 3256

s handled properly. Nanofiltration offers an elegant alternativeo this process provided adequate mass discrimination betweenhe excess reagents and the desired product.

. Experimental

.1. Reagents and materials

The organic liquids used were: methanol (MeOH) (HPLCrade, Merck, Whitehouse Station, NJ, USA) as solvent; methylcrylate (>99.0%, Fluka, Sigma–Aldrich, St. Louis, MO, USA)s excess reagent for PAMAM half-generation synthesis; ethy-ene diamine (Fisher, Pittsburgh, PA, USA) as excess reagentor PAMAM full-generation synthesis; and glacial acetic acidHAc) (Fisher) as a potential protonating agent for EDAnd PAMAM full-generations. Rhodamine B (RhB) (LC6100,ambda Physik, Coherent, Santa Clara, CA, USA) was useds a surrogate for PAMAM G−0.5 during the initial character-zation of the nanofiltration membrane. STARMEM 122 withMWCO = 220 at 55 bar and the accompanying fibrous back-

ng layer (Membrane Extraction Technologies) were used inoth the macroscale test fixture and the microfluidic nanofil-ration module. Hard polymer substrate candidates included:morphous polyethylene terephthalate (APET) (ALRO Plas-ics, Jackson, MI, USA); polysulfone (PSU) (K-Mac Plastics,

yoming, MI, USA); polyvinylchloride (PVC) (Hytek Plastics,orvallis, OR, USA); and the following obtained from a single

ource (McMaster-Carr, Los Angeles, CA, USA) polyethyleneerephthalate (PET), polyethylene terephthalate with cyclo-exane dimethanol copolymer (PETG), polyetherimide (PEI),olycarbonate (PC), polyether ether ketone (PEEK), poly-ethylmethacrylate (PMMA). Elastomer gasketing material

ested included: polydimethylsiloxane (PDMS) (Dow Corning,idland, MI, USA), Viton O-rings and ethylene polypropy-

ene copolymer O-rings (EPDM) (O-Rings West, Seattle WA,SA). A PTFE-encapsulated Viton O-ring was used to gasket

he macroscale test fixture (O-Rings West).

.2. Materials compatibility

A survey of possible polymer candidates was conducted todentify a substrate that would not be attacked by the variousolvent environments. The ideal hard polymer substrate would

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Both the macroscale test fixture and microfluidic nanofiltra-tion module were interfaced with the same fluidic control system(Fig. 2). For tests with the macroscale fixture, the path from the

J.T. Rundel et al. / J. Chrom

e: (1) thermoplastic for future embossing of microfeatures; (2)ransmissive in the visible for optical monitoring of the filtrationrocess and transmissive in the near infrared (NIR) for transmis-ion laser welding; (3) chemically resistant to the various organicolvent environments; and (4) machinable with minimal burringo allow for rapid prototyping in the design phase. Hard polymerselected for study included APET, PSU, PVC, PET, PETG, PEI,C, PEEK, PMMA. Elastomeric polymers selected for study

ncluded PDMS, Viton and EPDM.Polymer samples of interest were cut to size at 0.25 in. ×

.5 in. Solvent environments of 100, 10 and 1% (v/v) dilutionsn MeOH of EDA, MA and HAc were prepared in bulk andispensed as 2.0 ml aliquots into 4 ml glass vials. Samples ofach polymer were massed and placed in the respective solventnvironments. A control for each polymer type, not exposed tohe solvent environment, was left at ambient atmospheric con-itions in a sealed plastic bag for the duration of the test. After4 h exposure each sample was removed from the solvent envi-onment, rinsed with MeOH to remove any residual solvent, andlown dry with N2 gas to remove any residual MeOH. The sam-les were then weighed, placed in rough vacuum for 10 min andeighed again. The net loss or gain in mass was then calculated.isual inspection of the polymer samples provided qualitativebservations including haze, swelling, discoloration and etch-ng. Each sample was then returned to its respective solventnvironment for long-term exposure observations.

The performances of PET, PSU, STARMEM 122 (SMA) andEEK are shown in Fig. 1. PMMA, PC, PEI, PVC, PETG, PDMSnd Viton were rejected due to excessive changes in mass dueither to solvent uptake or dissolution of the polymer. PVC,ETG and PDMS also showed unacceptable discoloration andaze. PET was found to be the best hard polymer substrate. Opti-ally transparent PET is widely available in sheets thinner thanmm and ubiquitous as beverage containers and food packaging.owever, due to its semi-crystalline structure, larger thicknesses

re only available in the naturally white-opaque form. As a

esult, APET, available in optically transparent 1 mm sheets, waselected as the hard polymer substrate for the microfluidic mod-le. PSU was selected for the macroscale test fixture becausePET was not available in the required 0.5 in. thickness.

ig. 1. Compatibility of polymers selected for PAMAM purification in aicrofluidic nanofiltration module. Positive percent mass change indicates an

ptake of solvent whereas a negative percent mass change indicates dissolu-ion of the polymer substrate. Values greater than 20% absolute magnitude areruncated for clarity (SMA: STARMEM 122).

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r. A 1162 (2007) 167–174 169

STARMEM 122 was found to be compatible with MA andncompatible with EDA. Consequently, although this methods suitable for half-generation synthetic steps, in its currentorm it cannot be applied to purification following the synthesisf full-generation PAMAM dendrimers. Alternatively, ceramicembranes, available with excellent chemical compatibility and

eported MWCO = 450 (Inopor, Hermsdorf, Germany) coulde used in place of the polymer membrane. However, ceramicembrane thicknesses are typically limited to >800 �m making

hem less attractive for microfluidic applications. The extremelyigh melting points of most ceramics eliminate the possibilityf welding them to polymer substrates. Therefore, the strategyescribed here may not prove viable for integration of ceramicanofiltration membranes into a microfluidic format. A secondpproach would be a post-synthetic protonation of the amine-erminated EDA and full-generation PAMAM dendrimers viahe addition of an acid to the product stream. An organic acid,uch as acetic acid, would be expected to filter out along withhe excess protonated-EDA. However, the dendrimer productould need to be de-protonated prior to the next synthetic step.his would likely require the addition of yet another species to

he product stream that would either be tolerated as a bystanderuring the subsequent syntheses or somehow removed by yetnother process. It is hoped that a polymeric nanofiltration mem-rane will be developed soon to withstand exposure to EDA yetemain amenable to transmission laser welding.

.3. Fluidic control

ig. 2. Fluidic control schematic for the macroscale test fixture and the microflu-dic module. The pressure, supplied by the N2 cylinder, drives both theransmembrane flux and the flow from the feed reservoir to the retentate reser-oir. The bleed valve controls the pressure drop, and therefore the fluid flow,etween the feed and retentate reservoir.

170 J.T. Rundel et al. / J. Chromatogr. A 1162 (2007) 167–174

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ig. 3. SolidWorks rendering of the macroscale test fixture allowing dead end flse with fluidic interface (right panel).

xture to the retentate reservoir was removed and plugged tostablish dead end flow. For tests in the microfluidic module, theath to the retentate reservoir was preserved and an adjustableleed valve to atmosphere was used to control a pressure dropnd thereby fluid flow between the feedstock reservoir andetentate reservoir. The feedstock and retentate reservoirs wereressurized by supplying nitrogen gas to the headspaces. Theressure was adjusted and monitored at the tank cylinder regu-ator. Capillary rinse kits (Sigma–Aldrich) provided reservoirsor the feed, permeate and retentate. Swagelok 1/16 in. stainlessteel ball valves (Portland Valve & Fitting, Portland, OR, USA)ontrolled the routing of N2 gas. Luerlock fittings and 0.015 in..D. × (1/16) in. O.D. PEEK tubing (Upchurch Scientific, Oakarbor, WA, USA) were used for all fluidic connections between

omponents.

.4. Macroscale test fixture

A macroscale test fixture (Fig. 3), constructed of 0.5 in. PSU,as machined using a shop lathe and mill. The fixture wasesigned to provide a 10 cm2 membrane surface area and 1 mmhannel height above the membrane. The fluidic interface tohe macroscale test fixture was comprised of 1/4 in.-28 Delrinttings and Tefzel ferrules (Upchurch Scientific). The mem-rane and backing material were cut to size with a cylindricaltainless steel punch fabricated specifically for that purpose.asketing was achieved with a PTFE-encapsulated Viton O-

ing and mechanical support for the membrane was provided bysintered glass frit.

.5. Microfluidic nanofiltration module

A microfluidic nanofiltration module was designed and fab-icated for permeance and percent rejection testing (Fig. 4).he product stream channel was milled to a 100 �m depth in

mm thick APET with an inlet for the feed stream and anutlet for the retentate. A counter flow channel was milledo a depth of 250 �m into a second 1 mm thick APET sub-trate with an inlet for the sweep and an outlet for the

mcAr

sting of the membrane up to 7 bar (left panel). Macroscale test fixture shown in

ermeate. The channel was completely filled by a fibrous back-ng layer providing mechanical support to the membrane asell as a flow path for sweep and permeate. The resultingeometry provided 4 cm2 surface area available for filtra-ion.

The STARMEM 122 sheet, cut with scissors into a rect-ngular blank with dimensions larger than the product streamhannel, was laser welded to the APET substrate with the activeayer of the membrane toward the product stream channel. Thisormed a gasketing seal around the entire perimeter of the prod-ct stream channel. The excess membrane was removed byeeling away the area exterior to the gasketing weld therebychieving a clean edge. However, this peeling process delam-nated a backing layer component of the membrane whichormally provides mechanical support.

In order to achieve a uniform structural weld, a crush zoneith a height of 50 �m and width of 1 mm was milled along

he entire perimeter of the counter flow channel. The uppermosturface of the crush zone provided the only contact betweenhe two APET substrates thereby limiting the clamping force tohis region. During laser welding, the crush zone of the lowerubstrate melted along with the area of the upper substrate it wasn contact with. The applied clamping pressure forced the twoubstrates together promoting fusion of the structural weld uponooling.

All milling was performed on a Tool Crafter mill (CMS CNC,aguna Hills, CA, USA). The fibrous backing layer was cut toize using an ESI 5330 (Electro Scientific Industries, Portland,R, USA) laser trimming system. The entire assembly was gas-eted and structurally bonded via transmission laser weldingollowing the deposition of Clearweld LD120B (Gentex, Zee-and, MI, USA) along the weld lines. The laser welding waserformed with an IRAM 300 (Branson Ultrasonics, Danbury,T, USA) laser welding system retrofitted with a 940 nm vertical

tack diode array dispersed in a 1.8 mm × 80 mm swath. For the

embrane-to-APET weld, the diode array was supplied with a

urrent of 40.0 A resulting in 195 W of photonic power. For thePET-to-APET weld the diode array was supplied with a cur-

ent of 50.0 A resulting in 275 W of photonic power. Both welds

J.T. Rundel et al. / J. Chromatogr. A 1162 (2007) 167–174 171

F ed view (top left) and fully assembled (top right). Photographs of the microfluidicm ce (bottom right).

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ig. 4. SolidWorks rendering of the microfluidic nanofiltration module explododule (bottom left) and the microfluidic module mounted in the fluidic interfa

ere realized with a scan speed of 10 mm/s and a clamping forcef 1.3 kN (4.1 bar on a 64 mm diameter air cylinder).

A fluidic interface fixture for the microfluidic module (Fig. 4)as designed and fabricated to register the microfluidic module

nlets and outlets and to provide clamping support for the O-ing gaskets. The fluidic interface was comprised of Nanoportssemblies to accept 0.015 in. I.D. × (1/16) in. O.D. PEEK tub-ng (Upchurch Scientific) and M1x4 EPDM O-rings to provideolvent resistant gasketing of the nanoports. The fixture wasonstructed from 0.5 in. PC and stainless steel machine screws.

The microfluidic module was originally designed for aounter current flow in which pure methanol flows in theownstream channel to sweep away the permeate (Fig. 5). How-ver, the module was operated in a cross-flow geometry inrder to sample the permeate without dilution from a sweeptream and thereby more accurately assess the performance ofhe membrane. Future process development will move towardmplementation and characterization of the counter current flowperational mode.

.6. Permeance and percent rejection measurements

Prior to initial use, each membrane was rinsed a minimumf 10 times for 20 min at 1.4 bar in dead end flow until a stableux was observed. This preconditioning step is necessary toemove an impregnating oil that stabilizes the dry membraneuring shipment and storage.

Masses of the permeate and retentate were found by the massifference between the final and initial masses of the respec-ive reservoirs. Volumetric flux was calculated by the product of

ass flux and fluid density. For cross-flow modes, retentate and

ermeate volumetric flow rates were held equal by adjustmentf the bleed valve.

Rhodamine B was used as a surrogate for G−0.5 (MW = 404)ue to ease of detection, a similar molecular weight (MW = 479),

crf

ig. 5. Schematics showing a counter flow geometry (top) for which theicrofluidic module is designed for and a cross-flow geometry (bottom) in which

he module was tested in this study.

igh solubility in MeOH, and stability at standard conditions.he concentration of RhB was determined with a Luminescencehotometer (Perkin-Elmer, Waltham, MA) scanning from 350

o 700 nm with a fixed 25 nm offset between excitation and emis-ion wavelengths and 5 nm excitation and emission slit widths.he retentate and permeate signals were taken at 543 nm due to

he high signal strength and signal to noise ratio in that regionFig. 6).

G−0.5 was tested in the microfluidic module at a 1 mg/mloncentration in 4 mM MA in MeOH. These concentrations rep-esent the expected relative ratios of product and excess reagentollowing half-generation synthesis. However, the absolute con-

172 J.T. Rundel et al. / J. Chromatog

Fig. 6. Fluorescence scans of the retentate and permeate solutions. The peakswith maxima at 540 nm indicate the presence of the Rhodamine B surrogate inboth the retentate and permeate. The signals below 480 nm indicate the presenceo

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entrations are approximately two orders of magnitude moreilute than published values [10].

Absorbance of G−0.5 was scanned against a blank of 4 mMA on an Agilent 8453 UV–Visible Spectrophotometer (Agi-

ent, Santa Clara, CA) from 190 to 400 nm using a glass cuvetteith a 1 cm path length. The permeate signal was seen to beigher than expected possibly due to an unidentified interferrent.s a result, the relative concentration of G−0.5 in the permeateas calculated using the absorbance signal of the feedstock and

etentate Eq. (2), where C is the concentration; I the absorbanceignal; V the volume; and f, p, r indicate feed, permeate, reten-ate. Percent rejection was reported as an average of percentejections calculated in the range of 225–230 nm due to highignal strength and SN in that region.

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able 2verage permeances (L m−2 h−1 bar−1) of various solvent environments in the macro

olvent environment Macroscale test fixture average permeance

eOH 13.6 ± 0.2 −0.41 ± 1.10−7 M RhB:MeOH 13.1 ± 0.1 −1.37 ± 0.30% (v/v) MA:MeOH 8.1 ± 0.1 −1.27 ± 0.2

acroscale pressure range 1.4–6.9 bar, standard errors shown for slope and intercetandard deviation shown for the average permeance.

r. A 1162 (2007) 167–174

. Results and discussion

.1. Analysis

The microfluidic module was limited to a pressure of 1.4 barue to leakage through the membrane observed at higher pres-ures. This was most likely a consequence of module fabricationince the membrane itself is normally operated at 30–60 barnd no leakage was observed in the macroscale test fixture atressures up to 7 bar. Possible factors contributing to the failurenclude: (1) removal of the backing material following the laserelding of the membrane to APET substrate; (2) reduction inembrane tensile strength along the gasketing weld due to heat

ffects of the welding; (3) a compression of the fibrous backingaterial filling the counter flow channel resulting in a bowing of

he membrane into the counter flow channel and consequentlyn increase in the tensile and shear forces at the gasketing weldine; and (4) an observed outward bowing of the APET sub-trate of the bottom of the counter flow channel resulting in anncrease in the tensile and shear forces at the gasketing weldine. Efforts are underway to analyze and remedy these possibleailure modes.

The flux of the permeate across the membrane was found to beighly linear with pressure in both the macroscale test fixture andn the microfluidic nanofiltration module (Table 2). The slope ofux versus pressure describes the permeance of a given solventnvironment while the intercept describes the theoretical fluxt zero applied pressure. For applications on the microscale,nits of �L, cm2 and min offer a more relevant treatment. Forxample, a volumetric flow rate of 110 �L min−1 was observedor the permeate of a 10% (v/v) MA feedstock at 1.4 bar acrosshe 4 cm2 of membrane surface area in the microfluidic module.

Permeance in the macroscale test fixture was found to bereatest for pure MeOH and to decrease with the addition of RhBnd MA. Permeance in the microfluidic module at 1.4 bar showssimilar trend but with higher overall values. These permeancealues of ∼14–16 L m−2 h−1 bar−1 are somewhat greater thanhe 2–3 L m−2 h−1 bar−1 for MeOH across STARMEM 122 at0 bar reported in the literature [9].

Percent rejection for 10−7 M RhB at 1.4 bar was found to be5.7 ± 0.9% for dead end flow in the macroscale test fixture and0.6 ± 1.8% for cross-flow in the microfluidic module. Thesealues are significantly lower than the >90% rejection expected

rom 30 to 60 bar. Percent rejection versus pressure data in theacroscale test fixture shows a positive correlation from 1.4 to

.9 bar obtained using dead end flow (Fig. 7). Percent rejection

scale test fixture and the microfluidic module

Microfluidic module average permeance

8 16.2 ± 1.13 17.4 ± 0.99 12.4 ± 0.7

pt of volumetric flux vs. pressure. Microfluidic pressure fixed at 1.4 bar with

J.T. Rundel et al. / J. Chromatog

Fig. 7. Percent rejection vs. transmembrane pressure of 10−7 M RhB in MeOHiod

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n the macroscale test fixture showing a slightly positive correlation. A minimumf three data points were used for each data set. Error bars are ±1 standardeviation.

or G−0.5 at 1 mg/ml and 4 mM MA in the microfluidic moduleas found to be 55.2 ± 2.3%.

.2. Discussion

In this filtration process, low MW species such as MeOH andA are expected to move across the membrane at roughly the

ame rate [9]. As a result, the concentration of MA can onlye decreased by iterative filtration steps in which the volume ofhe retentate is significantly reduced in every step and dilutedith pure MeOH between steps. A theoretical treatment Eq.

3) can be developed using Eqs. (2a)–(2c) and (1); assumingis constant as the solvent/solute environment changes; and

efining n = number of filtration steps. By setting Vf/Vr = 10, wend that three filtration steps would be necessary to reduce theoncentration of a species with 0% rejection by three orders ofagnitude (Fig. 8). We also find that those same three filtration

ig. 8. Relative retention of species following subsequent filtration steps show-ng that high rejection values (>99%) are needed to minimize product loss andow rejection values (<60%) allow significant removal of unwanted species fromhe product stream. A 10:1 feed volume to retentate volume is assumed. Seriesre given as percent rejection of species.

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r. A 1162 (2007) 167–174 173

teps would reduce a species with 60% rejection by two ordersf magnitude. In fact, percent rejections >90% are necessary inrder to achieve retentions >10% after three filtration steps.

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Therefore, the removal of MA following the synthesis of−0.5 is not practical with this microfluidic module at 1.4 barue to the extensive product loss. Fortunately, G−0.5 representsworst-case scenario for microfluidic nanofiltration of PAMAMendrimers. Each subsequent generation is higher in mass and isxpected to experience higher percent rejection under the sameolvent and pressure conditions. Future work is needed to verifyhis expectation and to determine percent rejections for higherenerations at these conditions.

However, the relatively low percent rejection for G−0.5 coulde used as an advantage in purification of higher generations.ny excess MA remaining in the retentate from the precedingurification will react with EDA to produce G−0.5 as a side-roduct. Removal of this lower generation dendrimer is criticaln order to preserve monodispersity in subsequent generations.f the higher generation dendrimer has 99% rejection, only 23%f the higher generation dendrimer is expected to be lost in theermeate.

This microfluidic nanofiltration module is a front-end devicehat has tremendous utility as an on-line sample preparation toolrior to chromatographic and/or electrophoretic separations on ahip. Possible applications include but are not limited to: (1) pre-oncentration of high molecular mass analytes in large-volumenvironmental samples in the field; (2) more rapid sample pre-ared in the laboratory such as desalting of proteins or recoveryf drugs from biological samples; (3) fractionation of moleculesn flowing product streams for automated process analyses; and4) continuous purification of product streams to facilitate nano-aterials synthesis and analysis.

. Conclusion

Transmission laser welding of a polymeric nanofiltrationembrane within a hard polymer microfluidic device has been

emonstrated. The resulting module has been designed to behemically compatible with MeOH and 10% (v/v) MA and toperate at pressures up to 1.4 bar.

The microfluidic module has been evaluated for purifica-ion of the half-generations of PAMAM following synthesisn a microreactor. The first purification step for G−0.5 willikely experience significant product loss due to a low percentejection. However, product loss is expected to decrease withigher generations due to a predicted increase in percent rejec-ion resulting in an increase in efficiency. The relatively lowercent rejection of G−0.5 might prove to be advantageous inemoval of unwanted sideproducts following higher generation

yntheses.

A significant barrier remains in the purification of full-eneration PAMAM dendrimer due to incompatibility ofTARMEM 122 membrane with EDA and, by extension, the

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74 J.T. Rundel et al. / J. Chrom

mine-terminated full-generations themselves. Protonation ofDA and full-generations might achieve compatibility butould likely require the introduction of other species into theroduct stream. Ceramic membranes provide a possible alter-ative with excellent chemical resistance and low moleculareight cutoffs. However, integration of relatively thick ceramicembranes into hard polymer microfluidic devices remains a

hallenge.

cknowledgements

Financial support was provided by the W.M. Keck Foun-ation. The authors thank: Ted Hinke for invaluable technicaldvice and for machining the macroscale test fixture and thenterface for the microfluidic module; Steve Etringer for milling

he APET substrates used in fabrication of the microfluidic

odules; the Granwell Corporation of West Caldwell, NJ, forroviding APET samples; and the Gentex Corporation of Zee-and, MI, for providing samples of Clearweld.

[

[

r. A 1162 (2007) 167–174

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