Electromagnetic Valve Made in Low-Temperature Co-Fired Ceramics

Mateusz J. Czok,* Karol Malecha, and Leszek J. Golonka

Faculty of Microsystem Electronics and Photonics, Wrocław University of Technology, Wrocław 50-372, Poland

This article describes the technology of the electromagnetic valve made in the LTCC (low temperature co-fired ceramics).The actuation is supplied by mutual reaction between a 3D coil and a neodymium magnet bonded to flexible membrane. Suchsolution has many advantages like small power consumption and relatively great attractive forces. The valve is built in a hybrid

polymer-ceramic technology. The microfluidic and coil parts are fabricated in the LTCC technology. The flexible section is man-ufactured using a PDMS (poly(dimethylsiloxane)) membrane bonded to the ceramic part by plasma oxidation. The magnet isfixed on the membrane surface. The LTCC material was chosen for the sake of chemical durability and possibility of 3D struc-turation. The PDMS membrane provides simultaneously very good stability and high flexibility.

Introduction

Today’s miniaturization trend pushes ahead contem-porary science. One of the new fields of interest ismicrofluidics. It is a domain that interconnects variousbranches like chemistry, biology, or microtechnology.Microfluidics has a place among the fastest developeddomains of science. It can be said that the goal of thistechnology is to manufacture a microsystem, which willbe able to completely substitute a conventional labora-tory with qualified staff, sophisticated equipment, largeamount of analytical reagents, and long waiting time forresults. The most popular names of such device areLab-on-Chip (LoC) and Micro-Total-Analysis-System(lTAS). Microfluidic device consists of many compo-nents such as channels, mixers, pumps, valves, andothers.1–3 The microfluidic valve is one of the mostimportant parts of the modern LoC systems. It is utilizedto control fluid flow in the microfluidic system.

According to the type of work, valves can be classi-fied as passive or active devices. Passive microvalves canuse mechanical or nonmechanical moving parts, whileactive microvalves can use also external systems. Differenttypes of actuation are used to control microvalves. Checkvalve and capillary methods of actuations, such as ball,liquid-triggered, or hydrophobic, are used in passivevalves. On the other hand, for active microvalves, otheractuations as magnetic, electric, piezoelectric, thermal,electrochemical, laser, etc. are available.4–6

A silicon magnetic microfluidic valve was described in.7

A permanent magnet used in this solution interactedwith an electrodeposited layer of Co-Ni. Co-Ni layerwas deposited on a V-shaped, movable cantilever beam.

The channel was opened or closed by the deflectioncaused by the magnetic forces. The state of the valvedepended on the position of the magnet. Oh et al.8 pro-posed a magnetically driven in-line micro ball in a poly-mer tube as a microvalve. The magnetic actuator,attached along the x-direction, exerted a magnetic forceon the metal ball. As a result, attraction on the ballappeared what caused a fluid flow in the device. Anotherapproach to the construction of microfluidic valves wasdescribed by Desai et al.9 They proposed an electrostati-cally actuated microvalve. The support layer, membrane,and valve seat were made of poly(dimethylsiloxane)(PDMS). The electrodes were fabricated from thin filmsof multiwalled carbon nanotubes (MWNTs). Construc-tion of this valve consisted of two electrodes. A circular,flexible membrane with an embedded electrode was sus-pended above a microfluidic channel. A second electrodewas embedded just below the bottom of the microchan-nel. The electric potential applied between parallel elec-trodes induced an electrostatic attraction that pulled themembrane toward the channel floor and limited theflow.

However, only few constructions of ceramic-basedvalves can be found in the literature. Microfluidic valvecannot be easily fabricated as a monolithic ceramic struc-ture. Ceramic-based valves presented in the literature areusually made as a hybrid structures which consist ofmoving part (e.g., membrane) and a rigid substrate. Themoving part is made of silicon, steel, or polymer. Actua-tion of the membrane is accomplished using piezoelectricor electromagnetic principle. A piezoelectric initiallyopen LTCC (low temperature co-fired ceramics)-basedvalve presented by Soboci�nski et al.10 consisted of cera-mic substrate with fluidic channels, cavity, and valve seatand a steel membrane. A piezoelectric layer was depositedon the membrane forming a unimorph piezoactuator.

*mateusz.czok@pwr.wroc.pl

© 2013 The American Ceramic Society

Int. J. Appl. Ceram. Technol., 11 [3] 468–474 (2014)DOI:10.1111/ijac.12198

The applied electric field generated transverse expansionand contraction of the piezoelectric layer. These defor-mations created an internal bending moment and deflec-tion of the membrane. The LTCC-based piezoelectricvalve exhibited a quite fast actuation due to relativelyhigh stiffness and high force-delivering capability of thepiezoelectric actuator. The applied unimorph piezoactua-tor generated approximately 1.3 lm displacement. More-over, this solution was characterized by quite high powerconditions (250 V of supply voltage), low membranedeflection, and relatively high pressure drop of the fluidat the valve. A magnetic hydrogel nanocomposite valvefabricated in LTCC was presented by Satarkar et al.11

They have used a magnetic nanocomposites made ofN-isopropylacrylamide to develop a microfluidic valve.An alternating magnetic field applied to the structureheated the Fe3O4 nanoparticles in the hydrogel. As soonas the hydrogel temperature rose above the lower criticalsolution temperature, the hydrogel collapsed opening thechannel. A different approach was proposed by Gongora-Rubio and co-workers.12,13 They have developed anLTCC-based microfluidic valve with electromagneticactuation. It was built of multilayer coil and fluidicchannel fabricated in the LTCC substrate and flexiblemembrane made of silicon. A permanent magnet(SmCox) was bonded to top surface of the membrane.Using a magnet of 1 mm diameter, it was possible toachieve approximately 200 lm deflection of the 30 lmthick Si rectangular membrane. The described microflui-dic valve required 18 V of supply voltage and 150 mA,resulting in 2.7 W of power.

In this article, the technology and performance of thePDMS-LTCC-based microfluidic valve are presented. Thevalve is fabricated using a novel hybrid PDMS-LTCCtechnique. The technology takes advantage of both materi-als. On one hand, flexible and transparent actuators can befabricated using PDMS. On the other hand, a rigid sub-strate with integrated fluidic and electronic componentscan be manufactured using LTCC technology. In thisarticle application of the PDMS-LTCC bonding process isshown. The electromagnetic valve consists of a flexiblePDMS membrane with a glued permanent magnet andrigid LTCC microfluidic structure with an integrated coil.The polymeric membrane and ceramic substrate arebonded together using atmospheric pressure plasma dielec-tric barrier discharge (DBD).14

An electromagnetically actuated valve was selectedamong various actuation methods. The working principleof the chosen design is very simple. During actuation,the force generated between an electromagnetic microcoiland a magnet causes deformation of the flexible mem-brane, which stops the fluid flow within a channel.

The actuation is supplied by mutual reactionbetween a 3D coil and a NdFeB (Nd2Fe14B) magnet.The magnet is assembled to a flexible membrane. Such asolution fulfills assumed features such as fast reactiontime, small power consumption, and relatively greatforces. The microfluidic valve is built in a hybrid tech-nology because of the LTCC material limitations (lackof flexibility). The microfluidic and coil parts are fabri-cated in the LTCC technology. The flexible section ismanufactured using a PDMS membrane bonded to theceramic part by plasma oxidation method. The LTCCprovides chemical durability and possibility of 3D struc-turation. The PDMS membrane guarantees simulta-neously very good stability and high flexibility. Thescheme of electromagnetic valve is presented in Fig. 1.

DuPontTM (DP) 951 Green TapeTM (Durham, NC),165 lm thick, was chosen as a substrate material. Thewidth of the channel was designed to be equal to itsheight after sintering (140 lm). The length of the chan-nel is an arbitrary value and depends on the design. Theexternal dimension of the whole device was around22 mm 9 22 mm 9 2 mm.

Fluidic part has a significant importance in themicrofluidic valve construction. Fluid flow is relativelyeasy to achieve; however, fluid blockage is a greater chal-lenge. It is very difficult to introduce a moving elementand cut-off the flow in such tiny space. It is also verytough to avoid great dead volume. The design assumes achannel divided into two parts. A valve seat is placednearly in the central point of the structure and enablesobstruction of the flow. A design of the valve seat andits cross-sectional view are presented in Fig. 2. ThePDMS membrane is supported on the top layer of theelectromagnetic microfluidic valve. A central outletforming the valve seat is placed one layer below the topof the structure. During attraction between the coil andthe magnet, the central outlet is being blocked with thePDMS membrane. As a result the valve is closed.

Electromagnetic actuation provides fast responsetime and small power consumption. Thus, a microcoilwas designed as a source of magnetic field. It is a3D-embedded element. It exerts an influence on a mag-net upon application of current. It results from Amp�ere’s

Fig. 1. Scheme of the electromagnetically actuated valve (not to scale).

www.ceramics.org/ACT Electromagnetic Valve 469

circuital law which says that an electric current passingthrough a conductor produces a magnetic field. Themembrane, with a vertically magnetized permanentmagnet, is bonded to the surface of the ceramic structureand placed in a magnetic field induced by the coil. Thevertical force Fz acting on the magnet with magnetiza-tion Mz is given by 15 and can be seen in the Eq. (1).

Fz ¼ Mz

Zd

dzHzdV ð1Þ

where Hz is a vertical component of the magnetic fieldproduced by the coil, V is the magnet volume overwhich (1) is integrated.

Microcoils were designed in the form of two comple-mentary, square spiral microcoils of 11.5 9 11.5 mm(Fig. 3), and designed to produce an additive magneticfield. A width of coil’s lines was equal to 150 lm withthe same spacing between them. Each single layer has 15turns of winding. The membrane was manufacturedusing the PDMS material. This composition was chosenbecause of high flexibility, optical transparency, high ther-

mal resistance, inexpensiveness, biocompatibility, and easeof fabrication. The thickness of the membrane wasassumed during design. A mold made of LTCC tapeswas used to manufacture a desired PDMS membrane.The mold side walls were 300 lm high. The thickness ofthe membrane is much larger than its maximum deflec-tion. Therefore the equation for thick circular membranecan be used to describe the membrane deflection (2).16,17

x0 ¼ 3

16

R4M

d 3M

1� v2MEM

p ð2Þ

where x0 is the center deflection of the membrane, RMis the radius of the membrane, dM is the thickness of themembrane, p is pressure, EM is the elastic modulus ofthe membrane material, and vM is the Poisson’s ratio ofthe membrane material. The Young’s modulus and thePoisson’s ratio of the PDMS membrane are around1.8 MPa and 0.5, respectively.

Magnet dimensions were the main factors duringselection. The desired magnet should be small, but also“strong” enough to bend the membrane. NdFeB was cho-sen as the most suitable magnet material for such an elec-tromagnetically actuated valve. However, differentmagnet dimensions were selected to perform experiments.All magnets were delivered by MAGSY, Ltd. (Zlin, CzechRepublic). The most suitable magnet dimensions were5 9 5 mm2. The residual induction of the magnet wasaround 1350 mT with coercive field of 900 kA/m,tearing force of 11.3 N and energy density of 340 kJ/m3.

Experimental

Low temperature co-fired ceramics technology wasused to fabricate the device. The fabrication process wasdivided in three steps: LTCC substrate manufacture,PDMS membrane fabrication, and bonding process

Fig. 2. Vale seat construction and cross-sectional view.

Fig. 3. Design of microcoils, line width 150 lm.

470 International Journal of Applied Ceramic Technology—Czok, et al. Vol. 11, No. 3, 2014

using argon-oxygen DBD plasma. Moreover, the valvehousing was designed and manufactured using a polycar-bonate material.

The electromagnetic microfluidic valve was made offifteen 165 lm thick DP 951 LTCC green tapes.Patterning of green tapes was performed with a UV lasersystem (LPKF ProtoLaser U, Garbsen, Germany). Chan-nels, valve seat, vias, registration holes, and alignmentmarks were cut in appropriate layers. DP 6141 silver viafill paste was used for via filling. Then, DP 6142D Agconductor paste was used to deposit the microcoils on theceramic substrates. The nominal sheet resistance of thatcomposition is very low and equal to 3.3 mO/□ with alayer thickness of 9 lm. The screen printing process wasperformed with Aurel VS 1520A screen printer in a cleanand well-ventilated room at the temperature of about22°C. Then, all ceramic layers were stacked together andlaminated in an isostatic press. The lamination processwas divided in several stages. First, the 10 layers of greenceramic tapes with screen printed microcoils were stackedin appropriate order and laminated at the temperature of70°C and under pressure of 10 MPa for 10 min. Then,the fluidic part of the valve was laminated to the microcoilstructure using multistage lamination at lower pressure.First, the microfluidic channel layers were laminated tothe microcoil. Then, a valve seat layer was attached andlaminated. Finally, the last layer, forming distancebetween a membrane and the valve seat, was laminatedwith the structure. Thanks to this procedure, we haveavoided leakage under the valve seat, which was an issue inother approaches like one or two step lamination of thewhole structure. The lamination processes were carriedout at the temperature of 70°C and reduced pressure ofabout 5 MPa, for 10 min. The multistage lamination atlower pressure was used to provide proper bondingbetween layers and to avoid channels deformation. There-fore, no sacrificial volume material was needed. The pro-gressive lamination is one of the best solutions in suchstructures. Similar approaches were presented by Fournieret al.18 and Shafique et al.19 Then, the laminated LTCCstructure was co-fired in a box furnace at a recommendedfiring profile with a peak temperature of 850°C. Theco-fired LTCC valve is presented in Fig. 4. Afterward, aSG-683K glaze layer from Heraeus� (Hanau, Germany)was deposited onto the surface of the fabricated ceramicstructure using screen printing method. Then, wholestructure was dried and postfired in a belt furnace at onehour profile with a peak temperature of 850°C.

Two-component kit of SYLGARD 184 fromDowCorning� (Midland, MI) was used to prepare thePDMS membrane. The nominal weight mixing ratio ofcuring agent and polymer base of 1:10 was used. Such pre-

pared composition was thoroughly mixed and degassed ina vacuum desiccator. Then, the mixture was poured intothe mold to form a 300 lm thick membrane. To obtain auniform membrane, the excess of the poured PDMSmixture was smoothed using a stainless steel squeegee.Later, the PDMS was cured at temperature of 70°C for2 h. As a result, the thin PDMS membrane was fabricated.

A DBD plasma reactor from DORA PowerSystems (Wrocław, Poland) was used for PDMS mem-brane to glass-covered LTCC substrate bonding. Theplasma reactor was operated at 100 kHz and voltage ofabout 10 kV. A 20/80 oxygen/argon mixture was usedas a working gas. Pressure of the gas mixture was set to50 kPa. Both parts were modified separately for a timeof 10 s. Right after the DBD plasma modification pro-cess, the PDMS membrane and LTCC substrate wereput into contact and pressed, resulting in a permanentbond between both materials. The bonding processshould be realized within 5 min. After the bonding pro-cess, the permanent magnet was attached to the PDMSmembrane. Several attachment methods of the magnetto the membrane were evaluated. The magnets wereglued, flooded by PDMS, or trapped into two thinPDMS membranes. As a result, the NdFeB magnet wasglued to the PDMS membrane. Finally, fluidic portsmade of polyether ether ketone (PEEK) and perfluoro-elastomer (N-123H, Up-Church� Scientific, OakHarbor, WA) were glued as well. The magnet and flu-idic ports were glued using adhesive tape (N-100-01,Up-Church� Scientific). The adhesive tape was cured inan oven at the temperature of 170°C for 1 h. ThePDMS-LTCC microfluidic valve is presented in Fig. 5.

Results and Discussion

Characterization of the electromagnetic valve perfor-mance began from electrical and geometrical measure-

Fig. 4. The low-temperature co-fired ceramics part of the valve.

www.ceramics.org/ACT Electromagnetic Valve 471

ments. Dimensions of the manufactured LTCC-PDMSelectromagnetic valve were around 21.9 9 21.9 9 2.1mm3 of width, depth, and height. After lamination,average dimensions of the ceramic structure were25 9 25 9 2.5 mm3. Therefore, it was possible toevaluate shrinkage of our structure, which was around12.4 � 0.5% in x- and y-axis and 15.1 � 0.3% in zaxis. The large amount of coil metallization influencedthe shrinkage of the LTCC structure, which wasnegligibly lower in comparison with manufacturer’s data(12.7 � 0.3%). Hence, the dimensions of microfluidicparts were approximately equal to the designed values.Average surface roughness (Ra) of the channels made inthe fired LTCC structure was measured using optical pro-file measurement gauge (TalySurf CCI, Taylor HobsonPrecision, Leicester, U.K.). The Ra of the channel’sbottom surface was 0.296 lm.

Furthermore, the geometrical dimensions of themicrocoils deposited on the ceramic substrates wereexamined. The average line width, after screen printingprocess, was around 160 � 5 lm with spacing of140 � 5 lm. A fragment of the microcoil cross sectionand a picture of the deposited Ag layers are presented inFig. 6a and b, respectively.

Finally, the electrical measurements using HP 4263ALCR meter (Hewlett-Packard) were performed. The pre-sented electromagnetic valve made in the LTCC technol-ogy was characterized by an inductance of 100 lH andresistance of 46.5 O. Calculated sheet resistance of the mi-crocoil was around 2.7 mO/□. This is due to relativelyhigh average thickness of the deposited Ag layer.

A schematic view of the experimental setup is pre-sented in Fig. 7. It consisted of the LTCC microfluidicvalve, a water reservoir, an EA-PSI 8360-10T (Elektro-

Automatik Ltd., Viersen, Germany) power supply, and aVoyager V12145 (Ohaus, Nanikon, Switzerland) analyti-cal balance. A 90-mm-high liquid column of distilledwater was used during measurements. During the experi-ment, a constant hydrostatic pressure was used. It can beestimated with use of Eq. (3).

P ffi qgh ð3Þwhere P [Pa] is a hydrostatic pressure, q [kg/m3] is a liquiddensity, g [m/s2] is the gravity acceleration, and h [m] is aheight of the liquid column. The hydrostatic pressure wasequal to 0.9 kPa. The ratio of liquid weight, which flowedthrough the valve, and time, determined a fluid flow rate,which varied from 40 to 45 mL/min.

The experiment principle is based on the weight mea-surements of distilled water which flowed through thevalve in its open and closed state. The valve state waschanged every 2.5 min. Due to the normally open state ofthe presented electromagnetic valve, the hydrostatic pres-sure forces the liquid flow inside the microfluidic channel.As a result, the liquid is delivered to the analytical balance,which measures its weight. Then, the microcoil is power-supplied and attracts the NdFeB magnet bonded to the

Fig. 5. The poly(dimethylsiloxane)-low temperature co-firedceramics microfluidic valve.

(a)

(b)

Fig. 6. (a) A fragment of the microcoil cross section, (b) a pictureof the microcoil.

472 International Journal of Applied Ceramic Technology—Czok, et al. Vol. 11, No. 3, 2014

membrane, and closes the valve. Then, after 2.5 min, theweight of liquid is measured again.

The experiment was carried out for four differentvoltage conditions of 5, 7, 10, and 14 V, resulting in0.5, 1, 2, and 4 W of power, respectively. The obtainedweight measurement results were converted to volumetricunits. Moreover, the difference of the liquid volumebetween the closed and the open state was calculated andnormalized. Three series of measurements were per-formed, and obtained results were averaged. The devia-tion of measurements was below 10%. However, in thecase of the supply voltage equal to 10 V, the valve foreach measurement cycle was closed and no leakage wasobserved. The preliminary results of the performedexperiment are presented in Fig. 8.

As can be observed, the manufactured LTCC electro-magnetic valve worked as a flow regulator for supplyvoltages of 5 and 7 V. The flow rate was reduced by 20and 33%, respectively. Full closure of the microfluidicvalve was observed for the 10 V power supply. Neverthe-less, about 10% more of the liquid volume after valveclosing can be noticed in the graph. It is related with adead volume of the manufactured microfluidic valve,channel, and silicone tube volumes. Deflection of themembrane results in a pressure increase in a chamber ofthe valve seat and ejection of additional liquid volume.As a result, the weight of the fluid measured withanalytical balance increases.

Conclusions

An electromagnetic microfluidic valve was designedand manufactured. It was fabricated using novel PDMS-LTCC technology. Moreover, the PDMS membranefabrication and DBD plasma modification processes weredescribed.

Electrical and geometrical measurements of pre-sented construction were performed. The shrinkage ofthe structure was around 12.4 � 0.5% in x- and y-axisand 15.1 � 0.3% in z axis. The average line width, afterthe screen printing process, was around 160 � 5 lmwith spacing of 140 � 5 lm. The presented electromag-netic valve inductance and resistance were around 100lH and 46.5 O, respectively. The calculated sheet resis-tance of the microcoil (2.7 mO/□) was lower than nominaldue to high thickness of the Ag layer.

The performance of the fabricated microfluidic valvewas investigated using water hydrostatic pressure and ananalytical balance. The microfluidic valve was observed tofully close at a supply voltage of 10 V. An additional vol-ume of the liquid was observed after valve closing. How-ever, it is related with the PDMS membrane deflection.The presented electromagnetic valve is characterized withlow power consumption (2 W) in relation to similar con-struction presented in,12,13 which needs 2.7 W for opera-tion. Depending on the supply voltage, the presentedelectromagnetic valve can work as a flow regulator as well.

Further research will be focused on the optimizationof construction, performance, and integration withLTCC-based microfluidic devices.

Acknowledgments

The authors wish to thank National Science Centre(Grant No. DEC-2012/07/N/ST7/03480) and Wroclaw

Fig. 7. Schematic view of the experimental setup.

Fig. 8. Normalized fluid flow through the microfluidic valve; VC

– liquid volume in closed state of the valve, VO – liquid volumein open state of the valve.

www.ceramics.org/ACT Electromagnetic Valve 473

University of Technology (Grant Nos. B20011 andS20065) for the financial support. Author’s fellowship isco-financed by European Union within European SocialFund.

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