۱- Introduction

۲- Electrical micro-force sensors

۳- Optical micro-force sensors

۴- Discussion

۵- Conclusion

References

Abstract

Micro/nano manipulation for both mechanical and biological structures is currently a popular research field. To protect small-scale structures and acquire their mechanical properties, a micro-scale force sensor is needed. This paper focuses on reviewing the research on non-contact micro-force sensor parts that can be integrated in this manipulation system. The content involves the structure, working principle, resolution and sensitivity of different force sensor parts, including electrical and optical force sensors. The electrical force sensors include piezoresistive, piezoelectric, capacitive, electrothermal and strain gauge-based types; while the optical force sensors focus on but are not restricted to the optical fiber-based force sensors and the vision-based sensing systems. All of these sensors are analysed and compared. Electrical force sensors are currently widely used but are restricted by the sensing properties and size; optical force sensors have high sensitivity, small structure and anti-electromagnetic-interference properties, but they are hardly applied in micro/nano manipulation systems for force measurement. As a result, optical force sensors may become the new generation of sensors that can be integrated with micro/nano manipulation systems.

Introduction

Micro/nano manipulation is a popular interdisciplinary research field of this century. This approach is common for the fabrication of three-dimensional (3D) microstructures, the movement of nanostructures, the transportation and motion control of particles and, especially applications in biology [1,2]. Based on this technology, manipulation and characterization of biological cells, such as cell transfer, isolation, immobilization and injection have been widely used in many biological applications over the past decade [3]. As the handled biological cells can be easily damaged, the capability of precisely measuring small forces at the micro/nanoscale is necessary forproviding force feedbackduringmanipulation[4]. The measurement of force can also be used for the mechanical characterization of cells since genetic mutations and pathogens can result in changes to cell mechanical properties such as elasticity and viscosity [5]. In [6], changes in the mechanical properties of mouse zona pellucida post fertilization were tested for investigating the hardening process. Other than biological cells, force measurement is essential for sample protection and mechanical properties detection of micro/nano samples. Additionally, the force sensing system is vital for the experimental verification of the micro-force theoretical model. Different kinds of micro-force sensors were designed during the past decades, including piezoresistive, piezoelectric, capacitive, and electrothermal sensors and those based on visual detection. Reference [7,8] demonstrated two kinds of triaxial piezoresistive micro-force sensors with millinewton resolution, while reference [9,10] presented two piezoresistive force sensors with the piezoresistance attached to the membranes. In [11,12], force sensors were constructed using the polyvinylidene fluoride(PVDF) sensing material by measuring the electric charge of the PVDF membrane. Beyeler et al. [13] devised a six-axial force sensor using seven capacitors with a micronewton resolution. Bulut Coskun et al. [14] designed a micro-electrothermal force that used feedback control to nullify displacements within the device with a high resolution of 7.8 nN. Rajagopalan et al. [15] put forward a displacement-based force sensor with the resolution of 50 pN. The force was obtained by optically measuring the displacement of the probe with respect to a fixed reference beam.