Nonlinear sensors for biomaterials—Principles and applications

Certain laboratory testing methods for the characterization of biomaterials, which can be regarded as essentially nonlinear sensors, have become widely adopted in recent decades. These techniques, namely dynamic mechanical analysis (DMA), electrochemical impedance spectroscopy (EIS) and temperature-modulated differential scanning calorimetry (TMDSC), employ similar fundamental approaches in their different analyses, wherein materials are subjected to alternating (sinusoidal) stress, electrical potential and temperature change, respectively, and both the linear (in-phase) and nonlinear (out-of-phase) responses are analysed to yield novel information unavailable with the corresponding traditional linear analytical techniques. This review article discusses the general experimental procedures and underlying principles for these three nonlinear analytical techniques and the type of information obtained with each approach, compared with that from the corresponding linear tests, and presents examples from the literature about their wide-ranging application. While the focus is on dental biomaterials, additional examples for medical biomaterials are provided to illustrate further examples of the novel information that can be obtained from the use of these three nonlinear tests. The underlying philosophical similarity (analyses of nonlinear responses to different types of sinusoidal physical inputs) of these three sensor techniques is emphasized.