MEMS tri-axial force sensor with an integrated mechanical stopper for guidewire applications
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- Park, WT., Kotlanka, R.K., Lou, L. et al. Microsyst Technol (2013) 19: 1005. doi:10.1007/s00542-012-1691-x
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This paper describes the design and characterization of a micro-electro-mechanical systems tri-axial force sensor that can be mounted on the tip of an 1-French guidewire (0.014″). Piezoresistive silicon nanowires (SiNWs) are embedded into four beams forming a cross-shape to allow the detection of forces in three axes. The electrical resistance changes in the four SiNWs are used to decode arbitrary force applied onto the force sensor. Finite element analysis was used in the structural design of the force sensor. Robustness of the force sensor is improved due to the novel design of incorporating a mechanical stopper on the tip of the stylus. Flip chip bonding, using gold stud bumps, is used to mount the force sensor on a substrate for characterization and to simplify the assembly process. The sensor is robust enough to withstand normal forces higher than 20 gf. The proposed sensor can be used for new medical applications in vascular interventions and robotic surgeries.
Advances in surgical tools and their associated techniques advocate implementation of minimally invasive surgery (MIS). For MIS in cardio-vascular and thoracic interventional procedures, passing a guidewire through vascular vessel is the first step followed by the surgical procedures such as stenting. The ability to successfully treat a vascular lesion via endovascular methods is dependent on the ability to pass a guidewire across the lesion. Blockage of the vessel lumen in the range from 50 to 100 % makes passage of the guidewire a challenging task.
Microelectromechanical systems (MEMS) enabled the possibility of making sensorized guidewires and catheters for identifying and analyzing the stenosis without excessively using intravenous contrasts (Rebello 2004; Bonanomi et al. 2003; Tonino et al. 2009; Takizawa et al. 1999). MEMS sensors were first used to identify stenosis. Rebello (2004) reported that there is a change in temperature (approximately 3 °C) at the location of the stenosis, thus recommending the use of temperature sensor at the tip of the guidewire. Bonanomi et al. (2003) mentioned that force sensors can be used for identifying the stenosized location by obtaining the hardness information of the tissue. This is because the hardness of the calcified tissue at stenosis location is higher than the healthy vascular vessel. Once the stenosis is found, the degree of stenosis is important. Tonino et al. (2009) reported placing a pressure sensor near the tip of the guidewire and measuring fractional flow reserve (FFR, ratio of pressure before and after stenosis) to get the information of the blockage degree, can reduce the rate of composite re-stenosis symptoms and other complications.
Tactile sensing was also used for cardio-vascular MIS. To obtain the contact information of the catheter while making a touch to the vascular vessel, Takizawa et al. (1999) reported the assembly of three pressure sensors at an angle of 45° on the axis of the catheter. Tri-axial force sensor on the surgical scalpel for obtaining force applied by the surgeon for making incisions was demonstrated by Valdastri et al. (2006, 2007). Neuzil et al. (2010a) reported using a tactile sensor catheter to assess the contact force during radio frequency (RF) ablation, so sufficient force can be applied during the ablation procedure to reduce recurrence.
In MEMS force sensor area, four beam design is the most prominent (Fahlbusch et al. 1998; Beccai et al. 2005; Tibrewala et al. 2008; Jin and Mote 1998) as it allows force sensing in all the three axes with a relatively simple structure. For this design, piezoresistive transduction method is the most appropriate for implementing for force sensing. For all these sensors, robustness of the sensor was improved through packaging techniques making the whole system much bigger though the miniaturized sensor was used. Capacitive sensing tri-axial force sensors were also reported to show excellent resolution (Beyeler et al. 2009). Capacitive sensing generally requires signal conditioning to be close, because of the influence of parasitic capacitance in long connections. Piezoelectric force sensors can be used as well but have limitation in static changes.
In this paper, we report a piezoresistive tri-axial force sensor using a four beam design, with integrated force mechanical stopper to enable robust operation, small foot print, and simpler assembly process. We are proposing such tri-axial force sensor to be used for the sensorized guidewire application to sense tri-axial reacting force on contact, and to be used to assess the hardness of the contacting tissue.
2 Sensor design
2.1 Piezoresistive transduction and silicon nanowire piezoresistors
(R ≡ resistance, π ≡ piezoresistive coefficient, σ ≡ stress, l and t subscripts refer to longitudinal and transversal components).
F ≡ gauge factor, R ≡ resistance, \( \varepsilon \) ≡ strain).
Longitudinal component is defined by having stress and current in the same direction, and transversal component is defined as orthogonal direction to each other. Neuzil et al. (2010b) reported that piezoresistive effect of SiNW has been shown to be increasable up to GFs of 5,000 from 50 in bulk by shrinking cross-sectional dimensions and applying back-gate bias voltage. The physics behind this enhancement has been explained based on a stress induced shift of the surface Fermi level in depleted structures (Rowe 2008). Neuzil also reported the GF is similar to bulk value at zero bias, but gets the giant factor by the back-gate bias. In this paper, because of the complexity and stringent measurement conditions required by the high GF with back-gate bias, we chose to use ‘no-bias’ condition. This simplifies the connections and makes the sensor signal output more robust from noise. We still use silicon nanowire for the sensing element at no-bias condition but can adopt biased condition for future applications that require higher GF in a more controlled environment.
2.2 Tri-axial force sensor design
This graph shows the functionality of the stopper. The displacement in the cross beam increases until the stopper makes contact on the substrate rim. Further increased force on the stopper does not increase the displacement on the cross beam. This insures that the cross beam is protected from excessive force from the stopper.
As it is not possible to reduce the flip-chip contact force, manual direct placement of polystyrene bead on the silicon rod was done as described in Fig. 8b. Using the same micromanipulator and probe tip, epoxy is dispensed on the silicon rod, and then manually dropped polystyrene bead on the silicon rod. By this method, the load applied was assumed to be zero and oven curing is used to cure the epoxy. The alignment was a concern for this method, but samples with acceptable alignment have been successfully fabricated. Images from finished stopper attachment SEM and cross section are shown in Fig. 8c.
4.1 Experiment set-up for sensitivity
4.2 Sensitivity characterization
4.3 Stopper evaluation
For the normal force conditions, the stopper will eventually hit the rim of the force sensor, and stop (as shown in Fig. 6). But at a higher force the bond between the rod and the stopper should fail. For normal force test, only manual bonding method samples were used because only this method showed consistent shear test results. For 50 μm silicon rod, the samples under test were able to survive until 20–25 gf normal force before the mechanical stopper was broken at the base. SEM picture after normal test is shown in Fig. 13a. It is suspected that the cause of failure was overetching at the base of silicon rod which was the weakest region of the rod. 100 μm silicon rod could only achieve 10–15 gf in normal force test before the mechanical stopper was detached. However, the silicon rod was not broken as shown in Fig. 13b. Different from 50 μm silicon rod, it is suspected the failure was due to misalignment of the mechanical stopper on the silicon rod, making the force no longer perfectly aligned vertically to the silicon rod.
Herein we report a four beam tri-axial force sensor with a mechanical stopper design for tactile sensor applications in guidewire navigation. Detailed analysis on the stopper design and the relationship between the stopper sizes, the functional range of the sensor was provided. With the increase in the stopper size, the permissible displacement was reduced and hence limiting the functional range. For example, the functional range of the sensor was about 25 mN for 200 μm radius stopper and the functional range was 45 mN when the stopper radius size was reduced to 150 μm while rest of the features are kept same. The advantages of the stopper are that cantilevers will be protected from excessive deformation and also provide smooth contact to the lumen. However it is important to have good adhesion of the stopper to the silicon rod. The process improvement to increase bonding strength between mechanical stopper and silicon rod is still the focus on our ongoing research. These sensors can be further integrated with application specific integrated circuit and specialized biocompatible packaging to be used in real guidewire applications. The enhanced sensing capability in the guidewire can be used to explore new capabilities in minimal invasive surgery.
This work was supported in part by A*Star science and research council under Grant 0921480070. Authors would like to thank the support from Dr. Benjamin SY Chua, and Dr. C.N. Lee of the National University of Singapore, Department of Surgery.