Temperature-compensated optical fibre force sensing at the tip of a surgical needle

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Chapter 5 Experimental validation of the force sensing needle via in vivo tissue identification on mice

Introduction

In vivo experiments on animals plays a very significant part in medical device development, moving further towards practical application. In this chapter, ex and in vivo tissue identification experiments on mice using a tip force sensing needle were conducted. Firstly, a new sensing needle with a smaller outer diameter of 1.47 mm and greater length of 80 mm is introduced, with given calibration results. The experimental setup, subject preparation, and needle insertion experiment designs are then described. Two groups of mice experiments are presented. One is ex vivo tissue insertion, conducted on dead mice of different times of death to study the influence of time of death on needle tip force, as well as influence of temperature. The other is an in vivo tissue insertion experiment conducted on anaesthetized mice for a comparison with ex vivo experiments. A tip force database of the internal organs of mice was also obtained for the tissue identification via insertions of fresh mice organs which were anatomised within one minute after death. Two types of experiments, skin-tumour-skin insertions and mice torso insertions were carried out using manual insertion to assess the tissue identification performance of the sensing needle. The animal experiments followed protocols approved by the University of Auckland Animal Ethics Committee (AEC), having the AEC reference number: #1781. The operations conducted on alive mice, including mice preparation, tumour cell culture, tumour planting, mice anaesthesia, and in vivo needle insertions, were operated by Xinjian Mao (PhD candidate of Faculty of Medical and Health Sciences, the University of Auckland), who is named on the ethics approval. His research relates to tumour culture and observation. Dead mice were the disposed productions from his experiments.

Experimental Design

The sensing system

New sensing needle characterisation

As mice tissues have low stiffness, a new sensing needle with a smaller diameter needs to be fabricated for better sensation. To optimise the FPI sensor for a better sensitivity, new hollow round quartz capillaries (CM Scientific, UK) with an inner diameter of 0.15 and an outer diameter of 0.25 mm were used to fit a needle with a smaller diameter. A 17G tip force sensing needle with a diameter of 1.47 mm and a length of 80 mm was produced for mice tissue identification, as shown in Figure 5.1. The cavity length of the force FPI sensor and the reference FPI sensor are 31.25 µm and 37.5 µm, respectively.Using the same calibration methods mentioned in Chapter 3, its temperature compensation properties and force-intensity phase relationship were obtained, shown in Figure 5.2 and Figure 5.3, respectively. Figure 5.2 (a) shows the FPI interference light intensity change of two embedded FPI sensors induced by temperature change. Their intensity-phase changes were calculated, as well as their accumulated phase changes, shown in Figure 5.2 (b) and Figure 5.2 (c), respectively. The relationship between the accumulated phase changes of the two FPI sensors was obtained through curve fitting, as shown in Figure 5.2 (d). The temperature compensation then could be achieved based on this.By leveraging a commercial ATI force sensor, the relationship between phase change and applied force was finally gained, also via curve fitting, the result of which is shown in Figure 5.3. As the internal organs of mice have very little stiffness, compared to mouse skin, it is necessary to calibrate force under 1 N. Therefore, the sensing needle was calibrated with small force loading, the results of which are shown in Figure 5.4.

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Experimental setup

The experimental setup is shown in Figure 5.5. Apart from tip force sensing system introduced in previous chapters, a camera was used for recording the needle insertion procedure. As mice tissues are too soft to be fastened during needle insertions, it was placed on the table or held by hands. The tissue identification was based on manual operations to achieve varying insertion directions.

Mouse preparation and experimental design Mouse preparation

The tumour cells, the human colorectal adenocarcinoma cell line HCT116 cells from American Type Culture Collection (Manassas, VA), were prepared first. Specific pathogen-free female CD-1 homozygous nude mice (approximately 25g body weight) were used, derived from breeding mice supplied by Charle River Laboratories (Wilmington, MA). About 5,000,000 cultured tumour cells were injected into each mouse at their rear flank, shown in Figure 5.6. After three to four weeks, the surviving tumour tissue had grown to a diameter of about 20 mm.

Experimental design

In total, 28 mice with tumours and four mice without tumours were prepared for the needle insertion experiments. For the in vivo experiment, the mice were first anaesthetized, using gas anesthesia as shown in Figure 5.6. In the ex vivo experiments, euthanasia was carried out on mice just before the needle insertion experiments. To study the difference between in vivo and ex vivo insertion, skin-tumour-skin insertion experiments were conducted. Skin-tumour-skin insertions, lateral-direction abdomen insertions, insertions from anus to head, and internal organs insertions were carried out during ex vivo experiments to study various sensing objectives.

Chapter 1 Introduction
1.1 The need for force sensors at the tip of medical instruments
1.2 Research motivation
1.3 Objectives and scope
1.4 Contributions
1.5 Thesis outline
Chapter 2 Literature review
2.1 Needle insertions
2.2 Force sensor requirements in MIS
2.3 Overview of force sensing mechanisms
2.4 Fibre optical force sensor based on FPI
2.5 Summary and conclusion
Chapter 3 Temperature-compensated optical fibre force sensing at the tip of a surgical needle
3.1 Introduction
3.2 Sensor Design and Fabrication
3.3 Experimental Setup and Signal Processing
3.4 Experimental Results
3.5 Temperature monitoring during needle insertion
3.6 Conclusion
Chapter 4 Capability characterization of the force sensing needle via ex vivo experiments
4.1 Introduction
4.2 Experimental Design
4.3 Experiment Results
4.4 Conclusion
Chapter 5 Experimental validation of the force sensing needle via in vivo tissue identification on mice
5.1 Introduction
5.2 Experimental Design
5.3 Experimental Results
5.4 Performance assessment
5.5 Conclusion
Chapter 6 Epidural space identification on ex vivo porcine spines
6.1 Introduction
6.2 Sensor design
6.3 Epidural space identification
6.4 Conclusion
Chapter 7 Conclusion and future work
7.1 Research outcomes
7.2 Contribution to the scientific development of tip force sensing techniques
7.3 Limitations and future work