a bezier profiled horn for reducing penetration force with applications in surgery dung-an wang and...
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A Bezier Profiled Horn for Reducing Penetration Force with
Applications in Surgery
Dung-An WANG and Hai-Dang Tam NGUYEN
Graduate Institute of Precision Engineering, National Chung Hsing
UniversityTaiwan, ROC
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Motivation Reduced strain for patient’s pain reliefIncreased precision of cutting tissueReduce necrosis of patient’s tissue
Liao et al., 2012.
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Market for hard tissue removal
Millennium Research Group, 2010
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Industrial and medical interests Decrease cutting force (penetration force) Decrease thermal effects: temperature may
depend on Ultrasonic frequency Penetration force Cutting speed Geometry of cutting blade
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Aims Design of an ultrasonic horn with a new profile
To increase in tool-tip vibration amplitude, allowing a significant amount of material to be removed
To decrease the penetration force, reducing tissue necrosis
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Design Working frequency, 28.0 kHz
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Bézier profile Parametric curve based geometry is flexible
enough to give a much better control over the profile of horns for design purpose.
The profile of the horn is based on a cubic Bézier curve.
Four-point Bézier polygon Q0 Q1 Q2 Q3
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Parametric cubic Bézier curve
10)1(3)1(3)1()(
3
2
1
0
3223
t
P
P
P
P
tttttttP
Q
Q
Q
Q
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Design
The profile of the horn is optimized by allowing points Q1 and Q2 to move in the design space
0
3
0
Q
Q
u
uMMax
ffMin
Specify W120mm, W2=1.5mm, L=94mm. Thickness 1.2 mmf = 28.0 kHz
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Design Finite element analysis to obtain f0 and M of
the horn Material: SS41
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Optimized Bézier horn
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Comparison Catenoidal horn: same back and front end
widths and length as those of the proposed horn.
Stepped and linear horn: same back and front end widths of the proposed horn. The length of the stepped/linear horn is calculated to have the same working frequency of 28.0 kHz as the proposed horn.
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Comparison Normalized displacements along the
normalized length of the horns based on finite element computations
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Comparison The stepped horn: highest displacement
amplification, but high stress concentration at the step discontinuity
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Experiments A Bézier horn and a catenoidal horn are
fabricated by a laser cutting process from a stainless steel SS41
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Experiments Investigate the effects of ultrasound and horn
types on the penetration force of ultrasonic cutting
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Specimens A tissue stimulant, raw potato, as
representative for soft material A polymethylmethacrylate (PMMA) material as
representative for hard material Specimen: 70 mm x 35 mm x 5 mm
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Results Penetration speed 0.25 mm/s Five repeated trials Penetration force by the Bézier horn is 75% of
that of the catenoidal horn
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Results Penetration force as a function of penetration
speed Use Bezier horn Penetration depths: the tissue stimulant 2
mm, PMMA material 3 mm
Tissue stimulant PMMA material
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Mechanics Reduced penetration force is an effective
measure for increasing the critical cutting depth in ultrasonic cutting, and below the critical cutting depth, material can be removed plastically (Zhou et al., 2002).
When the plastic deformation is the predominant mode of deformation, a very smooth and fine surface can be obtained.
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Conclusions A planar Bézier horn with high displacement
amplification and low stress concentration is developed.
The displacement amplification of the Bézier horn is 30% higher than that of the traditional catenoidal horn with the same length and end surface widths
The penetration force by the Bézier horn is 75% of that by the catenoidal horn with a penetration speed of 0.25 mm/s during cutting of the tissue stimulant.