BIOCHIPS

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BioChips

By CHONG H. AHN, JIN-WOO CHOI, GREGORY BEAUCAGE, JOSEPH H. NEVIN,

I. INTRODUCTION

The development of lab-on-a-chip devices for biochemical analysis has seen an explosive growth over the past decadeInitial research in this area focused on developing the concepts of micro total analysis systems ( TAS), a parallel term to “lab on a chip,” and has rapidly evolved to applications in a number of biochemical analysis operations such as clinical analysis (blood gas analysis, glucose/lactate analysis, etc.), DNA analysis (including nucleic acid sequence analysis), proteomics analysis (proteins and peptides), combinatorial synthesis/analysis, immunoassays, toxicity monitoring, and even forensic analysis applications . A significant application area for this technology is clinical diagnostics.

Specifically for clinical diagnostics, diseases, including toxicity, can be diagnosed by performing various biochemical analyzes and by observation of symptoms. The early, rapid, and sensitive detection of the disease state is a vital goal for clinical diagnoses. The biochemical changes in the patient’s blood can signal organ damage or dysfunction prior to observable microscopic cellular damages or other symptoms. So there has been a large demand for the development of an easy-to-handle and inexpensive clinical diagnostic biochip using fully integrated plastic microfluidic chips, which has the sampling/identifying capability of fast and reliable measurements of metabolic parameters from a human bod with minimum invasion.

Most clinical diagnostics applications have focused on the detection of nucleotides and peptides that serve as early
indicators of disease. For instance, Dinh et al. describe a multifunctional biochip with nucleic acid and antibody probe receptors specific to the gene fragments of Bacillus anthracis and Escherichia coli, respectively . The detection of specific diseases or biological warfar agents is possible by incorporating biomarkers specific to such agents. Clinical diagnostic applications also include monitoring of regular metabolic parameters such as glucose
and lactate as demonstrated by the I-Stat analyzer.

The handheld analyzer provides point-of-care testing for monitoring a variety of clinically relevant parameters.
Immunosensing applications as a part of clinical diagnostics have also been demonstrated.

The range of applications for lab-on-a-chip systems is increasingly rapidly as more and more researchers become aware of the significant benefits of this technology. Most of these advantages are derived from the small sample and
reagent volume utilized in these systems. The advantages include low sample/reagent volume, rapid analysis times, less sample wastage, cost effectiveness (for sample usage), and possibility of developing disposable devices to name a few. Though the analyte can be manipulated in various physical forms such as liquid stream [1], liquid
droplets , and gaseous phase , microfluidic biochemical analysis (using liquid streams) have been the primary focus
of research efforts. One of the more significant challenges I developing a microfluidic biochemical analysis system has been the development of reliable microfluidic manipulation techniques. Various researchers have explored active control devices such as microvalves and micropumps for fluidic flow control . However, active microfluidic control has certain inherent disadvantages such as high cost, difficulty in integration, complex fabrication/assembly, and complex control circuitry.

Increasingly researchers are shifting their attention toward the use of passive microfluidic structures to regulate
microfluidic sequencing. A large number and variety of passive microfluidic devices have been successfully demonstrated
including passive valves, mixers , diffusion-based extractors , passive filters and membranes , and also a few passive actuation schemes . Passive microfluidic devices (or systems) offer some advantages specifically for biochemical analysis systems such as no external power requirement (for device operation), ease of integration, continuity in substrate material, rapid prototyping, low cost, and possibility of use without active control. Some of the challenges facing passive microfluidic devices/systems are that passive microfluidic systems are very application specific; they cannot be easily reconfigured; and they are strongly dependent on variances in the fabrication process and are not suitable for a wide range of fluidic mediums. Despite these challenges, the advantages of the passive control approach make it a viable approach for developing microfluidic platforms for biochemical analysis.
One of the most critical decisions for a TAS platform is the choice of substrate material. Most microfluidic
biochemical analysis systems have been fabricated using silicon (Si) or glass as substrates for the microfluidic
motherboards. There is considerable effort toward exploring substrates other than Si or glass, primarily toward
plastic/polymer-based motherboards . Plastic substrates, such as polyimide, polymethamethylacrylate
(PMMA), poly(dimethylsiloxane) (PDMS), polyethylene, or polycarbonate, offer a wide range of physical and chemical material parameters for the applications of biofluidic chips
generally at low cost using replication approaches. Polymers offer numerous advantages like low cost, rugged construction, ease of fabrication, and rapid prototyping. A significant advantage of using polymer substrates is the wide variety of surface properties that they offer. The surface properties of polymers can be readily modified to meet the fluidic and/or biocompatibility requirements of the biochemical analysis system . Also, polymer processing is a mature, established science and TAS researchers can readily exploit the significant data bank of the polymer experts to create multifunctional, low-cost, disposable microfluidic modules.
In this paper, the development of disposable smart plastic fluidic biochips for clinical diagnostics is reviewed. Fig. 1
shows a schematic sketch of the disposable lab on a chip with wristwatch-sized analyzer. The plastic fluidic chip includes a smart passive microfluidic manipulation system based on the structurally programmable microfluidic system (sPROMs) technology, allowing for preprogrammed sets of microfluidic sequencing with only an on-chip pressure source. The integration of the air-bursting detonator allows us to utilize a simple alternative fluid-driving source, thus eliminating costly, nondisposable active microfluidic pumps. The biochip also contains an integrated biosensor array for simultaneous detection of multiple clinically relevant parameters.
Thus, the disposable smart plastic biochip is compose of fully integrated modules of plastic fluidic chips for fluid
driving, sequencing, and biochemical sensors. The biochip is inserted into the analyzer unit where the microfluidic sequencing is initiated by a trigger signal from the electronic controller. After the sample solution (blood) is delivered to the biosensor array, the electrochemical detection circuitry on the analyzer is used to determine the concentrations of the various analytes. As a demonstration vehicle, the biochip has the specific goal to detect and identify three metabolic parameters: PO (partial pressure of oxygen), lactate, and
glucose from blood.

II. DISPOSABLE SMART LAB ON A CHIP

The development of a disposable, smart lab on a chip (or biochip) requires considerable research effort toward developing a clear understanding of the various components of the biochip. These include the microfluidic system, the biosensor arrays, and the fabrication techniques required to implement these in a feasible and economically viable fashion. The use of stand-alone TAS devices for remote and/or portable systems requires the development of a microfluidic control modality that can function reliably with minimal control signals. We have concentrated on the development of the sPROMs technology that offers the following features— passive fluidic manipulation, low-volume handling capability, low actuation power, low cost, disposability, robustness, and little or no feedback control. Furthermore, sPROMs devices with no moving parts are inherently more rugged and less fault prone. Since sPROMs-based devices can be readily implemented on low-cost plastic substrates, this technology is highly suitable for disposable microfluidi platform applications. Finally, this technology allows us to manipulate ultrasmall volumes [typically in the nanoliter (nL) to microliter L range] of fluids, thus exploiting maximum advantage of the TAS concept. We have extensively
developed the sPROMs technology to realize devices such as microfluidic multiplexers with integrated microdispenser that allows the transfer of precise aliquots of fluid from a single input to multiple outputs in a programmed sequence. This arrangement can then be extended to handle minute samples of multiple fluids simultaneously as a first step toward realizing a complete biochemical analysis system on chip. Fig. 2 shows an assembled biochip with the sPROMsbased multiplexer and integrated dispenser. A detailed operation
of this device is presented later in the paper.
Another critical element of the biochip is the on-chip air-bursting detonator. The air-bursting detonator uses pressurized gas, which is compressed and stored in a chamber capped with a thin membrane. The membrane has a heater lithographically defined to serve as the detonator. When a brief pulse of electrical energy is sent to the microheater, the heater temperature rises rapidly and melts the membrane. As soon as the membrane is broken the pressurized gas rushes out, pushing the fluid samples into the microchannel through the ruptured membrane. Low power consumption is guaranteed, since only pulsed power is used to burst the pressurized gas. This eliminates the use of complex micropumps as well as bulky batteries required to power the pumps. The use of a smart passive microfluidic control system with an on-chip power source allows for the development of fully integrated, yet low-cost disposable biochips. The following sections present detailed information about
various aspects of the biochip design, fabrication, and characterization.