Realistic imitation of mosquito`s proboscis: Electrochemically etched

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Sensors and Actuators A 165 (2011) 115–123
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical
journal homepage: www.elsevier.com/locate/sna
Realistic imitation of mosquito’s proboscis: Electrochemically etched sharp
and jagged needles and their cooperative inserting motion
Hayato Izumi a , Masato Suzuki a , Seiji Aoyagi a,∗ , Tsutomu Kanzaki b
a
b
Faculty of Engineering Science, Department of Mechanical Engineering, Kansai University, 3-3-35 Yamate-cho, Suita, Osaka 564-8680, Japan
Dainihon Jochugiku Co., Ltd., 1-1-11 Oguro-cho, Toyonaka, Osaka 561-0827, Japan
a r t i c l e
i n f o
Article history:
Available online 20 February 2010
Keywords:
Microneedle
Mosquito
Electrochemical etching
Biodegradable polymer
Injection molding
a b s t r a c t
Aiming at the use in low-invasive medical treatments, this paper proposes a realistic imitation of
mosquito’s proboscis. A silicon needle is electrochemically etched, making the three-dimensionally sharp
tip with finely smooth surface. The jagged shank shape is machined by a deep reactive ion etching (DRIE).
The combined needles comprising a central straight needle and two outer jagged needles are fabricated,
imitating a labrum and two maxillas of the mosquito, respectively. The cooperative motion of the three
needles imitating the mosquito’s motion is realized by applying PZT actuators independently to all the
needles. The effectiveness of inserting these needles cooperatively was experimentally confirmed. Considering practical medical application, a biodegradable polymer needle with three-dimensionally sharp
tip is also developed. The fabrication process based on micromolding is as follows: a nickel negative
cavity is made by electroplating on a silicon sharp needle, to which melted polymer is injected, and it
is finally released using a lost molding technique. The effectiveness of sharp tip for easy insertion was
experimentally proven.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
Over the last few years, several research groups have investigated the development of medical device in the field of Bio MEMS
and Micro Total Analysis System (␮-TAS), growing interest in health
and medical welfare. Among these various medical devices, a needle for blood collection is the most often used in medical practice.
A low-invasive needle is strongly desired in many medical treatments such as biopsy, transdermal drug delivery, neural interface,
lancets for puncturing and bleeding diabetics. In particular, diabetics have to collect their blood for the glucose level measurement,
which is indispensable for health monitoring. Their skin is punctured by solid metal lancet needle of straight shape to cause small
bleeding, which is painful and fearful.
The mosquito’s proboscis should be a good model for painless insertion. The proboscis is composed of several parts, which
are labium, labrum, pharynx, two maxillas, and two mandibles,
as shown in Fig. 1. Two maxillas have jagged shapes (Fig. 1(f)),
which is said to be functional for easy insertion [1,2]. Imitating
its jagged shape, the authors previously reported silicon and polymer jagged microneedles, which utilize anisotropically wet-etched
jagged groove on silicon surface in its fabrication process [2–4].
∗ Corresponding author. Tel.: +81 6 6368 0823; fax: +81 6 6330 3154.
E-mail address: [email protected] (S. Aoyagi).
0924-4247/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.sna.2010.02.010
Sharper tip is preferable to generate the stress concentration of the
skin, which assists the easy insertion: however, limited by abovementioned crystal silicon anisotropy, the tips of these needles were
not so sharp. The authors also previously fabricated a needle with
two-and-half-dimensional sharper tip by using deep reactive ion
etching (DRIE) [5,6]: however, a three-dimensional sharp tip, i.e.,
conically sharp tip, was still difficult to fabricate. To overcome
above-mentioned problem, in this paper, a silicon microneedle
having both the three-dimensional sharp tip and the harpoon-like
jagged shank is fabricated by employing electrochemical etching
technique, which has never used for sharpening the silicon needle
tip, although there is a report of sharpening the tip of Pt–Ir wire for
the probe of scanning tunnel microscope (STM) [7].
The authors previously observed the cooperative inserting
motion of mosquito’s proboscis, in which the central and the
outer needles are advanced alternatively, the vibration frequency
of which is at several dozens Hz, while the total three needles are
gradually moved forward [3]. In this paper, the combined needles
are practically fabricated, and the effectiveness of their cooperative
motions is experimentally investigated [8].
This paper also reports a biodegradable polymer needle with
three-dimensionally sharp tip considering practical medical application, which is fabricated based on micromolding as follows:
electroplating on a silicon sharp needle for a nickel negative cavity, injecting melting polymer to the cavity, and releasing it by lost
molding technique.
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Fig. 1. Mosquito’s proboscis comprising a center straight labrum and outer jagged maxillas: (a) schematic mosquito’s proboscis, (b) optical image of mosquito’s proboscis
by high-speed camera (optical magnification; 115), and (c)–(f) SEM images of mosquito’s proboscis.
2. Observation of penetrating motion of mosquito’s
proboscis
3. Fabrication
3.1. Principle and experimental setup
Several hypotheses of mosquito’s insertion mechanism have
been reported [1,9], however, none of them has been confirmed
either theoretically or experimentally at the present state. In
this section, penetrating motion of mosquito’s proboscis to a
transparent thin skin of laboratory rat was observed by using a magnifying lens system (Leica Corp., Ltd., max. optical magnification is
115, max. working distance is 39 mm) and a high-speed camera
(NAC Image Technology Inc., MEMRECAM fx-K5, sampling rate is
1000 flame/s). An example scene during insertion into the skin of
rat is shown in Fig. 2. Following facts were confirmed by the observation: (1) the tip of labium supports the bundle of other parts,
(2) the bundle is vibrated at several dozens Hz, (3) the labium and
two maxillas are inserted into the skin while keeping synchronous
motion to each other, i.e., they are advanced alternatively.
The electrochemical etching technique is known as a method of
polishing silicon surface, in which the silicon is used as an anode
electrode and its electrical potential is kept positive relatively to
that of a counter cathode electrode. HF/H2 O solution is used to etch
the silicon surface with considerable smoothness. An experimental
setup for electrochemical etching of silicon is schematically shown
in Fig. 3.
The principle of electrochemical etching of silicon material is as
follows: the surface of silicon is changed to silicon dioxide (SiO2 )
based on the anodic oxidization. Then, the SiO2 is etched by HF
solution. The formulae for these chemical reactions are described in
the inset of Fig. 3. The oxidization and HF etching are successively
repeated: finally, the silicon surface is smoothly polished. In this
Fig. 2. Observed penetrating motion of mosquito’s proboscis: (a) example scene during insertion captured by high-speed camera system, and (b)–(d) transition of cooperative
motion of mosquito’s needles (optical magnification; 46). The schematic motions of labrum and maxillas are shown in the insets.
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Fig. 3. Schematic of electrochemical etching for sharp needle.
experiment, 10% HF solution was used as an electrolyte solution,
and the applied voltage was set to 200 V.
3.2. Sharpening of silicon needle
Fabrication of a sharp silicon needle is the preliminary task, in
order to confirm the above-mentioned principle of polishing and
sharpening the tip. A two-and-half-dimensional silicon needle was
fabricated using DRIE. The specification of used silicon wafer is
as follows: n-type, crystal surface orientation: (1 0 0), resistivity:
several cm, thickness: 150 ␮m. Then, the tip of the needle was
electrochemically etched.
The progress of etching transition was observed by a zoom
microscope (OMRON Corp., VC1000, max. magnification is 400,
working distance is 64 mm, sampling rate is 30 flame/s), in which a
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conically sharp needle is formed from a two-and-half-dimensional
needle. The result of observation is schematically shown in Fig. 4,
which is as follows: the tip part of a silicon needle is soaked in
the electrolyte solution (Fig. 4(a)). The electrochemical etching proceeds rapidly at the position a little apart from the level surface of
electrolyte solution, where a constriction part is formed (Fig. 4(b)).
Then, the needle is cut at the constriction part (Fig. 4(c)). After that,
the remained tip is made thinner and further sharpened due to the
electrochemical etching reactions (Fig. 4(d)). Finally, the tip part is
so rounded that the edges of the needle pillar disappear, achieving
a conically sharp tip (Fig. 4(e)).
Precise mechanism of this electrochemical etching is still
unknown: however, an anticipated mechanism from many experiences and careful observations is as follows: the reaction on the
early stage is so hard to evaporate the etchant, making a hollow
space and lowering the level surface. After the needle is cut at the
constricted part, the reaction generally calms down. Eventually, the
smooth and sharp tip is obtained in the final product.
Scanning electron microscope (SEM) images of the fabricated
silicon microneedle are shown in Fig. 5. Looking at this figure, a
conically sharp tip with finely smooth surface due to electrochemical etching is realized. Tip angles of thirty electrochemically etched
needles were investigated. Their MEAN was 18◦ and SE (standard
error of mean) was 0.77◦ . A specification of sharpening silicon needle is shown in Table 1.
3.3. Combined three silicon needles
The combined three silicon needles consisting of a central
straight needle and two outer jagged needles are fabricated, imi-
Fig. 4. Observed transition of electrochemical etching on silicon needle: (a) soaked in electrolyte solution, (b) etched rapidly at position a little apart from level surface of
solution, (c) cut at the constriction part, (d) further sharpened, and (e) finally rounded to conically sharp tip.
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Fig. 5. SEM images of electrochemically etched silicon needle.
Table 2
Specification of three combined needles
Table 1
Specification of sharpened silicon needle
.
.
Needle length (L)
Width of needle (W)
Tip angle ()
Material
1 mm
150 ␮m
18◦ (SE: 0.77◦ )
Silicon
tating a labrum and two maxillas of the mosquito. The fabrication
process of combined microneedles is shown in Fig. 6, which is
almost the same as that of above-mentioned sole silicon needle.
The combined needles with large lateral intervals are fabricated
on a silicon wafer of 150 ␮m thickness using DRIE (Fig. 6(a)), followed by electrochemical etching of the tip part for sharpening the
needles.
SEM images of the fabricated combined three needles are shown
in Fig. 7. A specification of combined three silicon needles is shown
in Table 2. The width of central straight needle and outer jagged
ones are 30 ␮m and 15 ␮m, respectively. Conically sharp tips with
finely smooth surface are realized. The harpoon-like jagged shank
shape is also successfully realized, the size of which is almost the
same as the mosquito’s.
Effective needle length (L)
Width of central needle (W1 )
Width of outer jagged needle (W2 )
Tip angle of central needle ( 1 )
Tip angle of outer jagged needle ( 2 )
Material
1 mm
30 ␮m
15 ␮m
18◦
15◦
Silicon
4. Insertion experiment and results
4.1. Experimental equipment and condition
The fabricated device is set on three manual positioning stages
with PZT actuators (MESS-TEK Co., Ltd, MA-140XLS) as shown in
Fig. 8. Using these stages, connecting parts of combined three
needles are broken and the device is divided to three separated
needles. Then, the lateral intervals are adjusted to a small value of
approximately 10 ␮m under a stereoscopic microscope using manual positioning stages (Figs. 6(c) and 8). Dividing the combined
needles and then adjusting them makes the accurate position-
Fig. 6. Fabrication of three combined needles having electrochemically etched sharp tips: (a) device comprising three silicon needles fabricated by DRIE, (b) schematics of
electrochemical etching, and (c) device is set on three positioning stages (see Fig. 8). Using these stages, connecting pars are broken and the device is divided to three needles.
Their orientations and intervals are adjusted by the stages.
Fig. 7. SEM images of combined three needles.
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Fig. 8. Experimental system for imitating mosquito inserting motion (resolution: position 0.75 ␮m, force 0.5 ␮N).
Fig. 9. Transition of cooperative motion observed by a high-speed camera.
ing/orientating of three needles possible and easy, compared to
positioning/orientating originally separated three needles.
Three PZT actuators with positioning stages are on a computationally controlled linear stage, which realizes the mosquito-like
cooperative motion of independent three needles while all the needles gradually progress forward. Transition of cooperative motion
of fabricated needles observed by a high-speed camera is shown in
Fig. 9. For reference, let us compare this needle’s motion with that
of mosquito’s proboscis, which is already shown in Fig. 2(b)–(d).
Transition of resistance force during inserting the needles to an
artificial skin of silicone rubber (Tigers Polymer Corp., thickness is
1 mm, Young’s modulus is 2.2 MPa) was detected by a load cell (Tech
Gihan Corp., TGRV02-2N, the rated load is 2 N, the displacement at
the rated load is 4 ␮m, and the linearity is 0.5% to the full range). In
this experiment, moving speed of stage was set to a comparatively
low speed of 0.2 mm/s.
are independently moved and inserted to the object. In Modes B and
C, the vibration frequency at 30 Hz whose waveform is saw-tooth
was applied. Also, the amplitude of vibration was set to 140 ␮m,
which is experimental maximal value.
In Mode C, the phase of motions between central needle and
outer two jagged needles are adjusted by a function generator, so
the needles are synchronously moved. In concrete, after the central
needle is moved, the outer two jagged needles are simultaneously
moved with 180◦ phase delay to the central needle. These synchronous motions are repeated at 30 Hz, which is defined by the
function generator.
4.2. Effect of cooperative inserting motion
The modes of inserting motion tested here are shown in Fig. 10.
Three inserting modes are employed, which are non-cooperative
motion without vibration (referred to as Mode A), non-cooperative
motion with vibration (referred to as Mode B), and cooperative
motion (referred to as Mode C). In Mode A, all the three needles
are simultaneously inserted to the object. In Mode B, vibrations
are applied to all the three needles, while they are inserted to the
object. In Mode C, the central needle and outer two jagged needles
Fig. 10. Inserting mode (Mode A: non-cooperative without vibration, Mode B: noncooperative with vibration, and Mode C: cooperative).
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the needle is really inserted into the inside of silicone rubber. Once
a silicone rubber tissue was eroded, the resistance force decreased
drastically. Therefore, their force was saturated during inserting
needle into silicon rubber.
The above-mentioned transition of the relationship between the
needle and rubber is shown in schematic cross-sectional drawings
just above Fig. 11. In the present paper, the force at the inflection point is defined as the necessary force for puncture (called
as puncture force). This force was evaluated as the index for easy
insertion.
Here, silicone rubber presents viscoelastic characteristics, so the
resistance force is affected by the insertion speed of needle. The
experimental verification of these trends is the projected work.
Another concern is the relation between the resistance force and
the degree of pain. It is not cleared yet how the pain is caused by
needle insertion: however, it is easily expected that skin tissue is
more damaged as the larger resistance force of needle against the
skin is generated. Therefore, resistance force can be one of indexes
showing the degree of pain during needle insertion.
Fig. 11. Result of transition of resistance force during inserting the three needles to
silicone rubber. In Modes B and C, vibration frequency was set to 30 Hz. The vibration
amplitude was set to 140 ␮m.
The experimental result of the relationship between the displacement and the resistance force (referred to as a load curve)
is shown in Fig. 11. From careful observations at many times, the
transition of the relationship between the needle and the rubber,
which causes the trend of the load curve, is probably as follows: the
silicone rubber is gradually deformed profiling a concaved shape,
as the needle progresses to several hundred ␮m from the original silicone rubber surface. While this period, the resistance force
increases nonlinearly to the needle displacement at the beginning,
and linearly increases afterward, as shown in this figure. The reason
of nonlinear increase is that the contact area between needle and
silicone rubber rapidly increases, affected by not only the increase
of insertion depth but also the increase of the needle cross-section
area at the tip part. After tip part passes the original surface of rubber, the increase of contact area is only due to insertion depth,
since the cross-section area at the shank besides the tip is constant.
After the resistance force increases till some value, then the rate
of its increment against the displacement increment changes to a
comparatively low value, making a peak point (Mode A) or inflection point (Modes B and C). At the peak or inflection point of the
load curve, it seems that the shear fracture of silicone occurs, and
Fig. 12. Result of evaluation of puncture force.
Fig. 13. Process flow of sharp microneedle made of biodegradable polymer.
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Fig. 14. Fabricated nickel cavity by electroplating.
Looking at Fig. 11, the puncture force are about 21, 13, 6 gf for
non-cooperative without vibration, non-cooperative with vibration, cooperative, respectively. The effect of inserting mode on
puncture force is shown in Fig. 12. The puncture forces of three
needles were measured for each inserting mode. Looking at this figure, the experimental repeatability for three forces is good enough
to show the significant effect of insertion mode on puncture force.
From experimental results, it is proven that the cooperative alternative motion of the three needles is much effective for reducing
the necessary force for penetrating the skin, compared to the noncooperative motion with/without vibration.
5. Application to polymer needle
5.1. Purpose of using polymer material
As explained in Section 3, the authors fabricated a silicon needle with three-dimensional sharp tip using electrochemical etching
technique. Since silicon is brittle material, there is possibility that
the needle may be broken during insertion. The left pieces in human
organism may cause a fatal problem. Considering the safety to the
human body, the author has previously fabricated a solid needle
made of biodegradable polymer [3,4]. Development of polymer
microneedles is reported by several researchers [10–14]: however,
they are fabricated in the direction perpendicular to the substrate
surface, so the length is limited by the wafer thickness. By contrast,
our needle is fabricated in the direction parallel to the substrate
surface, so the comparatively long needle is possible.
Our previous polymer needle was formed by wet-etching a
groove on a silicon die, molding polymer into this groove, and
releasing it. The tip angle of needle, however, was fixed to rather
dull angles, which are 90◦ on the top surface and 54.7◦ on the
cross-section, restricted by anisotropic property of single crystal
silicon.
In this section, a conical sharp needle made of polymer material
such as biodegradable one, e.g., polylactic acid (referred to herein as
PLA), is fabricated, considering that mosquito’s proboscis is consisting of polymer chitin material. Polymer material is non-brittle and
flexible, which prevents the problem that a broken piece remains
inside the human body and it causes several diseases. In the case of
PLA, even if a piece is left, it is finally dissolved into safe H2 O and
CO2 inside the human body.
Micromolding method is used for this purpose, in which melted
PLA is injected to a nickel die negative to the electrochemically
etched sharp silicon needle. The fabrication of combined three needles made of polymer will be conducted in the projected work.
5.2. Fabrication and results
The fabrication process of PLA microneedle is shown in Fig. 13.
It is as follows: a fabricated sharpened silicon needle (Fig. 13(a)) is
set on a silicon substrate, on which Cr/Au (Cr: 0.1 ␮m, Au: 0.1 ␮m)
is sputtered as the seed layer for electroplating (Fig. 13(b)). Then,
nickel is electroplated using Watts bath solution, of which composition and temperature are NiSO4 : 36 g, NiCl: 6.75 g, B3 HO3 5.25 g,
H2 O: 150 ml, and 53 ◦ C (Fig. 13(c)). Next, silicon needle and substrate are etched away using KOH solution (20 wt%, 72 ◦ C, 12 h),
leaving a nickel cavity (Fig. 13(d)). Melted polymer is injected
to a space confined by the nickel cavity and the holder surface (Fig. 13(e)). Finally, the needle is released by lost molding
method, i.e., etching away of nickel using nitrohydrochloric acid
(HCl: NHO3 = 3:1, approximately 20◦ C of room temperature, 10 h)
(Fig. 13(f))).
The optical images of nickel cavity and SEM images of PLA needle
are shown in Figs. 14 and 15, respectability. The needle with almost
no burrs was realized.
5.3. Insertion experiment of sharp polymer needle
The resistance force of fabricated polymer needle during insertion to a silicone rubber was investigated. The experimental result
of transition of resistance force is shown in Fig. 16(a). To compare
the effects of tip shapes, this figure shows the data for both polymer needle (tip angle is 90◦ ) and commercial lancet needle (tip
angle is 40◦ ). For comparison, the same experiment was carried
out for the silicon needle, the result of which is shown in Fig. 16(b).
A schematic view of various needles used in the experiment, which
includes their detailed dimensions, is shown in Fig. 17.
Fig. 15. Optical images of PLA needle with three-dimensional sharp tip.
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Fig. 16. Transition of the resistance force during insertion of the needle into silicone rubber.
Fig. 17. Dimension of fabricated needles and commercial metal lancet needle.
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Looking at Fig. 16(a) and (b), the effectiveness of PLA needle
with three-dimensional sharp tip for easy insertion is confirmed.
The force necessary for puncturing object surface, which is referred
to as puncture force, is almost the same between PLA needle and
silicon needle. The puncture force of PLA or silicon needle is less
than that of the commercial metal needle.
In Fig. 16(b), the drop of force occurs in case of a silicon needle with two-and-half-dimensional tip, while it does not occur in
other cases. In our previous work on two-and-half-dimensional
silicon needles, the same drop was observed [5]. Two-and-halfdimensional tip is not so sharp, making its puncture force higher
than that of three-dimensional sharp tip. Therefore, the difference
between before and after the puncture is large, generating the drop
of force in its load curve. However, the drop of force is not observed
in case of a PLA polymer needle with two-and-half-dimensional tip,
and the resistance force after puncture keeps comparatively high
value, as shown in Fig. 16(a). We previously made clear in experimental way that the friction force between polymer and silicone
rubber is apparently larger than that between silicon and silicone
rubber [6]. The large friction force maybe prevents the force drop
at the puncture of polymer needle.
6. Conclusions
Imitating mosquito’s proboscis, a combined microneedles consisting of the central and the outer jagged needles were fabricated. A
three-dimensional sharp tip of needle was realized by electrochemical etching. Based on the observation of mosquito’s penetrating
motion, the effectiveness of the cooperative motion of fabricated
three needles was experimentally investigated by moving them in
several synchronous or independent modes using PZT actuators.
Further investigation of effectiveness of various inserting modes
by changing vibration frequency, amplitude, phase difference, etc.
is the projected work.
For practical application, a polymer needle with threedimensional sharp tip was fabricated by electroplating and
micromolding. The force of the polymer needle necessary for puncturing the silicone rubber was comparable with that of a silicon
needle, and it was less than that of a metal commercial needle.
The fabrication of combined three needles made of polymer is the
projected work.
Acknowledgements
This work was mainly supported by JSPS (Japan Society for the
Promotion of Science) KAKENHI (19310091). This work was partially supported by “High-Tech Research Center” Project for Private
Universities: Matching Fund Subsidy from MEXT, 2005–2009, the
Kansai University Special Research Fund, 2007–2009.
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Biographies
Hayato Izumi received his BE and ME and PhD degrees in mechanical engineering from Kansai University, Osaka, Japan, in 2004, 2006, and 2009, respectively.
He is currently a post doctoral fellow in the Mechanical Engineering Department
at the same university. His current research interest is MEMS, with an emphasis on FET sensor, microneedle, biomedical systems, such as trace blood collection
systems.
Masato Suzuki received his BE and ME and PhD degrees in semiconductor engineering from the Hiroshima University, Hiroshima, Japan, in 2002, 2004, and 2007,
respectively. From 2007 to 2008, he was with the Research Center for Nanodevices
and Materials at the same university as a post doctoral fellow. He is currently an
assistant professor of the Mechanical Engineering Department at Kansai University,
Osaka, Japan. His current researches are microsensors and microactuators.
Seiji Aoyagi received his BE, ME, and PhD degrees in precision machinery engineering from the University of Tokyo, Tokyo, Japan, in 1986, 1988, and 1994,
respectively. From 1988 to 1995, he was with the Mechanical System Engineering
Department at Kanazawa University, Kanazawa, Japan as a research associate and
an associate professor. He is currently a full professor in the Mechanical Engineering Department at Kansai University, Osaka, Japan. His current research interests are
robotics, mechatronics, MEMS, with an emphasis on sensors, and actuators for micro
robotics.
Tsutomu Kanzaki received his BE degree in environmental sciences from Shimane
University, Shimane, Japan, in 1978. In the same year, he joined Dainihon Jochugiku
Co., Ltd., Osaka, Japan, as a researcher. Currently, he is a manager of Research &
Development Laboratory at the same company. He is engaged in research and development of household insecticides.