List of symbols ................................................ xv
List of abbreviations ......................................... xxi
I Introduction and theory
1 Introduction ............................................... 3
1.1 Background and motivation .................................. 3
1.2 Hydrogels and their applications ........................... 5
1.2.1 Definition, properties, and classification .......... 5
1.2.2 Hydrogels for sensor applications ................... 7
1.3 State of the art of hydrogel-based sensors ................. 9
1.3.1 Fundamental principle of hydrogel-based sensors ..... 9
1.3.2 Conductometric sensors ............................. 10
1.3.3 Optical sensors .................................... 10
1.3.4 Resonance frequency-based sensors .................. 17
1.3.5 Mechanical sensors ................................. 18
1.3.6 Summary ............................................ 20
1.4 Scope of this work ........................................ 22
2 Theoretical foundations ................................... 25
2.1 Hydrogel swelling ......................................... 25
2.1.1 Thermodynamics of swelling equilibrium ............. 25
2.1.2 Kinetics of the swelling process ................... 32
2.1.3 Linear mechanical hydrogel models .................. 36
2.2 Piezoresistive pressure transducers ....................... 37
2.2.1 Piezoresistive effect .............................. 37
2.2.2 Influence of the geometric properties of the
flexure plate on sensitivity ....................... 39
2.2.3 Influence of local stiffening of the flexure
plate .............................................. 40
II Hybrid hydrogels as sensor material
3 Sensors based on porous hydrogels ......................... 45
3.1 Porous hydrogels .......................................... 45
3.1.1 Classification and features ........................ 45
3.1.2 Preparation methods ................................ 46
3.1.3 Hydrogel synthesis ................................. 47
3.2 Hydrogel characterization ................................. 48
3.2.1 Free-swelling measurements ......................... 48
3.2.2 ATR-FTIR spectroscopy .............................. 53
3.2.3 SEM measurements ................................... 54
3.2.4 Mercury intrusion porosimetry ...................... 55
3.3 Sensor integration ........................................ 57
3.3.1 Sensor layer fabrication ........................... 57
3.3.2 Sensor set-up ...................................... 60
3.4 Sensor characterization ................................... 61
3.4.1 Sensor conditioning ................................ 61
3.4.2 Transfer characteristics ........................... 62
3.4.3 Time-dependent sensor response ..................... 63
3.5 Conclusions and summary ................................... 65
4 Sensors based on clay nanocomposite hydrogels ............. 67
4.1 PNIPAAm hydrogel networks ................................. 67
4.1.1 Sensoric properties ................................ 67
4.1.2 Conventional gels .................................. 68
4.1.3 Strategies for preparation of mechanically stable
hydrogels .......................................... 69
4.2 Clay nanocomposite hydrogels .............................. 69
4.2.1 Layered silicates .................................. 69
4.2.2 Sensor layer fabrication ........................... 71
4.2.3 Structural characterization ........................ 72
4.2.4 Mechanical characterization ........................ 74
4.3 Sensor set-up ............................................. 79
4.4 Sensor characterization ................................... 80
4.4.1 Sensor conditioning ................................ 80
4.4.2 Temperature-induced sensor response ................ 82
4.4.3 Sensor response to organic co-solvents ............. 85
4.4.4 Sensor response to monovalent salt ions ............ 88
4.5 Conclusions and summary ................................... 90
III Hydrogel sensors with force compensation ................... 95
5 Force compensation method ................................. 95
5.1 Basic concept of compensation ............................. 95
5.2 General sensor model for deflection method ................ 97
5.2.1 Sensor model ....................................... 97
5.2.2 Quasi-static sensor behavior ...................... 100
5.2.3 Response time ..................................... 102
5.3 General sensor model for compensation method ............. 103
5.3.1 System model ...................................... 103
5.3.2 Stability requirements ............................ 107
5.3.3 Steady-state behavior ............................. 109
5.3.4 Dynamic system behavior ........................... 118
5.4 Conclusions and summary ................................. 125
6 Closed-loop hydrogel-based sensor ........................ 127
6.1 Sensor concept ........................................... 127
6.1.1 General sensor principle and sensor structure ..... 127
6.1.2 Theoretical implications for hydrogel sensors in
closed-loop configuration .......................... 129
6.1.3 Design specifications ............................. 130
6.2 Construction of the sensor element ....................... 130
6.2.1 Structural set-up ................................. 130
6.2.2 Fabrication of the sensor element components ...... 133
6.2.3 Assembly and packaging ............................ 141
6.3 Sensor in closed-loop configuration and its components ... 143
6.3.1 Model of the system structure ..................... 143
6.3.2 Measuring unit .................................... 144
6.3.3 Controller ........................................ 146
6.3.4 Actuator .......................................... 146
6.3.5 Hydrogel layer .................................... 147
6.4 System identification and analysis ....................... 148
6.4.1 Identification of model components ................ 148
6.4.2 Stability analysis and parameter estimation ....... 152
6.5 Measurement procedure and experimental results ........... 153
6.5.1 General measurement procedure ..................... 153
6.5.2 Reference pressure adjustment ..................... 154
6.5.3 Sensor conditioning ............................... 157
6.5.4 pH measurement .................................... 159
6.5.5 Noise suppression ................................. 164
6.5.6 Model validation .................................. 164
6.6 Conclusions and summary .................................. 165
IV Summary and future prospects
7 Summary .................................................. 169
7.1 Sensors based on hydrogels with improved material
properties ............................................... 169
7.2 Hydrogel sensors with force compensation ................. 171
8 Future prospects ......................................... 175
8.1 Sensors based on hydrogels with improved material
properties ............................................... 175
8.2 Hydrogel sensors with force compensation ................. 176
Appendices
A Hydrogel material ........................................ 181
A.l p(AAm-co-AA) hydrogels ................................... 181
A.1.1 Synthesis ......................................... 181
A.1.2 SEM characterization .............................. 183
A.2 NC-and BIS-PNIPAAm hydrogels ............................. 184
A.3 PVA/PAA hydrogels ........................................ 184
В Supplementary calculations for force compensation ........ 187
B.1 Application of the ROUTH criterion ....................... 187
B.2 Closed-loop sensor with time-dependent parameter
variations ............................................... 189
B.2.1 Parameter range for stability and minimum-phase
system for drift-afflicted sensors ................ 189
B.2.2 ISE quality criteria for different controller
types ............................................. 189
B.3 Redevelopment of denominator polynomial .................. 191
Bibliography .................................................. 193
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