Chapter 9 – Emerging Biophysics Techniques  419

One of the most promising emerging next-​generation sequencing technologies is single-​

cell sequencing, which we discussed briefly in Chapter 7. Here, advanced microfluidics can

be used to isolate a single cell on the basis of some biophysical metric. This metric is often, but

not exclusively, fluorescence output from the labeling of one or more specific biomolecules in

that cell, in much the same way as cells in standard fluorescence-​assisted cell sorting (FACS)

are isolated (see Chapter 3). The key differences here, however, are sensitivity and throughput.

Single-​cell sequencing demands a greater level of detection sensitivity to detect sometimes

relatively small differences between individual cells, and the method is therefore intrinsically

lower throughput than standard FACS since the process involves probing the biophysical

metric of individual cells computationally more intensively.

For example, an individual cell may potentially be probed using single-​molecule precise

fluorescence imaging to infer the copy number of a specific fluorescently labeled biomolecule

in that cell using a step-​wise photobleaching of the fluorescent dye (see Chapter 8). That

one cell can then be isolated from the rest of the population using advanced microfluidics,

for example, using piezo microvalves or potentially even optical tweezers (see Chapter 6) to

shunt the cell into a separate region of the smart flow cell. At this stage, the cell could, in prin­

ciple, then be grown to form a clonal culture and subjected to standard bulk level sequencing

technologies; however, the issue here is that such a cell population is never entirely “clonal”

since there are inevitably spontaneous genetic mutations that occur at every cell division.

A more definitive approach is to isolate the DNA of the one single cell and then amplify

this using PCR (see Chapter 7). However, the mass of DNA from even a relatively large cell

such as a human cell is just a few picograms (i.e., pg, or 10⁻¹² g), which is at the very low end

of copying accuracy for PCR, and so DNA replication errors during PCR amplification are

much more likely with current technology available. Even so, single-​cell sequencing offers

genuine potential to bridge the phenotype to genotype gap. The real goal here in terms of

personalized healthcare is to develop methods of very early-​stage diagnosis of diseases and

genetic disorders on the basis of detecting just a single cell from an individual patient sample.

Lower technology biophysics solutions to personalized diagnostics are also emerging and

are especially appealing due to their low cost but high potential gain. For example, a simple

card-​based origami optical microscope has been developed by researchers at the UC Berkeley

called the Foldscope (Cybulski et al., 2014) that can be assembled from a few simple folds of

card, using just one cheap spherical microlens and an LED, as a light source produces images

of sufficient quality to identify a range of different microbial pathogens up to a magnification

of ~2000. But it weighs just 8 g and costs only ~$1 to make. A motivation for this cheap and

low-​tech device is to enable earlier diagnosis of microbial infection of patients in developing

countries that may have no rapid access to microbial laboratory facilities.

9.4.2  NANOMEDICINE

The use of bionanotechnology applied to medicine is already emerging at the level of targeted

drug binding, for example, to develop pharmaceutical treatments that destroy specific dis­

eased cells such as those of cancers through the use of specific binding. These include radio­

active nanoparticles coated with specific antibody probes to act as “killer” probes. Specific

aptamers are used (see Chapter 7) to block key processes in specific diseases. Aptamers

have an important advantage over molecular recognition technologies in evoking a minimal

immune response, unlike the closest competing technology of antibody–​antigen binding

for which many antibodies evoke strong immunogenic reactions at relatively small doses.

Targeted binding can also be valuable for the visualization of diseased tissue (e.g., antibody-​

tagged QDs can specifically bind to tumors and assist in the discrimination between healthy

and nonhealthy cellular material).

Bionanotechnology is also being applied to assist in greater personalization of targeted

drug delivery, that is, increasing the specificity and efficacy of drug actually being internalized

by the cells in which they are designed to act. Established techniques in this area include

the delivery of certain drug compounds into specific cell types by piggybacking on the