MEMS for biomedical applications © Woodhead Publishing Limited, 2012 Related titles: Implantable sensor systems for medical applications (ISBN 978-1-84569-987-1) Intelligent sensor systems for medical applications can greatly improve quality of life and medical care This book discusses the core technologies needed for these sophisticated devices and their current and potential applications Chapters in Part I review a wide range of core technologies that are fundamental to intelligent medical sensor systems Part II reviews considerations for intelligent medical sensor systems and the final part discusses the various applications areas of intelligent sensor systems Biomedical imaging: Applications and advances (ISBN 978-0-85709-127-7) The development of imaging techniques is of great importance for the monitoring of medical implants, diagnosis of disease, and for strategies of personalized medicine Significant advances are being made in this technology and this book discusses the latest advances and developments in this increasingly important field Chapters in Part I provide readers with a wide ranging review of medical applications whilst the second set of chapters discusses biomaterials and processes Biosensors for medical applications (ISBN 978-1-84569-935-2) Biomedical sensors play an important role in the detection and monitoring of a range of critical medical conditions This publication provides readers with a comprehensive review of established, cutting edge, and future trends in biomedical sensors and their applications Chapters in Part I discuss principles and transduction approaches to biosensors Part II reviews a wide range of applications Details of these and other Woodhead Publishing books can be obtained by: • • • visiting our web site at www.woodheadpublishing.com, contacting Customer Services (e-mail: sales@woodheadpublishing.com; fax: +44 (0) 1223 832819; tel.: +44 (0) 1223 499140 ext 130; address: Woodhead Publishing Limited, 80 High Street, Sawston, 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Ansari Road, Daryaganj, New Delhi – 110002, India www.woodheadpublishingindia.com First published 2012, Woodhead Publishing Limited © Woodhead Publishing Limited, 2012 The authors have asserted their moral rights This book contains information obtained from authentic and highly regarded sources Reprinted material is quoted with permission, and sources are indicated Reasonable efforts have been made to publish reliable data and information, but the authors and the publishers cannot assume responsibility for the validity of all materials Neither the authors nor the publishers, nor anyone else associated with this publication, shall be liable for any loss, damage or liability directly or indirectly caused or alleged to be caused by this book Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microfilming and recording, or by any information storage or retrieval system, without permission in writing from Woodhead Publishing Limited The consent of Woodhead Publishing Limited does not extend to copying for general distribution, for promotion, for creating new works, or for resale Specific permission must be obtained in writing from Woodhead Publishing Limited for such copying Trademark notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation, without intent to infringe British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library Library of Congress Control Number: 2012938840 ISBN 978-0-85709-129-1 (print) ISBN 978-0-85709-627-2 (online) ISSN 2049-9485 Woodhead Publishing Series in Biomaterials (print) ISSN 2049-9493 Woodhead Publishing Series in Biomaterials (online) The publisher’s policy is to use permanent paper from mills that operate a sustainable forestry policy, and which has been manufactured from pulp that is processed using acid-free and elemental chlorine-free practices Furthermore, the publisher ensures that the text paper and cover board used have met acceptable environmental accreditation standards Typeset by Newgen Publishing and Data Services Printed by TJ International Ltd, Padstow, Cornwall, UK © Woodhead Publishing Limited, 2012 Contents Contributor contact details Woodhead Publishing Series in Biomaterials Introduction xi xv xix Part I Fundamentals of MEMS for biomedical applications 1 Microfabrication of polymers for bioMEMS P REZAI, W-I WU and P R SELVAGANAPATHY, McMaster University, Canada 1.1 1.2 1.3 1.4 1.5 Introduction Microfabrication Polymers and processes Conclusions References and bibliography Review of sensor and actuator mechanisms for bioMEMS P K SEKHAR and V UWIZEYE, Washington State University Vancouver, USA 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 Introduction: transducers Sensors Actuators Biomedical applications of sensors and actuators Optical biosensor Microrobotics in biomedical applications Conclusion References 34 35 46 46 47 55 62 65 71 75 76 v © Woodhead Publishing Limited, 2012 vi Contents Part II MEMS for biomedical sensing and diagnostic applications 79 MEMS for in vivo sensing S ARAVAMUDHAN, North Carolina A&T State University, USA 81 3.1 3.2 3.3 Introduction Overview of MEMS in vivo devices and sensors Challenges and possible solutions to in vivo sensing methodology Regulatory dimensions Conclusions and future trends References 81 83 3.4 3.5 3.6 MEMS and electrical impedance spectroscopy (EIS) for non-invasive measurement of cells D T PRICE, University of South Florida, USA 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 Importance of MEMS in cellular assays Impedimetric measurement theory Visualization and modeling Bioimpedance before MEMS: patch clamp measurements MEMS in bioimpedance applications Future trends Sources of further information and advice References MEMS ultrasonic transducers for biomedical applications R GULDIKEN and O ONEN, University of South Florida, USA 5.1 5.2 5.3 5.4 5.5 5.6 5.7 Introduction Modeling and design of capacitive micromachined ultrasonic transducers (CMUTs) Fabrication Integration Biomedical applications Conclusion and future trends References © Woodhead Publishing Limited, 2012 89 92 93 93 97 97 98 100 102 104 116 116 117 120 120 123 128 133 134 142 144 Contents 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.9 6.10 Lab-on-chip (LOC) devices and microfluidics for biomedical applications K W OH, University at Buffalo, The State University of New York (SUNY), USA Introduction Pressure-driven lab-on-chip (LOC) Capillary-driven LOC Electrokinetic-driven LOC Centrifugal-driven LOC Droplet-based LOC Electrowetting-based LOC Future trends Sources of further information and advice References Part III MEMS for tissue engineering and clinical applications vii 150 150 151 154 157 159 162 164 166 167 168 173 Fabrication of cell culture microdevices for tissue engineering applications J D CUIFFI, Draper Laboratory, USA 175 7.1 7.2 7.3 7.4 7.5 7.6 7.7 Introduction: cell culture microdevices Motivation for microdevice development Design and fabrication concepts for cell culture Applications of cell culture microdevices Future trends Sources of further information and advice References 175 175 179 185 188 188 189 MEMS manufacturing techniques for tissue scaffolding devices C-W LI and G-J WANG, National Chung-Hsing University, Taiwan 8.1 8.2 8.3 8.4 8.5 8.6 Introduction Tissue scaffold design Tissue scaffold fabrication using MEMS approaches Applications of MEMS-fabricated tissue scaffold Conclusion References © Woodhead Publishing Limited, 2012 192 192 193 198 210 213 213 viii Contents BioMEMS for drug delivery applications L KULINSKY and M J MADOU, University of California, Irvine, USA 218 9.1 9.2 9.3 9.4 9.5 9.6 9.7 Introduction Transdermal delivery Implantable systems Microfabricated drug delivery vehicles Conclusions Acknowledgement References 218 219 236 246 249 250 250 10 Applications of MEMS technologies for minimally invasive medical procedures K R OLDHAM, University of Michigan, USA 269 10.1 10.2 10.3 10.4 10.5 Introduction Microvisualization Micromanipulation Future trends and conclusions References 269 270 280 286 287 11 Smart microgrippers for bioMEMS applications Y Q FU, University of the West of Scotland, UK, J K LUO, University of Bolton, UK and A J FLEWITT and W I MILNE, University of Cambridge, UK 291 11.1 11.2 11.3 11.4 11.5 11.6 Introduction Microgripping and release strategies Microgripper demonstration: microcage Conclusions Acknowledgement References and bibliography 291 293 306 327 330 330 12 Microfluidic techniques for the detection, manipulation and isolation of rare cells M B SANO and R V DAVALOS, Virginia Tech – Wake Forest School of Biomedical Engineering, USA 12.1 12.2 12.3 12.4 12.5 Introduction Sized-based isolation Mass-based isolation Electrical-based isolation References © Woodhead Publishing Limited, 2012 337 337 338 343 346 355 Contents ix Part IV Emerging biomedical applications of MEMS 359 13 MEMS as implantable neuroprobes A V GOVINDARAJAN and M JE, Institute of Microelectronics, Singapore, W-T PARK, Seoul National University of Science and Technology, Korea and A K H ACHYUTA, The Charles Stark Draper Laboratory Inc., USA 361 13.1 13.2 13.3 Introduction – neuronal communication MEMS-based neuronal intervention devices Tissue response against implanted neural microelectrode interfaces Implantable wireless recording integrated circuit (IC) challenges References 361 363 13.4 13.5 382 387 388 14 MEMS as ocular implants W LI, Michigan State University, USA 396 14.1 14.2 14.3 14.4 14.5 14.6 Introduction Implantable MEMS for glaucoma therapy Integrated microsystems for artificial retinal implants Future trends Conclusion References 396 397 408 418 420 421 15 Cellular microinjection for therapeutics and research applications P KHANNA, Globalfoundries, USA 432 15.1 15.2 15.3 15.4 15.5 15.6 15.7 15.8 Introduction Significance of cellular injection Microinjection MEMS technologies for microinjection Future of mechanical microinjection Automating microinjection Conclusion References 432 433 435 437 441 442 444 444 16 Hybrid MEMS: Integrating inorganic structures into live organisms A J SHUM and B.A PARVIZ, University of Washington, USA 449 Introduction 449 16.1 © Woodhead Publishing Limited, 2012 460 MEMS for biomedical applications can perform some of the above functions such as microactuation (Mohebbi et al., 2001) and microsensing (Warneke et al., 2001) Other functions performed by Drosophila such as self-healing, energy storage/conversion, and flight in a miniature system measuring only a few millimeters have proven more difficult to replicate (Wu et al., 2003; Parviz et al., 2005) Despite the ability of solid-state microfabrication technology to construct advanced devices, the level of complexity in multifunctional system integration inherent to Drosophila is far beyond the state-of-the-art in microengineering Since construction of miniature robots as sophisticated as Drosophila will not be feasible in the near future, our group has taken a unique approach for modification of these currently superior nature-made systems: parallel microfabrication directly onto them (Shum et al., 2007a; Shum and Parviz, 2007b, 2007c) Our approach addresses many of the limitations of these prior approaches Interfacing engineered structures with insects has been primarily limited to large insects (meaning insects with dimensions on the order of a few centimeters), large engineered structures, and serial manufacturing In all of the preceding examples, a circuit or a microstructure was manually connected to the insect under a microscope This intricate, serial assembly approach can be exceedingly slow, limiting the throughput of production for functional microsystems Our approach is a parallel one, applying solid-state microfabrication processes to arrays of flies self-assembled on a silicon chip to manufacture in a massively parallel fashion, hybrid flying microstructures The feasibility of using Drosophila as a substrate for microfabrication involved an investigation into the tolerance of Drosophila to the harsh conditions characterized by semiconductor processing These microfabrication processes may involve low pressures, high temperatures, and toxic chemicals While Drosophila, like most insects, are somewhat rugged creatures, their tolerance to conditions specific to microfabrication processes was unknown We started our study with pressure tolerance since vacuum pressure requirements are common to many cleanroom procedures A typical Drosophila melanogaster fly lives to weeks Similar to the hawkmoth, the development cycle spans four distinct stages (Fig 16.7); (Day 1) (Day 5) (Day 11) Male Egg larval stages Pupa Female Adult 16.7 Typical development cycle of Drosophila at 25 °C (Source: Reprinted with permission from Shum and Parviz, 2007b.) © Woodhead Publishing Limited, 2012 Hybrid MEMS 461 each stage could be thought of as a possible point of intervention during which microfabrication on the substrate (the insect) may be performed The embryo or egg stage is the first lasting approximately day Eggs measure about 500 µm long and 150 µm in diameter During this time, the blueprint for the fly’s development is laid out and many biological pathways are set into motion, but the egg itself is immobile The second stage, lasting about days, is the larval stage During this time the worm-like fly will range between and mm in length and undergo molts At this time, the organism is active, constantly eating and growing The end of the larval stage is marked by a wandering away from the food source and a separation from the larval cuticle (or skin) The larval cuticle will serve as the pupal case, a protective covering that encases an immobilized fly as it undergoes metamorphosis, the drastic transition into the adult form On about the eleventh day, the adult fly will eclose, or emerge from the pupal case The exterior of the adult fruit fly, the cuticle, is a permanent structure Male adults are typically much smaller than female adults and can be identified by the dark spot on the underside of their tail Previously, methods for manipulating large quantities of Drosophila have been limited to the embryo phase (Bernstein et al., 2004; Furlong et al., 2001) Biology researchers routinely sort and transfer adult flies with minimal consequence to fly health using manual protocols involving exposure to either elevated doses of carbon dioxide or ether or to low temperatures We developed a container for loading flies into cleanroom processing equipment This manufactured part was multifunctional restraining flies in the process chamber without restricting fly surfaces from modification By using this fixture and tailoring process recipes to encourage fly survivability, we ultimately demonstrated the feasibility of microfabrication on a live organism with patterned thermal evaporation Drosophila tolerance to vacuum is essential for compatibility with many solid-state microfabrication processes A preliminary study was performed subjecting Drosophila at various stages of development to vacuum This study quickly revealed that the larval stages were the most vulnerable to decreased pressures Eggs, late-stage pupae, and adults could withstand at least a few minutes of exposure to decreased pressures Limited characterization for embryo and a comprehensive characterization for adult tolerance to vacuum were performed; a few pilot tests were also done on pupae A study of embryo survival versus time under low pressure was performed with sets of 60 embryos collected on 1-in diameter agar plates The agar is scored into sections with 10 embryos each Plates were placed in the vacuum chamber individually with the pump running for either 1, 1.5, 2, 2.75, 5, or 15 (Fig 16.8); the very next day, embryos were examined under a microscope to determine the number that had advanced to the first instar larval stage This was done by counting the number of vacant egg cases From the © Woodhead Publishing Limited, 2012 462 MEMS for biomedical applications Percentage survived (a) 100 90 80 70 60 50 40 30 20 10 0 10 12 14 Time under pressure (min) Pressure (m Torr) (b) 5000 4500 4000 3500 3000 2500 2000 1500 1000 500 0 10 15 Time (min) 16.8 (a) Percentage of Drosophila embryos that survived versus time under pressure Survival is defined as embryos that hatched into first instar larvae Sample size for each point is 60 embryos (b) Typical chamber pressure transient curve for experiment (Source: Reprinted with permission from Shum and Parviz, 2007b.) data, we see that survival is significantly compromised after with pressure reaching just below Torr A small sampling of pupae within day of eclosion was tested for up to First, nine pupae on a glass slide were subjected to vacuum pumping for 30 s with pressure reaching as low as 250 mTorr Seven of the nine later eclosed into flying fruit flies Since access to the final adult cuticle during microfabrication became our goal, we also checked pupa survival with only partial pupal cases Pupal cases were punctured manually with dissecting forceps to expose the abdomen and below of flies Sets of approximately 10 flies were subjected to 1, 2, 3, and of vacuum with pressures as low as 300, 300, 200, and 120 mTorr, respectively (Table 16.2) The pupal case dissection was quite tedious and caused some flies to eclose abnormally In these cases, wings did not inflate properly, and resulting flies were not © Woodhead Publishing Limited, 2012 Hybrid MEMS 463 Table 16.2 Percentage of Drosophila pupa with partial pupal cases that survived versus time under pressure Time (min) % Survived total % Survived normal 60 89 90 71 20 67 70 71 Source: Reprinted with permission from Shum and Parviz (2007b) Note: % survived total quantifies all flies that eclosed to mobile adults, % survived normal quantifies only flies eclosed with normal wings capable of flight capable of flight The irreproducible manual procedure is the likely cause for observing an anomalous increase in the survivals rate for and exposure periods in this data set Characterization of adult Drosophila tolerance to vacuum was particularly important since access to the final cuticle, the surface on which we will microfabricate, is readily available only during this phase of development Our study exposed sets of 10 Drosophila flies, collected in mesh covered Petri dishes, to pressures as low as mTorr vacuum for various durations After exposure to vacuum, all flies were found unconscious and transferred from the Petri dish to a food bottle Usually within an hour of removal from the vacuum chamber a number of them regained their mobility We monitored their activity over at least day to determine survivability Data was taken to characterize survival with respect to (1) time under pressure, (2) time away from food, (3) gender, and (4) age The fly survival versus time under vacuum curve is shown in Fig 16.9 In this experiment flies were collected days after eclosion The curve represents the average survival of four sets of flies for each 10 increment from 10 to 60 and of two sets for 70 The vertical error bars show the maximum and minimum survival measured for each time period In general as expected, survival rate declines with increasing time under vacuum Fly survival for 30 under vacuum versus time away from food was characterized mainly to check for any drastic dependencies since flies for the survival versus time under vacuum characterization may be collected into Petri dishes up to h before exposure to vacuum Data depicted in Fig 16.10 shows that average fly survival decreased by up to 20% when exposed to vacuum after h without food Fly survival versus time under vacuum curves separated by gender are shown in Fig 16.11 In this experiment, flies were collected in sets of ten days after eclosion The coordinates represent the average survival of four © Woodhead Publishing Limited, 2012 464 MEMS for biomedical applications Percentage survived (a) 100 90 80 70 60 50 40 30 20 10 0 10 20 30 40 50 60 70 80 Time under pressure (min) Pressure (mTorr) (b) 200 175 150 125 100 75 50 25 0 10 20 30 Time (min) 40 50 60 16.9 (a) Percentage of adult Drosophila that survived versus time under pressure Each point is the average of four samples of 10 flies Error bars indicate range of data taken (b) Typical chamber pressure transient curve for experiment (Source: Reprinted with permission from Shum and Parviz, 2007b.) sets of flies for each 30 increment from to 180 The vertical error bars show the maximum and minimum survival measured for each time period Female survival rate is much higher than male survival rate A typical pressure transient is given in Fig 16.12 Fly survival versus time under vacuum curves comparing two age groups are shown in Fig 16.13 In this experiment, the male and female data from Fig 16.11 is averaged together for Fig 16.13b Data for flies within day of eclosion was taken for Fig 16.13a This data however represents only half the sample size as that of 16.13b A typical pressure transient is the same as the previous experiment A drastic decrease in survival is observed for the older flies As a model microfabrication process, thermal evaporation was performed on adult Drosophila Thermal evaporation is a physical vapor deposition process in which materials are blanket coated typically as thin films onto flat substrates Pure metal sources are resistively heated on a filament under vacuum such that they evaporate causing their atoms to travel freely © Woodhead Publishing Limited, 2012 Hybrid MEMS 465 Percentage survived (a) 100 90 80 70 60 50 40 30 20 10 0 Time away from food (hr) 25 30 Pressure (m Torr) (b) 200 175 150 125 100 75 50 25 10 15 20 Time (min) 16.10 (a) Percentage of adult Drosophila that survived after 30 under pressure versus time away from food Each point represents survival of a sample of 10 flies with four points total for each time increment The line represents the best linear fit across the data (b) Typical chamber pressure transient curve for experiment (Source: Reprinted with permission from Shum and Parviz, 2007b.) in a highly directional path toward the target substrate Process variables include pressure, temperature, and distance between the source and target The pressure and temperature of a process is tightly correlated with the material to be deposited In general, lower pressure parameters enable the ability to evaporate at lower power and therefore lower temperature with a shorter distance between source and target Additionally, for evaporation onto fly surfaces, a container that could both restrain the flies and expose them to incoming atoms was necessary We initially attempted evaporation of gold that is useful as both a reactive surface for self-assembled monolayers and as a conductor However, evaporation following the conventional recipe at µTorr did not yield any live flies A control test using 20 flies in the evaporation chamber that was pumped down to 12 µTorr over 14 and then vented over yielded only one live fly Repeating evaporation with pressure at 100 mTorr also did not yield live, coated flies Target temperature of a nominal gold evaporation © Woodhead Publishing Limited, 2012 MEMS for biomedical applications Percentage survived (a) 100 90 80 70 60 50 40 30 20 10 0 30 60 90 120 150 180 Time (min) Percentage survived (b) 100 90 80 70 60 50 40 30 20 10 0 30 60 90 120 150 180 Time (min) 16.11 Percentage of adult Drosophila that survived versus time under pressure by gender Each point is the average of four samples of 10 flies Error bars indicate range of data taken All data was taken with flies that had eclosed days prior (a) Male fly data (b) Female fly data (Source: Reprinted with permission from Shum and Parviz, 2007b.) Pressure (m Torr) 466 200 180 160 140 120 100 80 60 40 20 0 10 20 30 40 50 60 Time (min) 16.12 Typical chamber pressure transient curve for experiments in Figs 16.11 and 16.13 Pressure is stabilized at mTorr by adjusting valve between pump and vacuum chamber (Source: Reprinted with permission from Shum and Parviz, 2007b.) © Woodhead Publishing Limited, 2012 Hybrid MEMS 467 Percentage survived (a) 100 90 80 70 60 50 40 30 20 10 0 10 20 30 40 50 60 70 80 90 100 60 70 80 90 100 Time (min) Percentage survived (b) 100 90 80 70 60 50 40 30 20 10 0 10 20 30 40 50 Time (min) 16.13 Percentage of adult Drosophila that survived versus time under pressure by age Error bars indicate range of data taken (a) Each point is the average of four samples of 10 flies on the day of eclosion Two samples are male and two samples are female (b) Each point is the average of eight samples of 10 flies days after eclosion Four samples are male and four samples are female (Source: Reprinted with permission from Shum and Parviz, 2007b.) was later measured with a thermocouple to be 48 °C This high temperature was the likely cause of fly fatality This was confirmed in a separate control experiment In the control experiment, we observed that 100% of a set of 10 flies died after only a few minutes in an oven set at 44 °C To address the Drosophila sensitivity to temperature and pressure, we decided to use indium as a source Indium can be evaporated at pressures as high as 47 mTorr with target temperature below 30 °C Our three step process lasted approximately 30 First, the chamber was pumped down with a mechanical pump to mTorr in about 10 Then indium was evaporated by applying power for approximately Finally, the chamber was vented over the next 10 to 15 Flies were unloaded from the evaporation container into a Petri dish and monitored for survivors Figure 16.14b shows a fly live after evaporation of indium Evaporation was done with 2800 W (140 A, 20 V) with minimum pressure of 560 àTorr and deposition â Woodhead Publishing Limited, 2012 468 MEMS for biomedical applications (a) (b) 16.14 (a) Optical microscope image of a normal fly (provided for comparison), (b) Photograph of a viable fly after the completion of indium evaporation The flies were etherized for taking the pictures The metallic tint of the indium thin film is clearly visible on the fly in (b) (Source: Reprinted with permission from Shum and Parviz, 2007b.) rate between and Å/s to yield a final thickness of 930 Å Coated flies appeared darker and more reflective than uncoated ones The above experiments verified our ability to vacuum deposit metal on live flies To achieve a patterned evaporation on a fruit fly, we devised a shadow masking process (Fig 16.15) A specialized fixture was machined consisting of a in inner diameter tube and a plunger with an adjustment screw to control its vertical position in the tube A template was constructed out of silicon to which flies would self assemble First an array of trapezoids was created by dry etching through a in diameter silicon wafer The wafer was then mounted on a second silicon wafer with wafer fragments as spacers to create deep recesses that each fit only one fly The template was then attached to the plunger A shadow mask was taped to the opening of the tube With the plunger fully removed, flies can be transferred to the container by first immobilizing them with carbon dioxide The plunger was then replaced, and slowly translated into the tube until flies were in close contact with the shadow mask The flies were then allowed to regain mobility As the plunger was brought closer to the shadow mask, flies would preferentially occupy template recesses providing control of fly positioning during evaporation The shadow mask can be made with an arbitrary pattern using SU-8, a negative photoresist The fabrication process flow is depicted in Fig 16.16a To fabricate the thin polymer mesh, we spin coat two polymer layers on a in diameter silicon wafer The first layer acts as a sacrificial layer and the second photopatternable layer as the structural material for the membrane The sacrificial bottom layer is AZ1512 positive novolac resist that is soluble in acetone The top photoresist, SU-8 2025, is approximately 25 µm and was insoluble in acetone The photoresist is then exposed with an UV source © Woodhead Publishing Limited, 2012 Hybrid MEMS 469 (a) Polymer membrane Silicon wafer with recesses (b) Metal evaporation through the patterned membrane onto flies in recesses Si wafer moved (c) Flies with metal patterns 16.15 (a–c) Schematic of self-assembly of the fly-array and metal evaporation on live flies in vacuum (Source: Copyright Wiley-VCH Verlag GmbH & Co KgaA Reproduced with permission from Shum et al., 2007a.) through a mask with the desired pattern Gradual temperature ramping during soft and post exposure bakes for SU-8 reduces stress buildup in the film and leads to good film quality for the final polymer membrane The SU-8 film is developed by submerging the wafer in SU-8 Developer and then rinsing with isopropyl alcohol In the final step, the SU-8 membrane is released from the silicon substrate by dissolving the positive photoresist with acetone The procedure yields a thin polymer mesh with a pattern of openings defined via photolithography Images of three patterns are shown in Fig 16.16b–d The silicon self-assembly template is fabricated primarily by a through wafer etch with deep reactive ion etching (DRIE) The fabrication process flow is depicted in Fig 16.17a A thick layer of AZ4620 resist (between 20 and 40 µm) is patterned and used as an etch mask DRIE was performed using the standard Bosch process at a rate of 2.9 àm/min until the patterns were etched â Woodhead Publishing Limited, 2012 470 MEMS for biomedical applications (a) (b) Deposit AZ 1512 Deposit SU-8 2025 (c) Photopattern SU-8 Release SU-8 (d) SU-8 2025 AZ 1512 Silicon 16.16 (a) Fabrication process flow, (b) SEM image of 50 μm cross pattern, (c) SEM image of 20 μm cross pattern, (d) optical microscope image of 50 μm circle pattern (Source: Reprinted with permission from Shum and Parviz, 2007b.) all the way through the wafer The photoresist mask is thoroughly removed with acetone and photoresist stripper Figure 16.17b shows an SEM image of a section of a wafer after DRIE Because the fly dimensions required the recesses to be 700–800 µm deep, we assembled the final template by gluing the etched wafer to a bare inch silicon wafer using small wafer fragments as spacers The template was then loaded onto the plunger Figure 16.17c shows a fly self-assembled into a silicon recess beneath the SU-8 shadow mask Evaporation of indium was performed using each of the three shadow mask patterns The evaporation process yielded a overall survival rate of approximately 30% Since flies were compressed, only dorsal and ventral surfaces were typically exposed An example of a fly, live after evaporation, is given in Fig 16.18a During this evaporation 25 out of 72 flies survived and thickness of the deposited indium was measured as 823 Å Thirteen of the 25 surviving flies contained patterns In general, patterns on the transparent wings were the most noticeable A collection of images of the different patterns on live flies are shown in detail in Fig 16.18b–d Critical steps toward building a hybrid manufacturing approach that allows the parallel modification of these organisms using solid-state © Woodhead Publishing Limited, 2012 Hybrid MEMS (a) 471 (b) Deposit AZ 4620 Photopattern AZ 4620 DRIE Etch (c) Strip AZ 4620 AZ 4620 silicon 16.17 (a) Silicon template fabrication process flow, (b) SEM image of silicon template, (c) optical microscope image of fly positioned in trapezoidal recess beneath SU-8 shadow mask (Source: Reprinted with permission from Shum and Parviz, 2007b.) microfabrication processes have been completed Characterization of fly survival rate in vacuum during various stages of development has elucidated when best to subject flies to these processes These experiments verify that fruit flies can survive under moderate vacuum conditions for an extended period of time opening up the possibility of developing hybrid manufacturing processes directly on them Ultimately, the first direct patterned vacuum metal deposition on a live organism was demonstrated The metal deposition process may be used later to form interconnects or binding sites to which micron-scale objects such as flight control circuitry, sensors, or wireless telecommunications components are attached Future work on hybrid fabrication with insects includes creating a library of microfabrication processes, for example plasma etching, that can be performed on live fruit flies Improving the self-assembly yield will allow the process to be extended to the controllable modification of thousands of fruit flies at once 16.4 Conclusions and future trends Integration of live organisms with microfabrication creates a new paradigm for system designers Nature’s organisms offer unique qualities that © Woodhead Publishing Limited, 2012 472 MEMS for biomedical applications (a) 600 μm (b) (c) (d) 16.18 Optical microscope images of various flies live after evaporation Flies were etherized before capturing images (a) Whole fly with 50 μm cross pattern, Wings patterned with (b) 50 μm cross pattern, (c) 20 μm cross pattern, (d) 50 μm circle pattern (Source: Reprinted with permission from Shum and Parviz, 2007b.) either cannot be replicated at all or with sufficient performance by synthetic approaches Rather than mimicking these qualities, hybrid systems aim to harness and use them directly In the above, relevant demonstrations of how organisms are integrated into such systems were presented Bacteria were incorporated into fuel cells and used to convert chemical energy into electrical energy Roundworms elucidated neurological pathways by being observed on chip in highly reproducible microfluidic channels Hawkmoth wing actuation was controlled by implants that stimulate flight muscles Fruit flies were modified directly and in a parallel fashion using microfabrication In each example, key integration issues were discussed The maintenance of the organism’s livelihood and its subsequent ability to perform its intended function during system operation is the primary concern Characterizing the limitations of the organism to different thermal, chemical, and social environments and determining when in the life-cycle to intervene are also crucial for successful integration As well, the system designer must take into account the manner in which the integration process itself will affect the organism If the integration requires manipulating the organism, the risk of any resulting level of detriment should be evaluated along with technique refinements that would optimize the impact to the organism For example, in some cases, integration may work better if the process is performed quickly, but in others, it may be advantageous to perform the process gradually © Woodhead Publishing Limited, 2012 Hybrid MEMS 473 The integration of organisms with microsystems is a concept that offers great potential The inherent scalability and reproducibility microfabrication processes offer can be used to accelerate the study of microorganisms many times over But perhaps even more important, is the knowledge that nature offers countless species of organisms with a myriad of useful qualities By tapping into this natural toolbox, microsystem design can quickly achieve the high level of complexity needed for complicated mechanisms such as flight 16.5 References Bachand G D, Hess H, Ratna B, Satir P and Vogel V (2009), ‘Smart dust biosensors powered by biomolecular motors’, Lab on a Chip, 9, 1661–1666 Bernstein R W, Zhang X J, Zappe S, Fish M, Scott M and Solgaard O (2004), ‘Characterization of fluidic microassembly for immobilization and positioning of Drosophila embryos in 2-D arrays’, Sensors and Actuators 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Bioimpedance before MEMS: patch clamp measurements MEMS in bioimpedance applications Future trends Sources of further information and advice References MEMS ultrasonic transducers for biomedical applications. .. The last decade has seen a significant maturation of MEMS for biomedical applications, as biomedical applications provide the perfect platform for integrating complex design and fabrication of articulation,... tissue formation in in vivo applications and high-throughput systems for biomedical applications (polymerase chain reactions (PCRs) and DNA probes to drug discovery platforms) As MEMS applications