Chemical and pH sensors based on the swelling behavior of hydrogels

Sensors and Actuators B 111–112 (2005) 555–561

Chemical and pH sensors based on the swelling behavior of hydrogels Gerald Gerlach a , Margarita Guenther a,∗ , Joerg Sorber a , Gunnar Suchaneck a , Karl-Friedrich Arndt b , Andreas Richter b b

a Institute for Solid-State Electronics, Dresden University of Technology, Helmholtzstr. 10, 01069 Dresden, Germany Institute of Physical Chemistry and Electrochemistry, Dresden University of Technology, Helmholtzstr. 10, 01069 Dresden, Germany

Available online 18 April 2005

Abstract Hydrogels show a strong swelling ability in dependence on pH value or solvent concentration in aqueous solutions. This behavior can be used for appropriate sensors if a suitable transducer transforms the volume change into an electrical output signal. In this work, piezoresistive sensors were used where the hydrogel led to a deflection of the silicon membrane within the sensor chip. This principle allows for a strict separation of the fluid from piezoresistors as well as from other electronic components at the sensor chip front side. Poly(vinyl alcohol)/poly(acrylic acid) (PVA/PAA) blend as well as poly(N-isopropylacrylamide) (PNIPAAm) have been applied and investigated for pH sensors and organic solvent concentration sensors, respectively. The output voltage versus pH value characteristics and the long-term signal stability of hydrogel-based sensors were investigated and the measurement conditions necessary for high signal reproducibility were determined. The influence of the preparation conditions of the hydrogel films on the sensitivity and response time of the chemical and pH sensors is discussed. © 2005 Elsevier B.V. All rights reserved. Keywords: Hydrogel; pH-sensitive; sensor; Solvent composition

1. Introduction The sensitivity of hydrogels to a large number of physical factors like temperature [1], electrical voltage [2,3], pH [4–11], concentration of organic compounds in water [12], and salt concentration [13,14] make them promising materials for a broad range of applications as microsensors [7–11,14] and microactuators [1,3,6,12] in MEMS devices. The following principles for the pH value detection are used in sensors based on the swelling behavior of hydrogels: changes of the holographic diffraction wavelength in optical Bragg grating sensors [9], shifts of the resonance frequency of a quartz crystal microbalance in microgravimetric sensors [10], a bending of micromechanical bilayer cantilevers [7], as well as a deflection of silicon membranes in piezoresistive pressure sensors [8,11]. The swelling ability of pH-sensitive hydrogels depends on the functional acidic or basic groups at the polymer back∗

Corresponding author. Tel.: +49 351 463 35378; fax: +49 351 463 32320. E-mail address: [email protected] (M. Guenther).

0925-4005/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2005.03.040

bone. Due to the dissociation of these groups and the influx of counterions, the concentration of ions in the hydrogel is higher than in the surrounding solution. This causes a difference in osmotic pressure and results in a solution flux into the hydrogel and, consequently, a swelling. The interaction and repulsion of charges along the polymer chain also lead to an increased swelling. Equilibrium of ionic gels occurs when the elastic restoring force of the polymer network balances the osmotic forces. During the swelling process hydroxide ions are transported into the neutral gel, while during the shrinkage protons diffuse into the gel and neutralize the negative charged acidic carboxylate groups. This ion diffusional flux induces an electrical potential difference that drives the electromigration of the ions in the direction opposite to that of the diffusion. In so called “Donnan equilibrium” the diffusional flux of the ions in one direction is equal to the electromigrational flux in the opposite direction, resulting in a net zero mass transport and a net zero charge transport. The change of the electrical potential at the gel–solution interface is a function of the pH value of the surrounding solution. A Nernst–Planck equation coupled with the Poisson and the me-

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Fig. 1. Operational principle of hydrogel-based sensors: (a) with PVA/PAAhydrogel layer, deposited onto the bending plate, (b) with PNIPAAm foil piece in a cavity. (1) Bending plate; (2) mechano-electrical transducer (piezoresistive bridge); (3) swellable hydrogel; (4) Si-chip; (5) socket; (6) tube; (7) interconnect; (8) solution; (9) grate.

chanical equilibrium equations can be used to describe the gel swelling/deswelling process [15]. Because the gel response is typically diffusion driven, the time response of the volume change approximately follows the square of the sample dimension. Scaling to micro-dimensions enhances the time response. Consequently, a reduction of the sample size improves the sensor performance. In the present work we discuss the influence of the gel kinetics on the response time and long-term signal stability of the proposed pH sensors.

2. Operational principle Chemical sensors serve for conversion of a chemical input value into a suitable measuring output signal. They are characterized by a material-recognizing element and a transducer. The transducer converts the non-electric measuring value into an electrical signal. Hydrogels are crosslinked polymers which swell in solvents to appreciable extent. The amount of solvent uptake depends on the polymer structure, and can be made responsive to environmental factors, such as solvent composition, pH value, temperature, etc. Hydrogels are capable to convert reversibly chemical into mechanical energy and therefore can be used as sensitive material for appropriate sensors. For the design of chemical and pH sensors a sensor chip with a distortable thin silicon membrane was used (Fig. 1). Commercially available pressure sensor chips (Aktiv Sensor GmbH, Stahnsdorf, Germany) were used as mechanoelectrical transducers for the transformation of membrane

deflections to an appropriate electrical output signal. The hydrogel itself was brought into a cavity at the backside of the silicon chip and closed with a cover. This cavity on the backside of the chip was wet etched, a silicon nitride mask was used as an etch resist. Therefore, only the backside of the chip came in contact with the measuring species, whereas the frontside with the electronic components was strictly protected from it. Since the sensor chips showed excellent stable properties, the long-term stability of the sensor was solely determined by the stability of the hydrogel characteristics. Fig. 1 illustrates the operational principle of hydrogelbased sensors. The swelling or shrinking processes of the hydrogel were monitored by a corresponding change in piezoresistance of an integrated Wheatstone bridge inside a rectangular silicon membrane. The membrane deflection caused a stress state change inside the membrane and therefore a change of the resistivity of the resistors. This changed proportionally the sensor’s output voltage [16]. The sensor chip was bonded to a socket with inlet and outlet flow channels. The aqueous solution to be measured was pumped through the inlet tubes into the silicon chip cavity coated with a 220 nm thick PECVD silicon nitride film to provide chemical protection.

3. Experimental The following hydrogel systems were used in the present work: 1. Poly(vinyl alcohol)/poly(acrylic acid) (PVA/PAA)blends: The swelling degree of these hydrogels changes steeply with the change of the pH value of the measuring species: it is at minimum in acids and takes its maximum in bases [4,5]. PVA and PAA polymers obtained from Aldrich Chemical Co. were dissolved separately in distilled water under stirring at 80 ◦ C (PVA 15 wt.% and PAA 7.5 wt.%). For hydrogel formation, the solutions are then mixed in such a manner that 80 wt.% were PVA and 20 wt.% PAA. This mixture is stirred for 1 h at 60 ◦ C to manufacture a homogeneous solution. Thin films of PVA/PAA blends were deposited onto the backside of the silicon membrane covered with a 220 nm thick PECVD silicon nitride film and with a 17 nm thick ␣-amino propyltriethoxysilane adhesion promoter layer (Fig. 1a). Finally, the dried hydrogel films were isothermally annealed in an oven at 130 ◦ C for 20 min. The 5, . . ., 50 ␮m thick PVA/PAA-hydrogel layer was located on the backside of the silicon membrane (Fig. 1a). 2. N-Isopropylacrylamides(NIPAAm): Hydrogels on PolyNIPAAm (PNIPAAm) basis react sensitively on changes of the concentration of organic components in aqueous solutions [12]. The crosslinked PNIPAAm-hydrogels were prepared by free radical polymerization of NIPAAm in water with N,N -methylene-bisacrylamide (BIS; BIS content: 4 mol%) as the crosslinking agent. A

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solution of NIPAAm, BIS (overall monomer concentration: 0.53 mol/l) and potassium peroxodisulfate as the initiator (3 × 10−3 mol/mol monomer) were cooled to 0 ◦ C and purged with nitrogen for 15 min. The N,N,N ,N -tetramethylethylenediamine as the accelerator (3 × 10−3 mol/mol monomer) was added and the solution was immediately transferred into a Petri dish to get hydrogel foils. After 17 h reaction time at 20 ◦ C, the gels were separated and washed with water for 1 week. For PNIPAAm chemical sensors, 250 ␮m thick hydrogel foil pieces were used (Fig. 1b). The dried PNIPAAm-foils were prepared by evaporation of water at room temperature and then cut into pieces of 1 mm × 1 mm. In order to estimate the mechanical stress caused by the hydrogel swelling, the silicon membrane deflection was measured by means of a two-beam laser interferometer (LIF) [8]. Two sets of experiments were performed. In first set the silicon membrane deflection was measured during the hydrogel swelling. In the second set the pressure controller was used to deflect the membrane. The deflection values of (1, . . ., 17.4) ␮m corresponded to the stress values of (0.6, . . ., 20) kPa. For the sensor signal calibration the buffered solutions of 0.2 M ionic strength were used which varied between pH values of 1 and 11.

4. Results and discussion 4.1. Swelling kinetics The sensor response time is determined by the thickness of the polymer layer as well as by the hydrogel swelling kinetics. The latter depends on the rates of diffusion of three species in the gel structure. i. Water must enter or leave the gel. ii. Base or acid must diffuse into the gel to spark its swelling or collapse. iii. Solutes which are not excluded from the gel will diffuse into the gel. The kinetics is complicated because usually all three of these different mass transfer processes occur simultaneously. The sensor’s output voltage was measured during the swelling of the PVA/PAA-hydrogel layer under influence of solutions with different pH values, as shown in Fig. 2. It was found that the time dependence of the swelling process consists of three parts. The first of them is the rapid swelling caused by the osmotic pressure. The time of this process was 1.2 s for the 6 ␮m thick and 80 s for 50 ␮m thick layer. This time corresponds to a diffusion coefficient of 3 × 10−7 cm2 /s for water in crosslinked PVA/PAA-hydrogel [4]. The mechanical quasi-equilibrium of gel occurs when the stress generated in the polymer network balances the osmotic pressure. A stress σ in the gel due to deviations from mechanical

Fig. 2. Sensor output voltage during the swelling process from pH1 to pH4.5.

equilibrium is described by a linear elastic constitutive law σ = Ke, where K is the bulk modulus of the gel and e the compressive deformation. The value of the mechanical bulk modulus of the crosslinked PVA/PAA polymer network was estimated as 42.3 kPa what is in good agreement with the value of 42.5 kPa obtained from the compression measurements at pH2 [5]. The second retarded part of the swelling process can be described by means of an exponential increase, which gives a time constant τ = 5 min for the swelling of a dry 50 ␮m thick layer in de-ionized water. This part of the swelling process is governed by the polymer chain mobility, which determines the mesh openings between polymer chains and consequently the size of the water-filled regions. Solute transport within hydrogel occurs primarily within these regions in the space delineated by the polymer chains. Any factor which reduces the size of these spaces will have an effect on the movement of the solute. Such factors include the size of the solute in relation to the size of the openings between polymer chains, polymer chain mobility, and the existence of charged groups on the polymer, which may bind the solute molecule. In general, the diffusivity of a solute within the crosslinked hydrogel increases as the crosslinking density decreases, as the size of the solute decreases, and as the volume fraction of water within the gel increases [17]. At pH values above the acid exponent of acrylic acid (pKa = 4.7), the interaction and repulsion of charges along the polymer chain leads to an additional solution sorption and swelling. A time constant τ = (1.5, . . ., 1.7) min for the 6 ␮m thick and τ = (50 ,. . ., 80) min for 50 ␮m thick layer was found for the relaxation part of the gel swelling in the electrolyte solutions. The ion diffusional flux induces a change of the electrical potential Ψ 0 = Ψ g − Ψ s in the polyelectrolyte gel and the formation of a Donnan quasi-equilibrium. At quasiequilibrium, a charge neutrality is maintained inside the gel: cg− + c− = c+ + cH+

(1)

where cg− is the concentration of the ionized carboxyl groups on the polymer backbone, c− and c+ are the concentrations of the mobile anions and cations diffused into the gel from

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Change of the electrical potential in the polyelectrolyte gel Ψ 0

Fig. 3. and the polymer fraction ϕp as a functions of pH value of surrounding solution.

the surrounding uni-univalent electrolyte solution and cH+ = 10−pH is the concentration of hydrogen ions related to the pH of the environment. Using a Boltzmann distribution for the mobile ions, one obtains
 (2) Ψ 0 = (RT/F ) sinh−1 (cH+ − cg− )/2c where c is the concentration of electrolyte in the surrounding solution, R the universal gas constant, T the absolute temperature, and F the Faraday constant. The concentration of ionized carboxyl groups in the swollen gel is determined as cg– = αϕp cg by the total concentration cg of ionizable groups in the gel before swelling, by the volume fraction ϕp of polymer in the swollen gel and by the dissociation degree of carboxyl groups α. Taking into account that for weak acid carboxyl groups (α  1) the dissociation behavior can be described as α = (Kapp /ϕp cg )1/2 , where Kapp is the apparent constant of dissociation (Ka = 10−4.7 for acrylic acid), one obtains Ψ 0 = (RT/F ) sinh−1 [(cH+ − (Kapp ϕp cg )1/2 )/2c]

(3)

Fig. 3 shows the calculated dependence Ψ 0 = f(pH) at cg = 2 mol/l, c = 0.2 mol/l and the dependence ϕp = f(pH) used for the calculations. The total concentration cg of ionizable groups in the gel was calculated from the PVA/PAA blend composition using the values of the density of the unswollen polymer blend, ρB = 1.288 g/cm3 , and the average molecular weight between crosslinks, Mc = 70 000 g/mol obtained in ref. [4]. The time-dependent change of the solution volume Vs in the swollen gel after the formation of the quasielectroneutrality can be described by a rate ν obtained from the experiment as Vs = Vs0 + νt. Taking into account that the volume fraction of the polymer in the swollen gel is given by ϕp = Vp /(Vp + Vs ), one obtains a time dependence of Ψ as: Ψ = Ψ 0 + (RT/F ) sinh−1 [(cH+ − (Kapp cg /(1 + (Vs0 + νt)/Vp ))1/2 )/2c]

(4)

Here Vp is the volume of polymer. Fig. 4 shows the calculated dependence Ψ = f(t) for the pH1 and pH11 solutions at Vp = 5 × 10−8 l and ν = 5.5 × 10−11 l/s obtained from the

Fig. 4. Time-dependent change of the electrical potential Ψ in pH1 and pH11 solutions.

experiment. As it is evident from Fig. 4, the additional solution sorption induces a change of the Ψ , which consists of two parts. The slow linear increase (2) causes the longtime drift in the swelling/deswelling hydrogel kinetics. In a water/NaOH solution the increasing water sorption leads to an increase in the degree of carboxyl groups dissociation in accordance with Ostwald’s dilution law and induces the subsequent water sorption and swelling, etc. The rate of the sensor signal long-time drift caused by this effect depends on the NaOH concentration in the solution and on the hydrogel swelling degree. In the pH11 solution, a maximal positive drift rate Udrift = 45 ␮V/min was found. The swelling reached its equilibrium at exposition times t ≥ 40 h. Existing crosslinks prevent a complete mixing of the polymer chains with the solvent by providing an elastic restoring force that counters the expansion of the network. Contrarily, in a aqueous HCl solution the dilution of the hydrogel–solution system with water leads to an increasing of the subsequent extraction of water from the hydrogel. In the pH1 solution a maximal negative drift rate Udrift = −65 ␮V/min was found. 4.2. Hysteresis behavior The drift of the sensor signal influences the signal value in the next measurements of solutions with other pH values and complicates the calibration procedure of the pH sensor. This influence can explain a sensor signal hysteresis which was obtained for cycling from acidic to basic conditions and vice versa and the hysteresis loop widening with increasing exposition time at pH11 and at pH1 accordingly [11]. Taking into account the interdependence between the experimentally obtained pKapp , the intrinsic pKa0 for the polyacid, and the change of the electrical potential Ψ on the polyelectrolyte chain as pKapp = pKa0 + 0.4343FΨ/RT [18], one can expect that the apparent pKapp is smeared out over a wide pH range and is also dependent on the ionic strength of the solution (which will set the charge screening effects inside the gel) [19].

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4.3. Swelling/deswelling behavior The PVA/PAA-hydrogels show the time-dependent mechanical properties during the swelling/deswelling in the electrolyte solutions with different pH values which can be described using a mechanical four-element Burger’s model for the viscoelastic polymers in combination with Boltzmann superposition principle. The Burger’s model consists of three parts in series: i. a Hooke’s spring (elastic, spontaneous, reversible) which corresponds to the rapid action of the osmotic forces inducing the gel deformation e0 = σ/K; ii. a Voigt–Kelvin’s spring and dashpot in parallel (viscoelastic, relaxing, time-dependent, reversible) corresponding to the retarded answer of the polymer chains to a stress σ generated in the polymer network and describing the deformation erel (t) = e[1 − exp(−t/τ)] where e = e∞ − e0 and e∞ is the deformation at t → ∞; iii. a Newton’s dashpot (viscous, time-dependent, irreversible) describing a continuous solution flow into/out of the gel with the rate (σ/η) where η is the viscosity. The Boltzmann superposition principle states that for a linear viscoelastic material: 1. a creep is a function of the entire loading history, 2. each loading step makes an independent contribution to the final deformation, 3. the total deformation is the sum of the individual contributions with a time-dependent creep compliance function. The time-dependent deformation is given by e(t) = e + 0

j 

[e0i + ei (1 − exp(−(t − ti )/τi ))

i=0

+σi (t − ti )/ηi ]

(5)

where e0 is the initial deformation, ti the reference time at which σ i is applied, and tj < t [20]. 4.4. Residual deformation The application of the Burger’s model and the Boltzmann’s superposition principle allowed us to calculate the residual deformation ε(t) of the hydrogel after measurement at different pH values. For example, after i cycles of alternating immersions in the solutions 1 and 2, ε(t) was obtained as εi (t) = e1 (t) − e2 (t − (it1 + (i − 1)t2 )) − (σ/η)2 (t − t2 ) = ε0 + ε + e2 exp(−t2 /τ2 ) − e1 exp(−i(t1 + t2 )/τ1 ) + i(t1 + t2 )[(σ/η)1 − (σ/η)2 ]

(6)

where ε0 = e01 –e02 , ε = e01 − e02 , t1 and τ 1 are the exposition and retardation times for the solution 1,

Fig. 5. Residual sensor signal Ures and calculated residual gel deformation ε at cycling between pH7, . . ., pH11 and pH1, . . ., pH11, respectively, in dependence on cycle number.

t2 and τ 2 for the solution 2, and i is the cycle number. As it is evident from Eq. (6), the residual deformation εi can increase or decrease with increasing number of measuring cycles in dependence on the relation of drift rate values. Fig. 5 shows the comparison between experimental results and calculations for cycling between pH11 and pH1 solutions at t1 = 150 min, t2 = 30 min, τ 1 = 26 min, τ 2 = 1.5 min, (σ/η)1 = 4.5 × 10−2 min−1 , (σ/η)2 = 6.5 × 10−2 min−1 , and for cycling between pH11 and pH7 solutions at t1 = t2 = 150 min, τ 1 = 55 min, τ 2 = 50 min, (σ/η)1 = 4.5 × 10−2 min−1 , (σ/η)2 = 3.6 × 10−3 min−1 for the 50 ␮m thick gel layer obtained from the fitting of the experimental curves. The set of experiments was performed to determine the measurement conditions necessary for high signal reproducibility. It was found that the influence of the negative drift after the exposition in HCl solutions can be neutralized only by an 0.1 mol/l sodium hydroxide solution. Then, the residual hydrogel swelling can be compensated by the exposition in de-ionized water for 6 h. No signal drift was found in deionized water. This fact confirms the conjecture that the drift is caused by the continuous change of the electrical potential at the gel–solution interface in consequence of the motion of mobile ions which influences the dissociation state of the fixed charge inside the hydrogel. The measurements in the solutions with pH < 3 and the large pH changes should be avoided to keep the high sensor sensitivity for a long period. A sufficient sensitivity of 2.5 mV/pH at a signal resolution of 0.16 mV was obtained in the pH range of 3–4 for a sensor with a 6 ␮m thick PVA/PAA layer. A very small signal drift rate Udrift = 0.7 ␮V/min was found in this pH range. A sensitivity of 13.2 kHz/pH at a signal resolution of 0.62 kHz has been demonstrated in the hysteresis free pH range of 2.55–3.45 for a microgravimetric sensor with a 390 nm thick PVA/PAA layer and with a sensor response time of about 500 ms [10]. A sensitivity of 60 kPa/pH at a signal resolution of 2 kPa has been obtained in the pH range of hydrogel volume phase transition for a pressure sensor with a 10 ␮m thick DMAEMA-co-HEMA hydrogel layer and with a sensor re-

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5. Conclusions

Fig. 6. Swelling degree of PNIPAAm and output voltage of a PNIPAAm based sensor in aqueous mixtures of ethanol.

sponse time of 10 min [21]. The sensor response time has been estimated in these works at 90% signal level without accounting for the long-time signal drift. A sensitivity of 18.3 ␮m/pH at a bending detection resolution of 10 nm has been demonstrated in the dynamic sensor range for a micromechanical cantilever with a 2.5 ␮m thick PMAA/poly(ethylene glycol) dimethacrylate hydrogel layer [7]. The use a low-noise, lowpower amplifier circuit for the compensation of the previous output signal value and for direct detection of the output signal variation generated by the pH change can increase the sensor sensitivity [22]. In this case an output signal of the sensor was obtained which linearly depended on the pH value. Additional steps should be done in further improving the sensor fabrication to retain the high sensor sensitivity for a long time and for measurements in a wide pH range. Efforts to improve the sensor design for practical use are currently underway in our institute. 4.5. Solvent concentration measurement The same sensor concept was also applied to chemical sensors for the measurement of solvent concentrations in aqueous solutions. For this purpose, the pH-sensitive PVA/PAA was replaced by PNIPAAm. The hydrogel was glued to a socket as a piece of foil. Fig. 6 shows the characteristic of such an organic solvent concentration sensor. The sensor’s output voltage was measured during the swelling of the hydrogel under influence of water solutions with different ethanol contents at T = 22 ◦ C. The volume phase transition occurs at about 20 vol.% of ethanol. This value corresponds to that one of PNIPAAm which was found for the swelling degree defined as the ratio between the masses of swollen and dry hydrogel [12]. The time constant describing the swelling behavior was determined to be about 60 min. The gel shrinkage observed at T = 22 ◦ C corresponds to the results obtained in ref. [23]. The addition of the organic compounds shifts the volume phase transition temperature Tc of the NIPAAm gel from Tc = 34 ◦ C (in pure water) to lower values.

We have demonstrated the operational principle of hydrogel-based chemical and pH sensors. In order to realize pH sensors, PVA/PAA blends with a pH value dependant swelling behavior were used as chemo-mechanical transducers. The hydrogel swelling leads to a bending of a thin silicon membrane and, by this, to an electrical output voltage of the sensor chip. The influence of the gel swelling/deswelling kinetics on the response time and long-term signal stability of proposed pH sensors was investigated. It was found that the observed signal drift, which complicates the calibration procedure of the pH sensor, depends on the pH value of the ambient solution and is caused by the slow continuous change of the electrical potential at the gel–solution interface. The influence of previous gel swelling states can be minimized by a prolonged rinsing in de-ionized water after every measurement at high pH values. Measurements in solutions with pH < 3 and large pH changes should be avoided in order to maintain a sufficient sensor sensitivity for a long time. In order to achieve high signal reproducibility of pH sensors, a compensation of previous output signal values should be used. A similar sensor design was also used for chemical sensors on PNIPAAm basis to measure the content of organic solvents in aqueous solutions. It was found that an initial conditioning in de-ionized water is necessary for high signal reproducibility. In order to shorten the sensor response time, the hydrogel film preparation and the film thickness have to be optimized.

Acknowledgement The authors gratefully acknowledge support for this work from the Deutsche Forschungsgemeinschaft (Collaborative Research Center (SFB) 287, Project C11).

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Biographies Gerald Gerlach received his M.Sc. and Ph.D. degrees in electrical engineering from the Dresden University of Technology in 1983 and 1987,

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respectively. He worked in research and development in the field of sensors and measuring devices and sensors at several companies. In 1993 he became a professor at the Department of Electrical Engineering at Dresden University of Technology. Since 1996 Professor Gerlach has been there the head of the Institute for Solid-State Electronics. His research is focused on sensor and semiconductor technology, simulation and modelling of micromechanical devices and the development of solidstate sensors, especially pyroelectric infrared sensors and piezoresistive humidity and gas sensors. From 1994 to 2000 Prof. Gerlach served as Deputy Dean and Dean of the Electrical Engineering Department at TU Dresden, respectively. Since 2000 he has been vice president of the German Society for Measurement and Automation Technology (GMA) and since 2002 member of the Presidency of the VDE (German Association of Engineers in Electrical Engineering, Electronics, Information Technology). Margarita Guenther received her Ph.D. in physics of semiconductors and dielectrics from Kiev State University, Ukraine, in 1988. Since 1999 she has been Research Scientist at the Institute for Solid-State Electronics at TUD. Her current research interests include the preparation of free-standing polymer films, polymer films on thin silicon membranes and silicon wafers, ion beam modification of active polymer layers for sensor function design, comprehensive characterization of polymer films (chemical–structural, mechanical and electrical properties; ellipsometry, FTIR, ESCA, SAW, AFM, nanoindentation measurements), application of sensitive polymer layers in gas and humidity piezoresistive sensors, chemical and pH-sensors based on hydrogels. Joerg Sorber received his Ph.D. in electrical engineering from Dresden University of Technology, Germany, in 1983. From 1978 to 1995 he worked in the field of the advanced driving systems at the Institute for Precision Engineering at TUD. Since 1996 he has been Senior Scientist at the Institute for Solid-State Electronics at TUD. His current research interests include the design of sensors and sensor systems including the simulation of their components as well as of complex systems, applications of sensitive hydrogels in chemical and pH sensors. Gunnar Suchaneck received his Ph.D. in physico-mathematical sciences from the Electrotechnical University, LETI, St. Petersburg, Russia, in 1983. Since 1984 he has been Senior Scientist at TUD, since 1997 with the Institute for Solid-State Electronics. His current research interests include solid state sensor technology, IR- and UV-thin film sensors, thin film sensors based on PZT thin films, ion implanted and low pressure plasma surface-modified polymer thin films, noises in thin film sensors, uncertainties of sensor property measurements. Karl-Friedrich Arndt studied physics at Dresden University of Technology. In 1975, he got the Ph.D. and in 1982 the Dr.rer.nat.habil. degree at TH Merseburg. Since 1990, he works as a full professor of Physical Chemistry of Polymers at TU Dresden. The main focus of his work are synthesis and characterization of soluble polymers and polymer networks with sensor and actuator properties. Andreas Richter received his Ph.D. in electrical engineering from TUD in 2002. Since 1996 he has been Scientific Assistant at TUD. His current research interests are in applications of “smart hydrogels” in sensorics, process/chemical engineering, medical engineering, biotechnology/microfluidics, and haptic/tactile man–machine communication. He received the following awards: Poster Award 5th Symposium on Polymers for Advanced Technologies, Tokyo 1999; Prof. Schwabe Award, Dresden 2002; Innovation Award 2003 of the Industry Club of Saxony.