Yan－Bo Wei，Li－Ping Shi，Xi－Wen Wei，Jie Huang

(School of Electronic Engineering，Heilongjiang University，Harbin 150080，China)

1 Introduction

With the development of science and technology，integration study of piezoelectric sensors and actuators has been widely applied in ultra-precision machining，micro-engineering，nanotechnology，biotechnology and other fields，especially in the past decade years，the role of micro-drive and micro-actuator based on the piezoelectric effect increasingly gained the attention of scholars in the field of biomedical engineering.In the fields of cell clones，cell hybridization，chromosomes and embryo transfer，drug injection of cells is the most common and basic operations［1－2］.Animal cell diameter is about 20 μm，and the nucleus diameter is about 5 μm.The ratio of length and diameter of glass microneedle is very large，and micro-needle needsto overcome larger capillary resistance［3］.In addition，the cell membrane is composed by phospholipid molecules and glycoproteins and it has great viscoelasticity，so micro-needle needs to penetrate the cell interior quickly and accurately in order to reduce the destruction of cell structure and increase the accuracy ofinjection position［4－5］.

Currently， the operation of cell micro-flow injection mainlyrelies on manual adjustment and microscopic observation，the accuracy of operation mainly depends on the operator’s experience and proficiency，control accuracy is difficult to achieve test requirements.Traditional methods have complex structure，difficult operation，costly，low success rate and otherissues.In recentyears，scholarshave conducted extensive research to cell microinjection and have many good research results［6－7］.Xiao and Li put forward mechanical structure and control system of cell drug micro-injection which is composed of piezoelectricdriven micro injection platform and motor-driven microcell stage［8］.Tian and Hou proposed micro-flow injection method which is used impact type inertial force generated by low-frequency vibration of piezoelectric ceramic to overcome micro-feed force generated by friction［9］.Liu proposed to use the inverse piezoelectric properties of piezoelectric ceramics to achieve micro-injection［10－11］.Xie and Sun used robots to realize cell microinjection［12－13］.

These micro-flow control methods are based on the basic characteristics and first inverse effect of piezoelectric ceramic，and the secondary piezoelectric effect has not yet been involved in micro-flow control.The paper is concerned with the micro-flow self-sensing actuators，the work of which is based on the secondary piezoelectric effect.The piezoelectric ceramic stack can yield micro-displacementdue to its firstinverse piezoelectric effect.Therefore，we apply this microdisplacement to cell micro-flow injection.Moreover，due to the charge of the secondary direct piezoelectric effect，the piezoelectric ceramic stack is able to detect the force and displacement in the injection by itself.It can achieve accurate measurements and precise positioning.

2 Principle of Micro-Flow Self-Sensing Actuator

The micro-flow self-sensing actuator is based on the first inverse and the secondary direct piezoelectric effect in this paper.In the same piezoelectric crystal，using an inverse piezoelectric effect as the design principle ofactuators，outputmicro-displacement.Then，using the charge generated by secondary direct piezoelectric effect as the design principle of sensors［14］.

In mechanicalfree boundary conditions，the external electric field Enis applied to piezoelectric ceramic stack，and external stress is zero.Then piezoelectric strainisgenerated through first inverse piezoelectric effect.Eq.(1)can be obtained from the first class piezoelectric equations［15］.

where dniis piezoelectric strain constant.

where emiis piezoelectric stress constant.

From Eqs.(1)and(2)，it can be seen that strain，displacementand external electric field Enare correspondence relationship.Therefore，when drug injection，by changing the applied electric field is able to control the feed amount of micro-flow self-sensing actuator，by measuring the charge generated by the secondary piezoelectric effect will be able to selfsensing the feed amount.That is，using straingenerated by the first inverse piezoelectric effect to achieve actuator function of micro-flow self-sensing actuator and using displacementgenerated by the secondary direct piezoelectric effect to achieve sensor function of micro-flow self-sensing actuator.

3 Experimental Study of Micro-Flow Self-Sensing Actuator

3.1 Experimental Study of First Inverse Piezoelectric Effect

In the first inverse piezoelectric effect experiment， the external electric field Enis applied to piezoelectric ceramic stack，and external stress is zero，and the boundary conditions is free.Experimental structure is shown in Fig.1;the relationship between driving voltage and displacement，that is，the relationship between driving voltage and the feed amount of microneedle is shown in Fig.2.DF1743003C provides driving voltage from 0 V to 60 V;MDS Series LVDT micrometer is used to measure micro-displacement; YE6230T-16 data acquisition device for data collection and transfer data to computer.

Fig.1 Experimental structure diagram of first inverse piezoelectric effect

Fig.2 Relation between voltage and displacement of ceramic stack under the mechanical free

The piezoelectric ceramic stack QDS-10×10－30 is used in the experiment，and its basic parameters are shown in Table 1.

Subjected to 0－60 V input，theoretical output displacement of piezoelectric ceramic stack can be calculated by Eq.(3)，and its theoretical value is shown in Table 2.

where δ，n，d33and V3is the displacement generated by first inverse piezoelectric effect，the layer of ceramics stack，piezoelectric coefficient and the driving voltage respectively.

Table 1 Basic parameters of piezoelectric stack QDS-10×10-30

Table 2 Comparison between theoretical and measured output values of first inverse effect of piezoelectric ceramics stack

Cell diameter is between 1－100 μm，and most of them in 20－30 μm，and the thickness of the cell membrane is 7－8 nm，subjected to 0－60 V input，and the piezoelectric ceramic stack can generate 13.45 μm displacement，which is fully able to pierce the cell membrane.First inverse piezoelectric effect analysis of the ceramic stack shows that output displacement and applied voltage has a good linear relationship，by controlling the input voltage can achieve cell drug quantitative injection.

3.2 Experimental Study of Secondary Direct Piezoelectric Effect

Piezoelectric ceramics can be regarded as an equivalent capacitor，when applied voltage to the piezoelectric ceramics， and piezoelectric ceramic equivalent capacitor Cnincludes two parts:one part is the capacitor C0as ordinary capacitor，and another part is increase equivalent capacitor C2because of the secondary direct piezoelectric effect.Therefore，the piezoelectric ceramic surface charge Qnalso contains two parts:one part is the charge QC0generated in capacitor C0，and another part is QC2generated in capacitor C2.Experimental study of secondary direct piezoelectric effect needs to measure charge QC2.The experimental structure is shown in Fig.3.

Fig.3 Experimental structure diagram of secondary direct piezoelectric effect

In Fig.3，ceramics stack can be viewed as an ordinary capacitor and its valueis equal to C0. Subjected to the input voltage V and－V，piezoelectric ceramic stack and capacitor C0generate the same amount of electric charges with different symbols.Therefore，charge integral circuit can collect the charge QC2， which is generated by secondary direct piezoelectric effect.

In the experiment，0－±60 V voltage is applied to piezoelectric ceramic stack and capacitor C0，and the step is 10 V，and the voltage generated by secondary direct piezoelectric effectismeasured by charge integral circuit，as shown in Fig.4.

Fig.4 Secondary direct piezoelectric effect of ceramic stack

When applied 0－60 V voltage to piezoelectric ceramic stack，theoreticaloutputvoltage can be calculated by Eq.(4)，and its theoretical value is shown in Table 3.

where V(2)is the output voltage of secondary direct piezoelectric effect;Q(2)is output charge of secondary direct piezoelectric effect;A is cross-sectional area of ceramics stack;e33is piezoelectric stress constant;C is equivalent capacitance of the piezoelectric ceramic stack;t is monolithic thickness of piezoelectric ceramic stack.

From Eq.(4)，one can see that the change of the output voltage V(2)of secondary direct piezoelectric effect is linearly proportional to the output displacement δ of first inverse piezoelectric effect.

Three measured outputvoltages have better repeatability in Table 3，but there is repeatability error of 1.52%.The main reason is the piezoelectric material inherent characteristic，such as hysteresis，creep，ferroelectric effect and so on.

Table 3 Comparison between theoretical and measured output voltage of secondary direct piezoelectric effect of ceramics stack

As shown in Table 4，the substitution of the measured voltageofsecondary directpiezoelectric effect into Eq.(4)yields the self-sensing displacement of secondary direct piezoelectric effect.

The self-sensing displacement of secondary direct piezoelectric effect and the output displacement of first inverse piezoelectric effect are equal to each other in terms of theory.However，experimental data has some errors，and the source of the errors is mainly the following two aspects. Firstly， the inherent characteristicsofpiezoelectric ceramics，such as nonlinearity，hysteresis，creep，can influence the experiment result to some extent.Secondly，in the experiment of secondary direct piezoelectric effect，capacitor C0and the value of ceramics stack as ordinary capacitor are equal to each other ideally，but there must be some inaccuracy in practice，and this is also the source of errors.

Table 4 Comparison between self-sensing displacement of secondary direct piezoelectric effect and measured displacement of first inverse piezoelectric effect

Fig.5 shows self-sensing displacement curve of piezoelectric ceramic stack.It can be seen that selfsensing displacement of secondary direct piezoelectric effect has good linearity.Therefore，by measuring the voltage due to the secondary direct piezoelectric effect can be better self-sensing the displacement due to the first inverse piezoelectric effect.That is，by measuring the voltage can perceive the injection volume of cell drug injection.

In Fig.1，using sensor LVDT to measure microdisplacement，and the circuit in Fig.3 can self-sensing micro-displacement， thereby eliminating sensor component parts，save space and improve the control accuracy of the system.

Fig.5 Self-sensing displacement curve of piezoelectric ceramic stack

4 Conclusions

Micro-flow self-sensing actuator which is based on the first inverse and secondary direct piezoelectric effect is presented.The micro-displacement of piezoelectric ceramic which is generated by using the first inverse piezoelectric effect can achieve cell microflow injection，and the charge generated by using the secondary direct piezoelectric effect can sense the force and displacement in the micro-flow injection process.The experimental results show that the displacement generated by the first inverse piezoelectric effect can achieve the cell micro-flow injection precisely，and the charge generated by secondary direct piezoelectric effect can better self-sensing the displacement of cellinjection.Thus，the driving voltage can be adjusted according to the feedback signal given by self-sensing displacement，and feed amount can be controlled precisely，then the injection flow of micro-flow drive can be controlled precisely.

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