- Email: [email protected]
Design, simulation and characterization of hydrogel-based thermal actuators
Accepted Manuscript Title: Design, simulation and characterization of hydrogel-based thermal actuators Author: Kangfa Deng Mathias Rohn Gerald Gerlach PII: DOI: Reference:
S0925-4005(16)30354-9 http://dx.doi.org/doi:10.1016/j.snb.2016.03.060 SNB 19865
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
29-11-2015 4-3-2016 15-3-2016
Please cite this article as: Kangfa Deng, Mathias Rohn, Gerald Gerlach, Design, simulation and characterization of hydrogel-based thermal actuators, Sensors and Actuators B: Chemical http://dx.doi.org/10.1016/j.snb.2016.03.060 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Design, simulation and characterization of hydrogel-based thermal actuators Kangfa Denga, Mathias Rohnb, Gerald Gerlacha,* a
Institute for Solid State Electronics, Technische Universität Dresden, D-01062 Dresden, Germany b Physical Chemistry of Polymers, Technische Universität Dresden, D-01062 Dresden, Germany
* Corresponding author. Tel.: +4935146332077. E-mail address: [email protected] Abstract This paper presents the design, assembly, simulation and testing of hydrogel-based thermal actuators. Actuators utilizing stimuli-responsive hydrogel materials have a number of advantages compared to conventional ones, including relatively simple fabrication and large force generation. The thermal actuator here utilizes a temperature-sensitive hydrogel to provide an adjustable force/pressure with a fast dynamic response. The actuator is used as part of a hydrogel-based pH sensor, where force compensation via this actuator keeps the sensor in its initial state and, hence, avoids relaxation and drift process. A finite element simulation with ANSYS is introduced to assess temperature distribution and maximum allowable actuating pressure in this actuator. Three actuators are assembled and tested. Two crucial factors, including the depth of the hydrogel reservoir and the filling factor of hydrogel, are investigated to improve the response time (1.9 min) and actuating pressure (65 kPa), respectively. The trade-off between actuating pressure and response time should be carefully considered in the further work. Keywords: hydrogel; thermal actuator; response time; thermoelectrical cooler; microelectromechanical systems; actuating pressure Highlights:
1.
A finite element simulation was introduced to assess the temperature homogeneity and maximum allowable actuating pressure in the hydrogel-based thermal actuator. The actuator’s response time could be improved by scaling down the dimension of the cavity and utilizing hydrogel microparticles. The actuator’s actuating pressure could be enhanced by heightening the filling percentage of hydrogel in the cavity. The fixed actuating pressure–response time tradeoff should be carefully considered in the actuator design.
Introduction
Hydrogels show a considerable volume expansion (up to 400 %) in response to certain stimuli, such as temperature, salt concentration, pH, etc.. Compared to other actuation materials, like piezoelectric [1], electrostatic [2], magnetostrictive [3], hydrogels possess a higher energy density and can be effectively applied as a new type of actuator [4]. These novel hydrogel-based actuators come with many advantages compared to the conventional ones, including excellent micro fabrication capability, larger force generation, higher displacement and lower response time. This makes them useful for lots of potential applications, such as hydrodynamic transistors [5], tunable micro-lenses [6], regulation valves in the hybrid MEMS–hydrogel flow control elements [7-9], pumps [10], autonomous drug delivery systems [11], etc.. Since the hydrogel has to be utilized in aqueous solution, slow diffusion of ions and molecules into the crosslinked polymer chains of the hydrogel often limits the response time 𝜏 of the actuator, which depends on the characteristic dimension 𝑙char of the hydrogel and the cooperative diffusion coefficient 𝐷coop : 𝑙char 𝜏~ . (1) 𝐷coop Two strategies have been utilized to improve the diffusion-limited response:
1. Modified hydrogel material properties. For example, synthesis of PNIPAAm nanocomposite gels based on
layered silicates instead of conventional PNIPAAm with organic crosskinker N, N-methylene bisacrylamide (BIS) [12, 13]. 2. Reduced hydrogel dimensions, such as porous gels with fast convective solution transport through the pores into the gels [14], microscale hydrogel particles with high surface-to-volume ratio instead of bulk material [15], spin-coated hydrogel thin film with small characteristic dimension 𝑙char [16, 17]. In this paper, we present a new type of hydrogel-based thermal actuator, which is part of a forcecompensated hydrogel-based pH sensor [18, 19]. The actuator utilizes a temperature-sensitive hydrogel to provide an adjustable force/pressure to compensate the swelling of a pH-sensitive hydrogel and to keep the pH sensor in a low-relaxation state (Fig. 1b). Two crucial aspects are considered: (1) a thermoelectrical cooler with a rapid temperature change is used to accelerate the hydrogel’s thermal response; (2) mechanically stable hydrogel micro-particles are provided to shorten the effective diffusion length, and consequently improve the response time. The synthesis and characterization of the thermosensitive hydrogel will be described. A thermal simulation is utilized to assess the temperature homogeneity while a mechanical simulation is used to estimate the maximum actuating pressure allowed in the actuator. The crucial actuator parameters including response time and actuating pressure will be characterized. Since the hydrogel is enclosed in the etched cavity of a silicon die, the influences of the depth of the cavity and the filling factor of hydrogel on the actuator parameters are studied to improve the actuator’s performance. 2.
Design and assembly
2.1. Working principle and structure The hydrogel-based thermal actuator (Fig. 1b) comprises a Peltier element, a silicon cap (Fig. 1d), a temperature-sensitive hydrogel and a piezoresistive pressure sensor (Fig. 1c). After being heated up or cooled down by the Peltier element, the hydrogel decreases or increases its volume, generating a swelling pressure 𝑝sens . The pressure sensor (EPCOS, Germany), which is employed as a mechano-electrical transducer, transforms the pressure into an electrical output voltage 𝑉out . The nanoporous Al2 O3 membrane (Whatman, USA) with a pore diameter of 200 nm permits water to penetrate into the hydrogel (Fig. 1e). In the case of the above mentioned pH sensor, the cavity of the pressure sensor is filled with the pH-sensitive hydrogel, which is shown as a dash square in Fig. 1b. The peltier element controls the temperature of the temperature-sensitive hydrogel of the actuator in such a way that the swelling pressure of the pH-sensitive hydrogel is compensated. Compensation is reached when the swelling pressures from both hydrogels are equal. 2.2. Fabrication 2.2.1. Temperature-sensitive hydrogel Poly (N-isopropylacrylamide) (PNIPAAm) is one of the most widely studied thermosensitive hydrogels [20, 21]. After being heated in water over the lower critical solution temperature (LCST), it undergoes a reversible phase transition from the swollen to the shrunken state, thereby losing about 90% of its volume. Aiming at raising LCST, PNIPAAm copolymer with hydrophilic monomer 3-acrylamidopropionic acid (AAmPA) is synthesized by free radical polymerization [20]. Table 1 lists the function and concentration of every component for this PNIPAAm copolymer. The basic monomer NIPAAm, AAmPA, the crosslinker BIS and the accelerator TMEDA are dissolved in deionized (DI) water in a flask at 0 °C. After nitrogen purges into the solution for 15 min, the initiator KPS is added at room temperature for the polymerization process. After 24 h, a gel is formed and washed through DI water several times to remove the residual molecules. Finally, the gel is milled into microscale particles. The swelling behavior of the hydrogel is tested at different temperatures by the following procedures:
1. Put the hydrogel into DI water and keep it at temperature 𝑇1 for 1 h. Then take out the gel and remove the solvent surrounding it. The weight of the gel is presented as 𝑊t . 2. Weight the gel (i.e. 𝑊d ) again after drying in vacuum for 1 day. 3. The weight degree 𝑄m of swelling is defined as the ratio of the weight 𝑊t of the swollen hydrogel over the dried copolymer network 𝑊d : 𝑊t 𝑄m = . (2) 𝑊d 2.2.2. PDMS protection layer Since the wet hydrogel comes directly in contact with electrical components of the pressure sensor, the electrical leakage has to be avoided by a protection/barrier layer. We use Polydimethylsiloxane (PDMS) (SYLGARD 184, Dow Corning Corporation, USA), which has excellent electrically insulating and water-resistant properties [22, 23], to coat on the membrane of the pressure sensor. 2.2.3. Thermoelectric cooler A thermoelectric cooler (TEC) serves as the thermal source for the thermosensitive hydrogel, so that the hydrogel could convert thermal energy into mechanical work. The TEC provides an efficient way to control the temperature precisely with fast response. It comprises a Peltier element (UWE electronic, Germany), a heat sink with fan, a heat spreader and a Pt100 temperature sensor (Fig. 2a). The heat spreader is glued to the Peltier element to improve the heat distribution. The Peltier element creates a heat flux between the p-n junctions of the semiconductors, making one side cooler while the other side hotter. By adjusting the flow direction of the DC current through the Peltier element, the heat spreader could selectively heat up or cool down the sample. This temperature control system has a rise and fall response time of 120 s and 146 s, respectively, with a temperature increment of 40 K (Fig. 2b). With a smaller variation of temperature like 5 K, the response time could reduce down to 30s. The study of the response time of TEC provides a guideline to estimate the final response time of the actuators. 2.2.4. Silicon cap The fabrication process of the silicon cap is based on a bulk micromachining process and illustrated in Fig. 3a: 1. RCA cleaning. The fabrication starts with a 3 inch, single crystal (100) silicon wafer (thickness 300 μm). This wafer is cleaned using the standard RCA clean process. 2. PECVD Si3 N4 . The Si3 N4 layer with a thickness of 250 nm is deposited on both sides of the substrate as hard masks for the wet etching process. 3. RIE Si3 N4 . Photolithography is taken to pattern the cavity, followed by a reactive ion etching (RIE) step to remove the related Si3 N4 layer. 4. Wet etching. The silicon structure is etched by potassium hydroxide (KOH) solution (concentration 31%) at 80 °C with an etching rate of 1.16 μm /min. Fig. 3b-c shows the silicon caps after the wet etching process. 2.3. Actuator assembly and packaging Fig. 4a depicts the assembly process of the actuator with the following steps: 1. PDMS coating on the pressure sensor (5 × 5 × 0.4 mm3 ). Firstly, the surface of the pressure sensor is modified by O2 plasma (400W, 20 Pa, 70s). Then the PDMS solution is spin-coated on the crosslinked in the oven at 70 °C for 1 h. Finally, a PDMS layer with a thickness of 10 μm is achieved. 2. Integration of the hydrogel reservoir (3.5 × 5 × 0.3 mm3 ) as well as the hydrogel particles on the top side of the pressure sensor. 3. Alignment and bonding of the Al2 O3 nanoporous membrane (3.5 × 5 × 0.06 mm3 ) and the water reservoir (3.5 × 5 × 0.3 mm3 ) with the hydrogel reservoir. Finally an actuator prototype (5 × 5 × 1.1 mm3 ) after the assembly process is shown in Fig. 4b.
3.
Simulation
Under the applied temperature from the peltier element, the temperature homogeneity of the temperaturesensitive hydrogel and thus the actuating-pressure homogeneity of the actuator are of interest. A finite element modeling with ANSYS could provide a useful tool to investigate the temperature distribution on the actuator, especially on the thermo-responsive hydrogel. Besides, the deformation and stress on the membrane of the pressure sensor are crucial factors to determine the maximum allowable filling factor of hydrogel during the assembly process. Both thermal simulation and mechanical simulation could facilitate prospective actuator design. 3.1. Thermal simulation As for hydrogel-based microsystems, the governing equations relating the heat transfer rate and the temperature gradient for the heat transfer mechanism include [24] thermal conduction 𝑄 d𝑇 ′′ (3) 𝑞cond = = −𝑘 , 𝐴 dx and thermal convection 𝑄 ′′ (4) 𝑞conv = = −ℎ(𝑇s − 𝑇f ), 𝐴 ′′ ′′ with Q the heat flow, 𝑞cond and 𝑞conv the conduction heat flux (W/m2 ) and the convection heat flux 2 (W/m ), respectively, 𝑘 the thermal conductivity (W/mK), ℎ the convective heat transfer coefficient (W/m2 K)，𝑇s the temperature of the surface, 𝑇f the temperature of the fluid ( air in this case), 𝑇s the absolute temperature (K) of the surface and A the section area (m2 ) for the heat transfer. A thermal simulation by ANSYS is performed to assess the temperature influence on this actuator (Fig. 5a). Since the cavity in the silicon cap is totally filled with hydrogel particles and water, hydrogel is presumed to have the same thermal conductivity as water. For the finite element simulation is assumed the following boundary conditions and loading (Fig. 5b): 1. The top side of the silicon cap is set to a high temperature due to the heat flow from the Peltier module. 2. The convection boundaries on the sidewalls of the pressure sensor and the silicon caps are set to be stagnant air (simplified case, 20 °C). Since all the components, like the pressure sensor, the silicon cap and the Al2 O3 membrane, have high thermal conductivities, this actuator has an almost homogenous temperature distribution with a tiny temperature difference ∆𝑇 (Fig. 5b). Fig. 5c summarizes ∆𝑇 as a function of the applied temperature from the peltier element and for two different reservoir depths of 160 and 360 μm. ∆𝑇 increases gradually with the applied temperature from 20 to 80 °C. Smaller value of the depth 𝑑rese of the hydrogel reservoir yields a shorter heat transfer path through the hydrogel which then decreases ∆𝑇. From those simulation results, an actuator with smaller 𝑑rese can improve the temperature homogeneity. 3.2 Mechanical simulation For the pressure sensor, the maximum displacement 𝑤max and the maximum stress 𝜎max are of interest [25]. The membrane of the pressure sensor serves as a square plate under a uniform pressure p (Fig. 6). The maximum displacement yields 𝑝𝑎4 (5) 𝑤max = 0.0138 3 , 𝐸ℎ where E the Young’s modulus of the silicon, h and a the thickness and length of the membrane, respectively. The maximum stress at the center point of the edge becomes 𝑝𝑎2 (6) 𝜎max = 0.3078 2 . ℎ For the finite element simulation in Fig. 7a, the sidewalls of the pressure sensor and silicon caps are set to be fixed boundaries. The hydrogel generates an actuating pressure 𝑝actu inside the hydrogel reservoir. The simulation result in Fig. 7b shows that the pressure sensor has a maximum deformation of 6.1 μm under 𝑝actu of 2 kPa. According to equation (5), the Al2 O3 membrane has a larger Young’s modulus and thickness than
the silicon membrane, leading to a main deformation drop on the silicon membrane. Fig. 7c shows the maximum stress and maximum deformation on the membrane as functions of 𝑝swel. Maximum stress increases with 𝑝swel. When 𝑝swel reaches 70 kPa, the maximum stress (i.e. 780 MPa) is over the fracture stress of silicon [26] where the silicon membrane would break. Those simulation results give a hint of the maximum actuating pressure allowed in the actuator. 4.
Test and discussion
4.1. Hydrogel characteristic The swelling behavior is strongly affected by the copolymerization ratio. Therefore, the influences of the concentration of the monomer AAmPA on the swelling degree of the hydrogel are studied. To be specific, by increasing the monomer amount AAmPA from 2 to 3.5 mol%, the carboxylic acid groups in the PNIPAAm copolymer increases, resulting in a higher degree of swelling 𝑄m as well as a high volume phase transition temperature (VPTT) from 50 to 60°C, as shown in Fig. 8a. Moreover, the optical appearance of the hydrogel varies from transparent to opaque as the temperature increases (Fig. 8b-e). Below VPTT, predominantly intermolecular hydrogen bonding between polymer chains and water molecules makes PNIPAAm polymer chains mostly extend, leading the hydrogel to exhibit a transparent morphology. Above VPTT, the waterpolymer hydrogen bonds are broken, resulting in a compact and collapsed conformation of PNIPAAm chains. Thus the gel reveals opaque at high temperature. Due to its higher 𝑄m as well as VPTT, PNIPAAm copolymer with 3.5 mol% AAmPA is selected for the thermal actuators. By gel milling, microscale hydrogel particles have been formed with a dimension between 10 and 50 μm (Fig. 9). Those hydrogel particles have a higher surface area exposed to the surrounding solution compared to the macroscale hydrogel layer, resulting in a faster stimuli response for the actuator. 4.2 Actuator characterization Three miniaturized thermal actuators S1, S2, S3 are assembled having different depths 𝑑rese of the hydrogel reservoir, which determine the volume of the cavity, and different filling factors 𝑉gel of the hydrogel in the cavity (Table 2). The influences of different 𝑑rese and 𝑉gel on the actuator performance, such as operation temperature range, response time, actuating pressure, are investigated. All those actuators are characterized by the measuring setup in Fig. 10. A Labview program sets temperature commands to the temperature controller (BelektroniG K10, Germany) as well as receives the voltage signals from the data acquisition device (Labjack U12, USA). 4.2.1. Response time Fig. 11a demonstrates that actuator S1 successfully regulates the actuating pressure/force in response to temperature. By gradually increasing the temperature with a step of 2 °C, the hydrogel decreases its volume, leading to a reduction of the actuating pressure and thus the output voltage. Besides, a pulse-wise change of the temperature between 30 and 35 °C reveals a stable sensor output without long-term drift of the output signal (Fig. 11b). Response time 𝑡90 , the time taken for the actuator response to achieve 90% of the step height, is utilized to evaluate the dynamic response of the actuator. 10 continuous cycles of measurement are performed with a temperature step of 2 K and 5 K, respectively. 𝑡90 decreases with increasing temperature from 20 to 36 °C (Fig. 12a). That is caused by 𝐷coop in equation (1), which changes proportionally with temperature. Therefore, hydrogel at higher temperature shows a higher 𝐷coop value, resulting in a lower response time. In addition, the temperature-decreasing process (i.e. hydrogel swelling) has a larger 𝑡90 than the temperature increasing-process (i.e. hydrogel shrinking) (Fig. 12b). This is caused by the shrinkage barrier effect during the hydrogel swelling process: Since the outer parts of the polymer network swell very fast, the diffusion profile decreases significantly from these swollen outer regions to the non-swollen inner parts of the polymer network, taking the hydrogel a longer time to swell completely. During the shrinking process, however, the inner and outer parts shrink at the same time in response to temperature, resulting in a quick shrinkage of the hydrogel.
4.2.2. Transfer characteristics Fig. 13a depicts the actuating pressure 𝑝actu as well as output voltage 𝑉out as functions of the temperature and for both T-increasing and T-decreasing processes. There is a nonlinear relationship 𝑝actu (𝑇) while the maximum 𝑝actu is around 33.3 kPa at 20 °C. Furthermore, the T-increasing and T-decreasing curves are slightly different, revealing a hysteresis effect of the actuator. The maximum actuating pressure hysteresis is 1.3% at 32 °C. As shown in Fig. 13b, the actuator sensitivity is determined to be in the range of -1 kPa/K to 3.2 kPa/K. The sensitivity at 36 °C is three times higher than that one at 20 °C. It is assumed that, with higher temperature, the hydrogel has a larger variance of the swelling degree 𝑄m during the free swelling test (Fig. 8a), leading to a larger change of the sensitivity. 4.2.3. Influence of depth 𝑑𝑟𝑒𝑠𝑒 of hydrogel reservoir Since the diffusion of a hydrogel into its surrounding solution is a rate-limiting factor governing the hydrogel’s swelling process, 𝑡90 follows the characteristic dimension 𝑙char . In order to shorten 𝑡90 , a lower depth 𝑑rese of the hydrogel reservoir with smaller 𝑙char and thus shorter diffusion path is preferred. The relationship between 𝑑rese and 𝑡90 is studied by comparing actuators S1 and S2 (Fig. 14). Compared to S1, the response time of S2 drops by 1.5 to 3.3 times, depending on the temperature. Besides, S2 owns a more homogenous temperature distribution in the hydrogel due to a smaller temperature difference, according to the thermal simulation results (Fig. 5c). However, because of a lower swelling pressure of the hydrogel, S2 has only 50% of the actuating pressure and 20% of the sensitivity relative to S1 (Table 2). Overall, this type of actuator is suitable for a low-scale actuating pressure range with fast dynamic response. 4.2.4. Influence of filling factor 𝑉𝑔𝑒𝑙 of hydrogel A lower 𝑉gel leads to a smaller deflection of the membrane in the actuator (i.e. lower 𝑝actu ) and a lower temperature at which 𝑝actu = 0, consequently limiting the operation temperature range (for example, 20…40 °C in actuator S1). In some cases, some practical applications require large actuating forces/pressures and wide operation temperature ranges, such as artificial muscles (1MPa) [27] and microvalves (200 kPa) [28]. In order to meet this demand, it is preferred to increase 𝑉gel . The relationship between 𝑉gel and 𝑝actu is studied by comparing actuators S1 and S3 (Fig. 14). The actuating pressure of S3 has twice the value of that of S1 (Fig. 14). The maximum 𝑝actu (i.e. 65 kPa) of S3 is lower than the maximum allowable actuating pressure from the mechanical simulation results (Fig. 8c), indicating that this actuator is in a robust operation state. Besides, the maximum operation temperature increases to 55°C (Fig. 15). It is believed that the operation temperature could reach VPTT (60°C) if dry hydrogel will completely fill the cavity (i.e. 𝑉gel = 100%). However, excessive amounts of hydrogel (𝑉gel > 100%) will lead to an offset actuating pressure, and even worse, break the silicon membrane in the actuator. In short, attribute to its high filling factor 𝑉gel of hydrogel, actuator S3 is advantageous to achieve high actuating pressure as well as wide operation temperature range. Modifying the hydrogel material properties could be an alternative method to improve the actuating pressure. To be specific, PNIPAAm hydrogel with larger amounts of AAmPA has both a higher swelling degree and VPTT, which leads to a higher actuating pressure and wider operation temperature range. Such hydrogel actuators above have a trade-off between response time and actuating pressure: smaller amount of hydrogel in the actuator could shorten the diffusion path as a means to reduce response time, but at the expense of a decreased actuating pressure and operation temperature range. Although the time response of the actuator with high actuating pressure (i.e. actuator S3) is relatively slow (Fig. 14), it is believed that changes in the hydrogel material properties and hydrogel structures can further improve the time response and make hydrogel actuation widely applicable to microsystems. 5.
Conclusion
In this paper, hydrogel-based thermal actuators, aiming at providing a precisely adjustable actuating pressure with fast dynamic response, has been designed, assembled, simulated and tested. Hydrogel microparticles with high surface-to-volume ratio were synthesized to improve the response time of the actuator. Three thermal actuators with different depths of the hydrogel reservoir and different filling factors of the hydrogel were assembled and tested. It was shown the response time and the actuating pressure decreased with rising temperature. The response time has been improved by a factor of 1.5…3.3 by scaling down the depth of the cavity. The actuating pressure has been enhanced by a factor of 2 by increasing the filling factor
of hydrogel in the cavity. Actuator S2 presents the lowest response time of 1.9 min while actuator S3 exhibits the maximum actuating pressure of 65 kPa. The experiments show that there is a trade-off relationship between response time and actuating pressure. By adjusting the actuator properties (concentration of AAmPA in PNIPAAm gels, hydrogel shape and size, depth of the cavity), those thermal actuators can be used in extensive potential applications. Acknowledgements This work was supported by the Deutsche Forschungsgemeinschaft within the Research Training Group Hydrogel-Based Microsystems (DFG-GRK 1865). Reference [1] Tzou H, Tseng C. Distributed piezoelectric sensor/actuator design for dynamic measurement/control of distributed parameter systems: a piezoelectric finite element approach. J Sound Vib. 1990;138:17-34. [2] Xu C, Wang S, Tang G, Yang D, Zhou B. Sensing characteristics of electrostatic inductive sensor for flow parameters measurement of pneumatically conveyed particles. J Electrostat. 2007;65:582-592. [3] Kwun H, Bartels K. Magnetostrictive sensor technology and its applications. Ultrasonics. 1998;36:171178. [4] Gerlach G, Arndt KF. Hydrogel sensors and actuators - engineering and technology. Urban G, editor. Heidelberg: Springer; 2009. [5] Peters EC, Svec F, Fréchet JMJ. Thermally responsive rigid polymer monoliths. Adv Mater. 1997;9:630-633. [6] Kim J, Serpe MJ, Lyon LA. Hydrogel Microparticles as Dynamically Tunable Microlenses. J Am Chem Soc. 2004;126:9512-9513. [7] Arndt KF, Kuckling D, Richter A. Application of sensitive hydrogels in flow control. Polym Advan Technol. 2000;11:496-505. [8] Eddington DT, Beebe DJ. Flow control with hydrogels. Adv Drug Deliver Rev. 2004;56:199-210. [9] Liu RH, Yu Q, Beebe DJ. Fabrication and characterization of hydrogel-based microvalves. J Microelectromech Syst. 2002;11:45-53. [10] Richter A, Klatt S, Paschew G, Klenke C. Micropumps operated by swelling and shrinking of temperature-sensitive hydrogels. Lab Chip. 2009;9:613-618. [11] Yuandong G, Baldi A, Ziaie B, Siegel RA, editors. Modulation of drug delivery rate by hydrogelincorporating MEMS devices. 2nd Annual International IEEE-EMBS Special Topic Conference on Microtechnologies in Medicine and Biology; 2002. [12] Schulz V. Advanced hydrogel-based chemical microsensors. Dresden: TUDpress; 2013. [13] Ferse B, Richter S, Eckert F, Kulkarni A, Papadakis CM, Arndt KF. Gelation Mechanism of Poly(Nisopropylacrylamide)-Clay Nanocomposite Hydrogels Synthesized by Photopolymerization. Langmuir. 2008;24:12627-12635. [14] Schulz V, Zschoche S, Zhang HP, Voit B, Gerlach G, editors. Macroporous smart hydrogels for fastresponsive piezoresistive chemical microsensors. Procedia Engineering 25; 2011. [15] Pelton R. Temperature-sensitive aqueous microgels. Advances in Colloid and Interface Science. 2000;85:1-33. [16] Kuckling D, Harmon ME, Frank CW. Photo-cross-linkable PNIPAAm copolymers. 1. Synthesis and characterization of constrained temperature-responsive hydrogel layers. Macromolecules. 2002;35:63776383. [17] Kuckling D, Hoffmann J, Plotner M, Ferse D, Kretschmer K, Adler HJP, Arndt KF, Reichelt R. Photo cross-linkable poly(N-isopropylacrylamide) copolymers III: micro-fabricated temperature responsive hydrogels. Polymer. 2003;44:4455-4462. [18] Deng K, Rohn M, Guenther M, Gerlach G, editors. Thermal microactuator based on temperaturesensitive hydrogel. Procedia Engineering 120; 2015. [19] Deng K, Gerlach G, Guenther M, editors. Force-compensated hydrogel-based pH sensor. Proc SPIE 9431, Active and Passive Smart Structures and Integrated Systems 2015; 2015; San Diego, United States.
[20] Kuckling D, Richter A, Arndt KF. Temperature and pH-dependent swelling behavior of poly(Nisopropylacrylamide) copolymer hydrogels and their use in flow control. Macromol Mater Eng. 2003;288:144-151. [21] Kuckling D, Adler HJP, Arndt KF, Ling L, Habicher WD. Temperature and pH dependent solubility of novel poly(N-isopropylacrylamide) copolymers. Macromol Chem Physic. 2000;201:273-280. [22] Kersey L, Ebacher V, Bazargan V, Wang R, Stoeber B. The effect of adhesion promoter on the adhesion of PDMS to different substrate materials. Lab Chip. 2009;9:1002-1004. [23] Satyanarayana S, Karnik RN, Majumdar A. Stamp-and-stick room-temperature bonding technique for microdevices. J Microelectromech Syst. 2005;14:392-399. [24] Pitts DR, Sissom LE. Heat transfer. New York: McGraw-Hill; 1998. [25] Liu C. Foundations of MEMS. New Jersey: Pearson Prentice Hall; 2006. [26] Sooriakumar K, Chan W, Savage TS, Fugate C, editors. A comparative study of wet vs. dry isotropic etch to strengthen silicon micro-machined pressure sensor. Electrochem Soc Proc; 1995. [27] Shahinpoor M, Kim KJ, Mojarrad M. Artificial muscles: applications of advanced polymeric nanocomposites: CRC Press; 2007. [28] Wang J, Chen ZY, Mauk M, Hong KS, Li MY, Yang S, Bau HH. Self-actuated, thermo-responsive hydrogel valves for lab on a chip. Biomed Microdevices. 2005;7:313-322.
Biographies Kangfa Deng received the M.Sc. degree in Microelectronics Engineering and Solid State Electronics from Peking University, Beijing, China, in 2013, where he is a research assistant and currently pursuing the Ph.D. degree in Solid-State Electronics Laboratory, Technische Universität Dresden. His interests include design, assembly, and the improvement of force-compensated hydrogel-based chemical sensors. Mathias Rohn received the M.Sc. degree in Chemistry from Potsdam University and Fraunhofer Institute for advanced polymerscience Potsdam-Golm, Germany. In 2014 he is a research assistant and currently pursuing the Ph.D. degree in physical chemistry of polymers, Technische Universität Dresden. His interests include synthesis and characterization of hydrogels, with the focus on relationship between structure and properties of hydrogels. Gerald Gerlach received M.Sc. and Ph.D. degrees in electrical engineering from the Dresden University of Technology in 1983 and 1987, 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 full professor at the Department of Electrical and Computer Engineering at Technische Universität Dresden (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 modeling of micromechanical devices and the development of solid-state sensors, e.g. pyroelectric infrared sensors and piezoresistive chemical sensors. From 2007 to 2010 he served as President of the German Society for Measurement and Automatic Control (GMA). From 2007 to 2008 he was Vice President and President of EUREL (The Convention of National Societies of Electrical Engineers of Europe), respectively. Since 2013 he has been Chairman of the German Association of Technical-scientific Societies (DVT). Prof. Gerlach is Associate Editor-in-Chief of the IEEE Sensors Journal and Chief Editor of JSSS, the Journal of Sensors and Sensor Systems.
Fig. 1. Hydrogel-based thermal actuator: (a) working principle and (b) cross-section structure. Microscope photos of (c) the pressure sensor, (d) the silicon cap, SEM images of (e) the nanoporous Al2 O3 membrane separating the water reservoir and (f) the temperature-sensitive hydrogel.
Fig. 2. (a) Prototype and (b) temperature response of the thermoelectrical cooler (TEC)
Fig. 3. (a) Fabrication process of the silicon cap, photographs of silicon caps after wet etching process with (b) full view and (c) close-up view
Fig. 4. (a) Assembly steps of the thermal actuator and (b) actuator prototype after the assembly process
Fig. 5. Thermal simulation of the thermal actuator: (a) boundary conditions, (b) temperature distribution after an applied temperature (60 °C) from the peltier element and (c) relationship between temperature difference ∆𝑇 and applied temperature
Fig. 6. Square plate under a uniform stress. (a)Top view and (b) side view with point of the maximum stress 𝜎max and the maximum displacement 𝑤max , respectively
Fig. 7. Mechanical simulation of the thermal microactuator: (a) boundary conditions, (b) deformation distribution under an actuating pressure 𝑝actu of 2 kPa and (c) relationship between the maximum stress, maximum deformation and actuating pressure
Fig. 8. (a) Weight degree 𝑄m of swelling of PNIPAAm with 2, 3.5 mol% AAmPA when the hydrogel is immersed into DI water and (b-e) photographs of PNIPAAm with 3.5 mol% AAmPA between 30 and 75 °C
Fig. 9. SEM images of the temperature-sensitive hydrogel particles with different magnification
Fig. 10. Measuring setup for the hydrogel-based thermal actuators
Fig. 11. Temperature-dependent output signal of the actuator S1: (a) for incremental temperature step between 20 and 36 °C and (b) for pulse-wise temperature excitation between 25 and 30 °C
Fig. 12. Response time 𝑡90 of the actuator S1 with a temperature step of (a) 2 °C and (b) 5 °C.
Fig. 13. (a)Transfer characteristics and (b) sensitivity as functions of temperature for the thermal actuator S1.
Fig. 14. Response time and actuating pressure for three thermal actuators S1 (360 μm, 60%), S2 (160 μm, 60%), S3 (360 μm, 95%)
Fig. 15. Actuating pressure and sensitivity as functions of temperature for the thermal actuator S3.
Table 1. Components for the PNIPAAm hydrogel Component Nisopropylacrylamid e (NIPAAm) 3acrylamidopropioni c acid (AAmPA) N,N-methylene bisacrylamide (BIS) Potassium peroxodisulfate (KPS) N,N,N',N'tetramethyl-ethylenediamine (TEMED)
Purpose
mol%
Hydrogel network monomer
92.3…9 3.8
Hydrophilic monomer
2…3.5
Crosslinker
3.65
Initiator
0.27
Accelerator
0.27
Table 2. Comparison of actuator parameters of thermal actuators S1 (360 μm, 60%), S2 (160 μm, 60%), S3 (360 μm, 95%) Parameter/actuator Depth 𝒅𝐫𝐞𝐬𝐞 of hydrogel reservoir (μm) Filling factor 𝑽𝐠𝐞𝐥 of hydrogel* (%) Operation temperature range (°C) Sensitivity (kPa/°C)
S1 360
S2 160
S3 360
60
60
95
20…40
20…40 20…55
-1…-3.2
-0.5…- -1.4…1.0 2.6 0…23 0…65
Actuating pressure range 0…33.3 (kPa) Response time @ 25…35 2.7…8.7 1.9…2. 6.3…7. °C (min) 5 4 * Percentage of cavity volume filled with dry hydrogel particles
S0925-4005(16)30354-9 http://dx.doi.org/doi:10.1016/j.snb.2016.03.060 SNB 19865
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
29-11-2015 4-3-2016 15-3-2016
Please cite this article as: Kangfa Deng, Mathias Rohn, Gerald Gerlach, Design, simulation and characterization of hydrogel-based thermal actuators, Sensors and Actuators B: Chemical http://dx.doi.org/10.1016/j.snb.2016.03.060 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Design, simulation and characterization of hydrogel-based thermal actuators Kangfa Denga, Mathias Rohnb, Gerald Gerlacha,* a
Institute for Solid State Electronics, Technische Universität Dresden, D-01062 Dresden, Germany b Physical Chemistry of Polymers, Technische Universität Dresden, D-01062 Dresden, Germany
* Corresponding author. Tel.: +4935146332077. E-mail address: [email protected] Abstract This paper presents the design, assembly, simulation and testing of hydrogel-based thermal actuators. Actuators utilizing stimuli-responsive hydrogel materials have a number of advantages compared to conventional ones, including relatively simple fabrication and large force generation. The thermal actuator here utilizes a temperature-sensitive hydrogel to provide an adjustable force/pressure with a fast dynamic response. The actuator is used as part of a hydrogel-based pH sensor, where force compensation via this actuator keeps the sensor in its initial state and, hence, avoids relaxation and drift process. A finite element simulation with ANSYS is introduced to assess temperature distribution and maximum allowable actuating pressure in this actuator. Three actuators are assembled and tested. Two crucial factors, including the depth of the hydrogel reservoir and the filling factor of hydrogel, are investigated to improve the response time (1.9 min) and actuating pressure (65 kPa), respectively. The trade-off between actuating pressure and response time should be carefully considered in the further work. Keywords: hydrogel; thermal actuator; response time; thermoelectrical cooler; microelectromechanical systems; actuating pressure Highlights:
1.
A finite element simulation was introduced to assess the temperature homogeneity and maximum allowable actuating pressure in the hydrogel-based thermal actuator. The actuator’s response time could be improved by scaling down the dimension of the cavity and utilizing hydrogel microparticles. The actuator’s actuating pressure could be enhanced by heightening the filling percentage of hydrogel in the cavity. The fixed actuating pressure–response time tradeoff should be carefully considered in the actuator design.
Introduction
Hydrogels show a considerable volume expansion (up to 400 %) in response to certain stimuli, such as temperature, salt concentration, pH, etc.. Compared to other actuation materials, like piezoelectric [1], electrostatic [2], magnetostrictive [3], hydrogels possess a higher energy density and can be effectively applied as a new type of actuator [4]. These novel hydrogel-based actuators come with many advantages compared to the conventional ones, including excellent micro fabrication capability, larger force generation, higher displacement and lower response time. This makes them useful for lots of potential applications, such as hydrodynamic transistors [5], tunable micro-lenses [6], regulation valves in the hybrid MEMS–hydrogel flow control elements [7-9], pumps [10], autonomous drug delivery systems [11], etc.. Since the hydrogel has to be utilized in aqueous solution, slow diffusion of ions and molecules into the crosslinked polymer chains of the hydrogel often limits the response time 𝜏 of the actuator, which depends on the characteristic dimension 𝑙char of the hydrogel and the cooperative diffusion coefficient 𝐷coop : 𝑙char 𝜏~ . (1) 𝐷coop Two strategies have been utilized to improve the diffusion-limited response:
1. Modified hydrogel material properties. For example, synthesis of PNIPAAm nanocomposite gels based on
layered silicates instead of conventional PNIPAAm with organic crosskinker N, N-methylene bisacrylamide (BIS) [12, 13]. 2. Reduced hydrogel dimensions, such as porous gels with fast convective solution transport through the pores into the gels [14], microscale hydrogel particles with high surface-to-volume ratio instead of bulk material [15], spin-coated hydrogel thin film with small characteristic dimension 𝑙char [16, 17]. In this paper, we present a new type of hydrogel-based thermal actuator, which is part of a forcecompensated hydrogel-based pH sensor [18, 19]. The actuator utilizes a temperature-sensitive hydrogel to provide an adjustable force/pressure to compensate the swelling of a pH-sensitive hydrogel and to keep the pH sensor in a low-relaxation state (Fig. 1b). Two crucial aspects are considered: (1) a thermoelectrical cooler with a rapid temperature change is used to accelerate the hydrogel’s thermal response; (2) mechanically stable hydrogel micro-particles are provided to shorten the effective diffusion length, and consequently improve the response time. The synthesis and characterization of the thermosensitive hydrogel will be described. A thermal simulation is utilized to assess the temperature homogeneity while a mechanical simulation is used to estimate the maximum actuating pressure allowed in the actuator. The crucial actuator parameters including response time and actuating pressure will be characterized. Since the hydrogel is enclosed in the etched cavity of a silicon die, the influences of the depth of the cavity and the filling factor of hydrogel on the actuator parameters are studied to improve the actuator’s performance. 2.
Design and assembly
2.1. Working principle and structure The hydrogel-based thermal actuator (Fig. 1b) comprises a Peltier element, a silicon cap (Fig. 1d), a temperature-sensitive hydrogel and a piezoresistive pressure sensor (Fig. 1c). After being heated up or cooled down by the Peltier element, the hydrogel decreases or increases its volume, generating a swelling pressure 𝑝sens . The pressure sensor (EPCOS, Germany), which is employed as a mechano-electrical transducer, transforms the pressure into an electrical output voltage 𝑉out . The nanoporous Al2 O3 membrane (Whatman, USA) with a pore diameter of 200 nm permits water to penetrate into the hydrogel (Fig. 1e). In the case of the above mentioned pH sensor, the cavity of the pressure sensor is filled with the pH-sensitive hydrogel, which is shown as a dash square in Fig. 1b. The peltier element controls the temperature of the temperature-sensitive hydrogel of the actuator in such a way that the swelling pressure of the pH-sensitive hydrogel is compensated. Compensation is reached when the swelling pressures from both hydrogels are equal. 2.2. Fabrication 2.2.1. Temperature-sensitive hydrogel Poly (N-isopropylacrylamide) (PNIPAAm) is one of the most widely studied thermosensitive hydrogels [20, 21]. After being heated in water over the lower critical solution temperature (LCST), it undergoes a reversible phase transition from the swollen to the shrunken state, thereby losing about 90% of its volume. Aiming at raising LCST, PNIPAAm copolymer with hydrophilic monomer 3-acrylamidopropionic acid (AAmPA) is synthesized by free radical polymerization [20]. Table 1 lists the function and concentration of every component for this PNIPAAm copolymer. The basic monomer NIPAAm, AAmPA, the crosslinker BIS and the accelerator TMEDA are dissolved in deionized (DI) water in a flask at 0 °C. After nitrogen purges into the solution for 15 min, the initiator KPS is added at room temperature for the polymerization process. After 24 h, a gel is formed and washed through DI water several times to remove the residual molecules. Finally, the gel is milled into microscale particles. The swelling behavior of the hydrogel is tested at different temperatures by the following procedures:
1. Put the hydrogel into DI water and keep it at temperature 𝑇1 for 1 h. Then take out the gel and remove the solvent surrounding it. The weight of the gel is presented as 𝑊t . 2. Weight the gel (i.e. 𝑊d ) again after drying in vacuum for 1 day. 3. The weight degree 𝑄m of swelling is defined as the ratio of the weight 𝑊t of the swollen hydrogel over the dried copolymer network 𝑊d : 𝑊t 𝑄m = . (2) 𝑊d 2.2.2. PDMS protection layer Since the wet hydrogel comes directly in contact with electrical components of the pressure sensor, the electrical leakage has to be avoided by a protection/barrier layer. We use Polydimethylsiloxane (PDMS) (SYLGARD 184, Dow Corning Corporation, USA), which has excellent electrically insulating and water-resistant properties [22, 23], to coat on the membrane of the pressure sensor. 2.2.3. Thermoelectric cooler A thermoelectric cooler (TEC) serves as the thermal source for the thermosensitive hydrogel, so that the hydrogel could convert thermal energy into mechanical work. The TEC provides an efficient way to control the temperature precisely with fast response. It comprises a Peltier element (UWE electronic, Germany), a heat sink with fan, a heat spreader and a Pt100 temperature sensor (Fig. 2a). The heat spreader is glued to the Peltier element to improve the heat distribution. The Peltier element creates a heat flux between the p-n junctions of the semiconductors, making one side cooler while the other side hotter. By adjusting the flow direction of the DC current through the Peltier element, the heat spreader could selectively heat up or cool down the sample. This temperature control system has a rise and fall response time of 120 s and 146 s, respectively, with a temperature increment of 40 K (Fig. 2b). With a smaller variation of temperature like 5 K, the response time could reduce down to 30s. The study of the response time of TEC provides a guideline to estimate the final response time of the actuators. 2.2.4. Silicon cap The fabrication process of the silicon cap is based on a bulk micromachining process and illustrated in Fig. 3a: 1. RCA cleaning. The fabrication starts with a 3 inch, single crystal (100) silicon wafer (thickness 300 μm). This wafer is cleaned using the standard RCA clean process. 2. PECVD Si3 N4 . The Si3 N4 layer with a thickness of 250 nm is deposited on both sides of the substrate as hard masks for the wet etching process. 3. RIE Si3 N4 . Photolithography is taken to pattern the cavity, followed by a reactive ion etching (RIE) step to remove the related Si3 N4 layer. 4. Wet etching. The silicon structure is etched by potassium hydroxide (KOH) solution (concentration 31%) at 80 °C with an etching rate of 1.16 μm /min. Fig. 3b-c shows the silicon caps after the wet etching process. 2.3. Actuator assembly and packaging Fig. 4a depicts the assembly process of the actuator with the following steps: 1. PDMS coating on the pressure sensor (5 × 5 × 0.4 mm3 ). Firstly, the surface of the pressure sensor is modified by O2 plasma (400W, 20 Pa, 70s). Then the PDMS solution is spin-coated on the crosslinked in the oven at 70 °C for 1 h. Finally, a PDMS layer with a thickness of 10 μm is achieved. 2. Integration of the hydrogel reservoir (3.5 × 5 × 0.3 mm3 ) as well as the hydrogel particles on the top side of the pressure sensor. 3. Alignment and bonding of the Al2 O3 nanoporous membrane (3.5 × 5 × 0.06 mm3 ) and the water reservoir (3.5 × 5 × 0.3 mm3 ) with the hydrogel reservoir. Finally an actuator prototype (5 × 5 × 1.1 mm3 ) after the assembly process is shown in Fig. 4b.
3.
Simulation
Under the applied temperature from the peltier element, the temperature homogeneity of the temperaturesensitive hydrogel and thus the actuating-pressure homogeneity of the actuator are of interest. A finite element modeling with ANSYS could provide a useful tool to investigate the temperature distribution on the actuator, especially on the thermo-responsive hydrogel. Besides, the deformation and stress on the membrane of the pressure sensor are crucial factors to determine the maximum allowable filling factor of hydrogel during the assembly process. Both thermal simulation and mechanical simulation could facilitate prospective actuator design. 3.1. Thermal simulation As for hydrogel-based microsystems, the governing equations relating the heat transfer rate and the temperature gradient for the heat transfer mechanism include [24] thermal conduction 𝑄 d𝑇 ′′ (3) 𝑞cond = = −𝑘 , 𝐴 dx and thermal convection 𝑄 ′′ (4) 𝑞conv = = −ℎ(𝑇s − 𝑇f ), 𝐴 ′′ ′′ with Q the heat flow, 𝑞cond and 𝑞conv the conduction heat flux (W/m2 ) and the convection heat flux 2 (W/m ), respectively, 𝑘 the thermal conductivity (W/mK), ℎ the convective heat transfer coefficient (W/m2 K)，𝑇s the temperature of the surface, 𝑇f the temperature of the fluid ( air in this case), 𝑇s the absolute temperature (K) of the surface and A the section area (m2 ) for the heat transfer. A thermal simulation by ANSYS is performed to assess the temperature influence on this actuator (Fig. 5a). Since the cavity in the silicon cap is totally filled with hydrogel particles and water, hydrogel is presumed to have the same thermal conductivity as water. For the finite element simulation is assumed the following boundary conditions and loading (Fig. 5b): 1. The top side of the silicon cap is set to a high temperature due to the heat flow from the Peltier module. 2. The convection boundaries on the sidewalls of the pressure sensor and the silicon caps are set to be stagnant air (simplified case, 20 °C). Since all the components, like the pressure sensor, the silicon cap and the Al2 O3 membrane, have high thermal conductivities, this actuator has an almost homogenous temperature distribution with a tiny temperature difference ∆𝑇 (Fig. 5b). Fig. 5c summarizes ∆𝑇 as a function of the applied temperature from the peltier element and for two different reservoir depths of 160 and 360 μm. ∆𝑇 increases gradually with the applied temperature from 20 to 80 °C. Smaller value of the depth 𝑑rese of the hydrogel reservoir yields a shorter heat transfer path through the hydrogel which then decreases ∆𝑇. From those simulation results, an actuator with smaller 𝑑rese can improve the temperature homogeneity. 3.2 Mechanical simulation For the pressure sensor, the maximum displacement 𝑤max and the maximum stress 𝜎max are of interest [25]. The membrane of the pressure sensor serves as a square plate under a uniform pressure p (Fig. 6). The maximum displacement yields 𝑝𝑎4 (5) 𝑤max = 0.0138 3 , 𝐸ℎ where E the Young’s modulus of the silicon, h and a the thickness and length of the membrane, respectively. The maximum stress at the center point of the edge becomes 𝑝𝑎2 (6) 𝜎max = 0.3078 2 . ℎ For the finite element simulation in Fig. 7a, the sidewalls of the pressure sensor and silicon caps are set to be fixed boundaries. The hydrogel generates an actuating pressure 𝑝actu inside the hydrogel reservoir. The simulation result in Fig. 7b shows that the pressure sensor has a maximum deformation of 6.1 μm under 𝑝actu of 2 kPa. According to equation (5), the Al2 O3 membrane has a larger Young’s modulus and thickness than
the silicon membrane, leading to a main deformation drop on the silicon membrane. Fig. 7c shows the maximum stress and maximum deformation on the membrane as functions of 𝑝swel. Maximum stress increases with 𝑝swel. When 𝑝swel reaches 70 kPa, the maximum stress (i.e. 780 MPa) is over the fracture stress of silicon [26] where the silicon membrane would break. Those simulation results give a hint of the maximum actuating pressure allowed in the actuator. 4.
Test and discussion
4.1. Hydrogel characteristic The swelling behavior is strongly affected by the copolymerization ratio. Therefore, the influences of the concentration of the monomer AAmPA on the swelling degree of the hydrogel are studied. To be specific, by increasing the monomer amount AAmPA from 2 to 3.5 mol%, the carboxylic acid groups in the PNIPAAm copolymer increases, resulting in a higher degree of swelling 𝑄m as well as a high volume phase transition temperature (VPTT) from 50 to 60°C, as shown in Fig. 8a. Moreover, the optical appearance of the hydrogel varies from transparent to opaque as the temperature increases (Fig. 8b-e). Below VPTT, predominantly intermolecular hydrogen bonding between polymer chains and water molecules makes PNIPAAm polymer chains mostly extend, leading the hydrogel to exhibit a transparent morphology. Above VPTT, the waterpolymer hydrogen bonds are broken, resulting in a compact and collapsed conformation of PNIPAAm chains. Thus the gel reveals opaque at high temperature. Due to its higher 𝑄m as well as VPTT, PNIPAAm copolymer with 3.5 mol% AAmPA is selected for the thermal actuators. By gel milling, microscale hydrogel particles have been formed with a dimension between 10 and 50 μm (Fig. 9). Those hydrogel particles have a higher surface area exposed to the surrounding solution compared to the macroscale hydrogel layer, resulting in a faster stimuli response for the actuator. 4.2 Actuator characterization Three miniaturized thermal actuators S1, S2, S3 are assembled having different depths 𝑑rese of the hydrogel reservoir, which determine the volume of the cavity, and different filling factors 𝑉gel of the hydrogel in the cavity (Table 2). The influences of different 𝑑rese and 𝑉gel on the actuator performance, such as operation temperature range, response time, actuating pressure, are investigated. All those actuators are characterized by the measuring setup in Fig. 10. A Labview program sets temperature commands to the temperature controller (BelektroniG K10, Germany) as well as receives the voltage signals from the data acquisition device (Labjack U12, USA). 4.2.1. Response time Fig. 11a demonstrates that actuator S1 successfully regulates the actuating pressure/force in response to temperature. By gradually increasing the temperature with a step of 2 °C, the hydrogel decreases its volume, leading to a reduction of the actuating pressure and thus the output voltage. Besides, a pulse-wise change of the temperature between 30 and 35 °C reveals a stable sensor output without long-term drift of the output signal (Fig. 11b). Response time 𝑡90 , the time taken for the actuator response to achieve 90% of the step height, is utilized to evaluate the dynamic response of the actuator. 10 continuous cycles of measurement are performed with a temperature step of 2 K and 5 K, respectively. 𝑡90 decreases with increasing temperature from 20 to 36 °C (Fig. 12a). That is caused by 𝐷coop in equation (1), which changes proportionally with temperature. Therefore, hydrogel at higher temperature shows a higher 𝐷coop value, resulting in a lower response time. In addition, the temperature-decreasing process (i.e. hydrogel swelling) has a larger 𝑡90 than the temperature increasing-process (i.e. hydrogel shrinking) (Fig. 12b). This is caused by the shrinkage barrier effect during the hydrogel swelling process: Since the outer parts of the polymer network swell very fast, the diffusion profile decreases significantly from these swollen outer regions to the non-swollen inner parts of the polymer network, taking the hydrogel a longer time to swell completely. During the shrinking process, however, the inner and outer parts shrink at the same time in response to temperature, resulting in a quick shrinkage of the hydrogel.
4.2.2. Transfer characteristics Fig. 13a depicts the actuating pressure 𝑝actu as well as output voltage 𝑉out as functions of the temperature and for both T-increasing and T-decreasing processes. There is a nonlinear relationship 𝑝actu (𝑇) while the maximum 𝑝actu is around 33.3 kPa at 20 °C. Furthermore, the T-increasing and T-decreasing curves are slightly different, revealing a hysteresis effect of the actuator. The maximum actuating pressure hysteresis is 1.3% at 32 °C. As shown in Fig. 13b, the actuator sensitivity is determined to be in the range of -1 kPa/K to 3.2 kPa/K. The sensitivity at 36 °C is three times higher than that one at 20 °C. It is assumed that, with higher temperature, the hydrogel has a larger variance of the swelling degree 𝑄m during the free swelling test (Fig. 8a), leading to a larger change of the sensitivity. 4.2.3. Influence of depth 𝑑𝑟𝑒𝑠𝑒 of hydrogel reservoir Since the diffusion of a hydrogel into its surrounding solution is a rate-limiting factor governing the hydrogel’s swelling process, 𝑡90 follows the characteristic dimension 𝑙char . In order to shorten 𝑡90 , a lower depth 𝑑rese of the hydrogel reservoir with smaller 𝑙char and thus shorter diffusion path is preferred. The relationship between 𝑑rese and 𝑡90 is studied by comparing actuators S1 and S2 (Fig. 14). Compared to S1, the response time of S2 drops by 1.5 to 3.3 times, depending on the temperature. Besides, S2 owns a more homogenous temperature distribution in the hydrogel due to a smaller temperature difference, according to the thermal simulation results (Fig. 5c). However, because of a lower swelling pressure of the hydrogel, S2 has only 50% of the actuating pressure and 20% of the sensitivity relative to S1 (Table 2). Overall, this type of actuator is suitable for a low-scale actuating pressure range with fast dynamic response. 4.2.4. Influence of filling factor 𝑉𝑔𝑒𝑙 of hydrogel A lower 𝑉gel leads to a smaller deflection of the membrane in the actuator (i.e. lower 𝑝actu ) and a lower temperature at which 𝑝actu = 0, consequently limiting the operation temperature range (for example, 20…40 °C in actuator S1). In some cases, some practical applications require large actuating forces/pressures and wide operation temperature ranges, such as artificial muscles (1MPa) [27] and microvalves (200 kPa) [28]. In order to meet this demand, it is preferred to increase 𝑉gel . The relationship between 𝑉gel and 𝑝actu is studied by comparing actuators S1 and S3 (Fig. 14). The actuating pressure of S3 has twice the value of that of S1 (Fig. 14). The maximum 𝑝actu (i.e. 65 kPa) of S3 is lower than the maximum allowable actuating pressure from the mechanical simulation results (Fig. 8c), indicating that this actuator is in a robust operation state. Besides, the maximum operation temperature increases to 55°C (Fig. 15). It is believed that the operation temperature could reach VPTT (60°C) if dry hydrogel will completely fill the cavity (i.e. 𝑉gel = 100%). However, excessive amounts of hydrogel (𝑉gel > 100%) will lead to an offset actuating pressure, and even worse, break the silicon membrane in the actuator. In short, attribute to its high filling factor 𝑉gel of hydrogel, actuator S3 is advantageous to achieve high actuating pressure as well as wide operation temperature range. Modifying the hydrogel material properties could be an alternative method to improve the actuating pressure. To be specific, PNIPAAm hydrogel with larger amounts of AAmPA has both a higher swelling degree and VPTT, which leads to a higher actuating pressure and wider operation temperature range. Such hydrogel actuators above have a trade-off between response time and actuating pressure: smaller amount of hydrogel in the actuator could shorten the diffusion path as a means to reduce response time, but at the expense of a decreased actuating pressure and operation temperature range. Although the time response of the actuator with high actuating pressure (i.e. actuator S3) is relatively slow (Fig. 14), it is believed that changes in the hydrogel material properties and hydrogel structures can further improve the time response and make hydrogel actuation widely applicable to microsystems. 5.
Conclusion
In this paper, hydrogel-based thermal actuators, aiming at providing a precisely adjustable actuating pressure with fast dynamic response, has been designed, assembled, simulated and tested. Hydrogel microparticles with high surface-to-volume ratio were synthesized to improve the response time of the actuator. Three thermal actuators with different depths of the hydrogel reservoir and different filling factors of the hydrogel were assembled and tested. It was shown the response time and the actuating pressure decreased with rising temperature. The response time has been improved by a factor of 1.5…3.3 by scaling down the depth of the cavity. The actuating pressure has been enhanced by a factor of 2 by increasing the filling factor
of hydrogel in the cavity. Actuator S2 presents the lowest response time of 1.9 min while actuator S3 exhibits the maximum actuating pressure of 65 kPa. The experiments show that there is a trade-off relationship between response time and actuating pressure. By adjusting the actuator properties (concentration of AAmPA in PNIPAAm gels, hydrogel shape and size, depth of the cavity), those thermal actuators can be used in extensive potential applications. Acknowledgements This work was supported by the Deutsche Forschungsgemeinschaft within the Research Training Group Hydrogel-Based Microsystems (DFG-GRK 1865). Reference [1] Tzou H, Tseng C. Distributed piezoelectric sensor/actuator design for dynamic measurement/control of distributed parameter systems: a piezoelectric finite element approach. J Sound Vib. 1990;138:17-34. [2] Xu C, Wang S, Tang G, Yang D, Zhou B. Sensing characteristics of electrostatic inductive sensor for flow parameters measurement of pneumatically conveyed particles. J Electrostat. 2007;65:582-592. [3] Kwun H, Bartels K. Magnetostrictive sensor technology and its applications. Ultrasonics. 1998;36:171178. [4] Gerlach G, Arndt KF. Hydrogel sensors and actuators - engineering and technology. Urban G, editor. Heidelberg: Springer; 2009. [5] Peters EC, Svec F, Fréchet JMJ. Thermally responsive rigid polymer monoliths. Adv Mater. 1997;9:630-633. [6] Kim J, Serpe MJ, Lyon LA. Hydrogel Microparticles as Dynamically Tunable Microlenses. J Am Chem Soc. 2004;126:9512-9513. [7] Arndt KF, Kuckling D, Richter A. Application of sensitive hydrogels in flow control. Polym Advan Technol. 2000;11:496-505. [8] Eddington DT, Beebe DJ. Flow control with hydrogels. Adv Drug Deliver Rev. 2004;56:199-210. [9] Liu RH, Yu Q, Beebe DJ. Fabrication and characterization of hydrogel-based microvalves. J Microelectromech Syst. 2002;11:45-53. [10] Richter A, Klatt S, Paschew G, Klenke C. Micropumps operated by swelling and shrinking of temperature-sensitive hydrogels. Lab Chip. 2009;9:613-618. [11] Yuandong G, Baldi A, Ziaie B, Siegel RA, editors. Modulation of drug delivery rate by hydrogelincorporating MEMS devices. 2nd Annual International IEEE-EMBS Special Topic Conference on Microtechnologies in Medicine and Biology; 2002. [12] Schulz V. Advanced hydrogel-based chemical microsensors. Dresden: TUDpress; 2013. [13] Ferse B, Richter S, Eckert F, Kulkarni A, Papadakis CM, Arndt KF. Gelation Mechanism of Poly(Nisopropylacrylamide)-Clay Nanocomposite Hydrogels Synthesized by Photopolymerization. Langmuir. 2008;24:12627-12635. [14] Schulz V, Zschoche S, Zhang HP, Voit B, Gerlach G, editors. Macroporous smart hydrogels for fastresponsive piezoresistive chemical microsensors. Procedia Engineering 25; 2011. [15] Pelton R. Temperature-sensitive aqueous microgels. Advances in Colloid and Interface Science. 2000;85:1-33. [16] Kuckling D, Harmon ME, Frank CW. Photo-cross-linkable PNIPAAm copolymers. 1. Synthesis and characterization of constrained temperature-responsive hydrogel layers. Macromolecules. 2002;35:63776383. [17] Kuckling D, Hoffmann J, Plotner M, Ferse D, Kretschmer K, Adler HJP, Arndt KF, Reichelt R. Photo cross-linkable poly(N-isopropylacrylamide) copolymers III: micro-fabricated temperature responsive hydrogels. Polymer. 2003;44:4455-4462. [18] Deng K, Rohn M, Guenther M, Gerlach G, editors. Thermal microactuator based on temperaturesensitive hydrogel. Procedia Engineering 120; 2015. [19] Deng K, Gerlach G, Guenther M, editors. Force-compensated hydrogel-based pH sensor. Proc SPIE 9431, Active and Passive Smart Structures and Integrated Systems 2015; 2015; San Diego, United States.
[20] Kuckling D, Richter A, Arndt KF. Temperature and pH-dependent swelling behavior of poly(Nisopropylacrylamide) copolymer hydrogels and their use in flow control. Macromol Mater Eng. 2003;288:144-151. [21] Kuckling D, Adler HJP, Arndt KF, Ling L, Habicher WD. Temperature and pH dependent solubility of novel poly(N-isopropylacrylamide) copolymers. Macromol Chem Physic. 2000;201:273-280. [22] Kersey L, Ebacher V, Bazargan V, Wang R, Stoeber B. The effect of adhesion promoter on the adhesion of PDMS to different substrate materials. Lab Chip. 2009;9:1002-1004. [23] Satyanarayana S, Karnik RN, Majumdar A. Stamp-and-stick room-temperature bonding technique for microdevices. J Microelectromech Syst. 2005;14:392-399. [24] Pitts DR, Sissom LE. Heat transfer. New York: McGraw-Hill; 1998. [25] Liu C. Foundations of MEMS. New Jersey: Pearson Prentice Hall; 2006. [26] Sooriakumar K, Chan W, Savage TS, Fugate C, editors. A comparative study of wet vs. dry isotropic etch to strengthen silicon micro-machined pressure sensor. Electrochem Soc Proc; 1995. [27] Shahinpoor M, Kim KJ, Mojarrad M. Artificial muscles: applications of advanced polymeric nanocomposites: CRC Press; 2007. [28] Wang J, Chen ZY, Mauk M, Hong KS, Li MY, Yang S, Bau HH. Self-actuated, thermo-responsive hydrogel valves for lab on a chip. Biomed Microdevices. 2005;7:313-322.
Biographies Kangfa Deng received the M.Sc. degree in Microelectronics Engineering and Solid State Electronics from Peking University, Beijing, China, in 2013, where he is a research assistant and currently pursuing the Ph.D. degree in Solid-State Electronics Laboratory, Technische Universität Dresden. His interests include design, assembly, and the improvement of force-compensated hydrogel-based chemical sensors. Mathias Rohn received the M.Sc. degree in Chemistry from Potsdam University and Fraunhofer Institute for advanced polymerscience Potsdam-Golm, Germany. In 2014 he is a research assistant and currently pursuing the Ph.D. degree in physical chemistry of polymers, Technische Universität Dresden. His interests include synthesis and characterization of hydrogels, with the focus on relationship between structure and properties of hydrogels. Gerald Gerlach received M.Sc. and Ph.D. degrees in electrical engineering from the Dresden University of Technology in 1983 and 1987, 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 full professor at the Department of Electrical and Computer Engineering at Technische Universität Dresden (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 modeling of micromechanical devices and the development of solid-state sensors, e.g. pyroelectric infrared sensors and piezoresistive chemical sensors. From 2007 to 2010 he served as President of the German Society for Measurement and Automatic Control (GMA). From 2007 to 2008 he was Vice President and President of EUREL (The Convention of National Societies of Electrical Engineers of Europe), respectively. Since 2013 he has been Chairman of the German Association of Technical-scientific Societies (DVT). Prof. Gerlach is Associate Editor-in-Chief of the IEEE Sensors Journal and Chief Editor of JSSS, the Journal of Sensors and Sensor Systems.
Fig. 1. Hydrogel-based thermal actuator: (a) working principle and (b) cross-section structure. Microscope photos of (c) the pressure sensor, (d) the silicon cap, SEM images of (e) the nanoporous Al2 O3 membrane separating the water reservoir and (f) the temperature-sensitive hydrogel.
Fig. 2. (a) Prototype and (b) temperature response of the thermoelectrical cooler (TEC)
Fig. 3. (a) Fabrication process of the silicon cap, photographs of silicon caps after wet etching process with (b) full view and (c) close-up view
Fig. 4. (a) Assembly steps of the thermal actuator and (b) actuator prototype after the assembly process
Fig. 5. Thermal simulation of the thermal actuator: (a) boundary conditions, (b) temperature distribution after an applied temperature (60 °C) from the peltier element and (c) relationship between temperature difference ∆𝑇 and applied temperature
Fig. 6. Square plate under a uniform stress. (a)Top view and (b) side view with point of the maximum stress 𝜎max and the maximum displacement 𝑤max , respectively
Fig. 7. Mechanical simulation of the thermal microactuator: (a) boundary conditions, (b) deformation distribution under an actuating pressure 𝑝actu of 2 kPa and (c) relationship between the maximum stress, maximum deformation and actuating pressure
Fig. 8. (a) Weight degree 𝑄m of swelling of PNIPAAm with 2, 3.5 mol% AAmPA when the hydrogel is immersed into DI water and (b-e) photographs of PNIPAAm with 3.5 mol% AAmPA between 30 and 75 °C
Fig. 9. SEM images of the temperature-sensitive hydrogel particles with different magnification
Fig. 10. Measuring setup for the hydrogel-based thermal actuators
Fig. 11. Temperature-dependent output signal of the actuator S1: (a) for incremental temperature step between 20 and 36 °C and (b) for pulse-wise temperature excitation between 25 and 30 °C
Fig. 12. Response time 𝑡90 of the actuator S1 with a temperature step of (a) 2 °C and (b) 5 °C.
Fig. 13. (a)Transfer characteristics and (b) sensitivity as functions of temperature for the thermal actuator S1.
Fig. 14. Response time and actuating pressure for three thermal actuators S1 (360 μm, 60%), S2 (160 μm, 60%), S3 (360 μm, 95%)
Fig. 15. Actuating pressure and sensitivity as functions of temperature for the thermal actuator S3.
Table 1. Components for the PNIPAAm hydrogel Component Nisopropylacrylamid e (NIPAAm) 3acrylamidopropioni c acid (AAmPA) N,N-methylene bisacrylamide (BIS) Potassium peroxodisulfate (KPS) N,N,N',N'tetramethyl-ethylenediamine (TEMED)
Purpose
mol%
Hydrogel network monomer
92.3…9 3.8
Hydrophilic monomer
2…3.5
Crosslinker
3.65
Initiator
0.27
Accelerator
0.27
Table 2. Comparison of actuator parameters of thermal actuators S1 (360 μm, 60%), S2 (160 μm, 60%), S3 (360 μm, 95%) Parameter/actuator Depth 𝒅𝐫𝐞𝐬𝐞 of hydrogel reservoir (μm) Filling factor 𝑽𝐠𝐞𝐥 of hydrogel* (%) Operation temperature range (°C) Sensitivity (kPa/°C)
S1 360
S2 160
S3 360
60
60
95
20…40
20…40 20…55
-1…-3.2
-0.5…- -1.4…1.0 2.6 0…23 0…65
Actuating pressure range 0…33.3 (kPa) Response time @ 25…35 2.7…8.7 1.9…2. 6.3…7. °C (min) 5 4 * Percentage of cavity volume filled with dry hydrogel particles