1
Microfluidics in Nanomedicine

YongTae Kim1 and Robert Langer2

1Georgia Institute of Technology, George W. Woodruff School of Mechanical Engineering, Wallace H. Coulter Department of Biomedical Engineering, Institute for Electronics and Nanotechnology, Parker H. Petit Institute for Bioengineering and Bioscience, 345 Ferst Drive, Atlanta, GA 30318, USA

2Massachusetts Institute of Technology, Department of Chemical Engineering, Harvard-MIT Division of Health Sciences and Technology, David H. Koch Institute for Integrative Cancer Research, 500 Main Street, Cambridge,MA 02139, USA

1. 1 Introduction
2. 1.1 Nanomedicine Development
3. 1.2 Microfluidics Technology
4. 2 Microfluidic Assembly of Nanomedicines
5. 3 Microfluidic Characterization of Nanomedicines
6. 4 Microfluidic Evaluation of Nanomedicines
7. 4.1 Mimicking Physiological Environments
8. 4.2 Endothelial Cell Systems
9. 4.3 "Organ-On-A-Chip"
10. 4.4 Renal Toxicity and Hepatotoxicity
11. 4.5 Live Tissue Explants
12. 4.6 Intact Organisms
13. 5 Challenges and Opportunities
14. 6 Concluding Remarks
15. Acknowledgments
16. References

Keywords

Microfluidics

The science and technology that involves the manipulation of nanoscale amounts of fluids in microscale fluidic channels for applications that include chemical synthesis, and biological analysis and engineering.

Nanotechnology

The manipulation of matter on atomic and molecular scales.

Nanomedicine

The medical application of nanotechnology for the advanced diagnosis, treatment and prevention of a number of diseases.

Biomimetic microsystem

A microscale device that mimics biological systems and is used to probe complex human problems.

Clinical translation

Clinical translation involves the application of discoveries made in the laboratory to diagnostic tools, medicines, procedures, policies and education, in order to improve the health of individuals and the community.

Nanomedicine is the medical application of nanotechnology for the treatment and prevention of major ailments, including cancer and cardiovascular diseases. Despite the progress and potential of nanomedicines, many such materials fail to reach clinical trials due to critical challenges that include poor reproducibility in high-volume production that have led to failure in animal studies and clinical trials. Recent approaches using microfluidic technology have provided emerging platforms with great potential to accelerate the clinical translation of nanomedicine. Microfluidic technologies for nanomedicine development are reviewed in this chapter, together with a detailed discussion of microfluidic assembly, characterization and evaluation of nanomedicine, and a description of current challenges and future prospects.

1
Introduction

Nanomedicine4 is the medical application of nanotechnology that uses engineered nanomaterials for the robust delivery of therapeutic and diagnostic agents in the advanced treatment of many diseases, including cancer [1-3], atherosclerosis [4-6], diabetes [7-9], pulmonary diseases [10 11] and disorders of the central nervous system [12 13]. One key advantage of nanomedicine is the ability to deliver poorly water-soluble drugs [14-16] or plasma-sensitive nucleic acids (e.g., small interfering (si)RNA [17 18]) into the circulation with enhanced stability. Nanomedicine is also capable of providing contrast agents for different imaging modalities and the targeting of specific sites for the delivery of drugs and/or genes [19-23]. Engineered nanomaterials, developed as particulates that are widely referred to as nanoparticles (NPs), have been formulated using a variety of materials that includes lipids, polymers, inorganic nanocrystals, carbon nanotubes, proteins, and DNA origami [24-36]. The ultimate goal of nanomedicine is to achieve a robust, targeted delivery of complex assemblies that contain sufficient amounts of multiple therapeutic and diagnostic agents for highly localized drug release, but with no adverse side effects [37 38], and a reliable detection of any site-specific therapeutic response [39 40].

1.1
Nanomedicine Development

Typical nanomedicine development processes for the clinical translation include benchtop syntheses, characterizations, in-vitro evaluations, in-vivo evaluations with animal models, and scaled-up production in readiness for clinical trials. Although, previously, several NPs have been reported as superior platforms, many are still far from their first stages of patient clinical trials due to several critical challenges [41 42]. Such challenges mainly result from batch-to-batch variations of NPs produced in the benchtop synthesis process, and from insignificant outcomes in the in-vitro evaluation process under physiologically irrelevant conditions. These limitations ultimately lead to highly variable results in the in-vivo evaluation, or to failure in clinical trials. In order to address these challenges, the following methodologies need to be established in the nanomedicine development process:

• Nanomedicine needs to be continuously produced in a high-throughput fashion. The large-scale, continuous production of nanomedicines will allow a robust supply of highly reproducible materials for the in-vitro and in-vivo evaluation stages and clinical trials, ultimately increasing the success rate in clinical trials.
• Nanomedicines synthesized using large-scale, continuous production methods also need to be characterized in a high-throughput manner. Rapid characterization will create an efficient production cycle for an optimized nanomedicine via feedback loops between the synthesis and characterization stages.
• The in-vitro evaluation of nanomedicine must be conducted in more physiologically relevant environments. Highly repeatable results obtained from these biomimetic conditions will allow the obviation of a number of simple screening experiments in animal studies, not only saving costly animal models but also accelerating the clinical translation.

1.2
Microfluidics Technology

Microfluidics technology provides highly compatible platforms to create a new nanomedicine development pipelines that include the required methodologies introduced above. Basically, microfluidics presents a number of useful capabilities to manipulate very small quantities of samples, and to detect substances with a high resolution for a wide range of applications, including chemical syntheses [43 44] and biological analysis [45 46]. More importantly, the adaptability of microfluidics allows its integration with many other technologies, such as micro/nanofabrication, electronics, and feedback control systems [47-52]. Recently, microfluidic platforms integrated with control systems and advanced microfabrication technologies have been used to address the critical challenges in nanomedicine [53-57]. For example, the continuous synthesis of NPs in microfluidics has demonstrated a versatility to produce a variety of NPs with different sizes, shapes, and surface compositions [58 59]. Several advances have recently been made in the label-free detection, characterization and identification of single NPs [60]. The confluence of microfluidics and biomimetic design has enabled the creation of physiologically relevant microenvironments for the evaluation of drug candidates [61-63]. The key microfluidic technologies in nanomedicine, including microfluidic assembly, and the characterization and evaluation of nanomedicines, are discussed in the following sections (see Fig. 1), and their current challenges and future research directions are highlighted.

2
Microfluidic Assembly of Nanomedicines

The bulk synthesis of NPs typically has strong dependencies on nonstandard multistep processes which are time-consuming, difficult to scale up, and depend heavily on specific synthetic conditions in the laboratory. This reliance of NPs on such nonstandard multistep processes inevitably causes high batch-to-batch variations in their physico-chemical properties [64-69]. Batch size is also subject to custom protocols that vary among laboratories, leading to difficulties in screening and identifying optimal NP physico-chemical characteristics for enhanced drug delivery. Furthermore, the introduction and combination of multiple materials for creating multicomponent NPs compromises the expected functionality of the individual elements. This is largely because of an inability to precisely control the continuous assembly process in various conventional bulk syntheses that involve the macroscopic mixing of precursor solutions [58 70]. As the micrometer- and nanometer-scale interactions of precursors will direct the characteristics of NPs, it is essential that their composition is fine-tuned in order to attain the...