Annals of Biomedical Engineering, Vol. 39, No. 2, February 2011 (Ó 2010) pp. 600–620 DOI: 10.1007/s10439-010-0218-9 From Cleanroom to Desktop: Emerging Micro-Nanofabrication Technology for Biomedical Applications 1 2,3 TINGRUI PAN and WEI WANG 1Micro-Nano Innovations (MiNI) Laboratory, Department of Biomedical Engineering, University of California, Davis, CA, USA; 2Institute of Microelectronics, Peking University, Beijing 100871, China; and 3National Key Laboratory of Science and Technology on Micro/Nano Fabrication, Beijing 100871, China (Received 25 August 2010; accepted 20 November 2010; published online 14 December 2010) Associate Editor Scott I. Simon oversaw the review of this article. Abstract—This review is motivated by the growing demand reactions in a high-throughput fashion, to provide for low-cost, easy-to-use, compact-size yet powerful micro- parallel multiplexed functionality, as well as to reduce nanofabrication technology to address emerging challenges consumption of expensive reagents. Micro-nanofabri- of fundamental biology and translational medicine in regular laboratory settings. Recent advancements in the field benefit cation technology has played a central role in such an considerably from rapidly expanding material selections, implementation. Benefiting from the rapid-expanding ranging from inorganics to organics and from nanoparticles microelectronic industry, the cleanroom-based micro- to self-assembled molecules. Meanwhile a great number of nanofabrication has evolved at a remarkable pace as novel methodologies, employing off-the-shelf consumer elec- predicted by Moore’s law.132 The current industrial tronics, intriguing interfacial phenomena, bottom-up self- assembly principles, etc., have been implemented to transit standard ensures highly reliable processing to produce micro-nanofabrication from a cleanroom environment to a trillions of basic electrical elements—transistors—with desktop setup. Furthermore, the latest application of micro- lithographic resolution of tens of nanometers within nanofabrication to emerging biomedical research will be one square inch area. Using the established micro- presented in detail, which includes point-of-care diagnostics, nanofabrication techniques, microchips for various on-chip cell culture as well as bio-manipulation. While significant progresses have been made in the rapidly growing biological and clinical applications have been success- field, both apparent and unrevealed roadblocks will need to fully demonstrated in the recent years, including be addressed in the future. We conclude this review by implantable neural probes,202 microfluidics-based offering our perspectives on the current technical challenges biological systems,21 physiological pressure sensors,4,97 and future research opportunities. and cochlear and retinal implants.69,203 One major advantage of using the conventional cleanroom-based Keywords—Biomedical engineering, Lithography, Microfab- fabrication technology is the capacity to directly rication, Nanofabrication, Out-of-cleanroom. integrate the biochips with powerful electronic pro- cessing units underneath, which eliminates extensive electrical wiring and further reduces overall system INTRODUCTION dimensions. However, insurmountable obstacles are Development of micro-nanoscale devices and sys- presented when applying the conventional techniques tems has been one of the most noticeable trends in to fast-growing biological and clinical applications. many areas of biomedical research over the past dec- These include the limited access to cleanroom facili- ties, incompatible chemical and thermal treatments, ades. The momentum toward building smaller instru- ments and more compact analytical systems is driven complicated and inflexible process flows, and by the ability to offer high-precision assessment of restricted material selection options, in addition to minute biological components (e.g., single cells, DNA high-maintenance and operation costs of fabrication strands, and viral particles), to facilitate biochemical equipment. A group of emerging lithography-based techniques, such as soft lithography, have partially addressed the Address correspondence to Tingrui Pan, Micro-Nano Innovations (MiNI) Laboratory, Department of Biomedical Engi- aforementioned concerns by reducing the micro- neering, University of California, Davis, CA, USA. Electronic mail: nanofabrication complexities while offering flexible [email protected] processing schemes and material selections. Since the 600 0090-6964/11/0200-0600/0 Ó 2010 The Author(s). This article is published with open access at Springerlink.com From Cleanroom to Desktop 601 original introduction, soft lithography techniques have number of new functional materials have been intro- been increasingly explored in a wide range of biomed- duced to micro-nanofabrication society. In addition to ical applications, including integrated cell sorters,54 the conventional silicon-based materials, those new in situ cellular biomechanical analysis,177 stationary/ materials, as listed in Table 1, facilitate the biomedical dynamic bio-molecular gradients,87 and on-chip geno- research in an easy, fast, low-cost, multifunctional, mic analysis.152 Recent trends lead to exploiting the and more importantly, out-of-cleanroom manner.155 powerful performance of high-precision consumer For instance, flexible polymeric materials have been electronics, intriguing interfacial chemical/physical extended to fabricate and/or functionalize biological phenomena, or self-assembled micro-nanobuilding micro-nanosystems.115 Nanoscale building elements, blocks to further extend micro-nanofabrication capac- such as nanowires and nanoparticles, are widely used ity to an out-of-cleanroom laboratory environment. to provide specific biological cues as well as to form For instance, off-the-shelf printers and digital projec- scaffolds for tissue engineering.167,212 Moreover, tors have been extensively employed as inexpensive and emerging environmentally responsive materials can rapid micropattern generators in place of conventional find their applications in biosensing and bio-manipu- photomask exposure systems.191,199,214 Micro-nano- lation platforms.127 scopic chemical and physical phenomena at interfaces, such as elastic deformation (wrinkling and collapsing), Thermoset Polymers have been frequently used to fabricate specific micro-nanopatterns.27,146 Furthermore, the bottom-up Thermoset polymers refer to polymers that irre- self assembly techniques can be of particular use to versibly cure through a thermal, chemical, or photo- construct integrated functional micro-nanosystems for chemical reaction. With fluidic properties and one-step biomedical application.125,147 In addition, the rapid curing, thermoset polymers have been frequently used development of novel functional materials, ranging as inexpensive and durable masters for hot emboss- from inorganics to organics and from nanoparticles to ing99 and replica molding.52 In particular, as the most self-assembled molecules, offers emerging alternatives commonly used thermoset polymer in micro-nano- to conventional material processing.115,127,155,167,212 fabrication, polydimethylsiloxane (PDMS) can be Overall, the latest research activities in micro-nano- processed by mixing two components, a base and a fabrication enlightens this consistent evolution from curing agent, at a given weight ratio (e.g., 10:1) and the conventional cleanroom-based techniques toward cross-linked at a room or slightly elevated temperature. easy-to-use, low-cost, compact-size, rapid-prototyping The uncured pre-polymer of PDMS is fluidic and can and out-of-cleanroom processing for specific applica- flow into micro/nanostructures, which makes it an tions (e.g., biologically oriented use), which follows the excellent material option for the molding process. very similar trend to microelectronics and internet Combining excellent mechanical (e.g., elastic and booming over the past decades. flexible), optical (e.g., transparent) and biological (e.g., In this review, we will highlight the recent achieve- biocompatible and non-toxic) properties, together with ments in the rapid-expanding direction with a focus on its low price, PDMS is a popular material selection for out-of-cleanroom techniques for biological and medical biomedical microdevices.6,49,53,126 Besides its dominant applications, followed by featuring a few promising use for microfluidics and bio-patterning,201 the elas- examples from active research, including point-of-care ticity of PDMS has been particularly explored in bio- diagnostics, integrated cell culture as well as micro- mechanical applications. Chen and coworkers have first nanoscopic bio-manipulation. Although tremendous utilized a PDMS substrate decorated with a densely efforts have been made in such an implementation with packed micropost array to investigate cellular adhesion appreciable advantages over the conventional coun- mechanics,177 on which different levels of bending of terparts (e.g., low cost, fast turnout, custom configu- posts indicate the localized stress experienced by indi- rability, easy operation and maintenance, etc.), current vidual cells. More recently, a similar system has been technical challenges with potential solutions and adopted to quantitatively measure mechanical forces at opportunities (e.g., feature resolution, process reliabil- cell–matrix and cell–cell contacts by the same group.116 ity, and system integration) will be further discussed. Furthermore, molecular structure of PDMS