Branched, dendritic, and hyperbranched polymers in liquid biopsy device design

Abstract

The development of minimally invasive tests for cancer diagnosis and prognosis will aid in the research of new treatments and improve survival rates. Liquid biopsies seek to derive actionable information from tumor material found in routine blood samples. The relative scarcity of tumor material in this complex mixture makes isolating and detecting cancerous material such as proteins, circulating tumor DNA, exosomes, and whole circulating tumor cells a challenge for device engineers. This review describes the chemistry and applications of branched and hyperbranched to improve the performance of liquid biopsy devices. These polymers can improve the performance of a liquid biopsy through several mechanisms. For example, polymers designed to increase the affinity of capture enhance device sensitivity. On the other hand, polymers designed to increase binding avidity or repel nonspecific adsorption enhance device specificity. Branched and hyperbranched polymers can also be used to amplify the signal from small amounts of detected material. The further development of hyperbranched polymers in liquid biopsy applications will enhance device capabilities and help these critical technologies reach the oncology clinic where they are sorely needed. This article is categorized under: Diagnostic Tools > Biosensing Diagnostic Tools > Diagnostic Nanodevices.

Keywords: biosensor; bottlebrush polymer; dendrimer; hyperbranched polymer; liquid biopsy.

Figure 1.

Tumors regularly shed material into the bloodstream, including circulating tumor DNA (ctDNA), exosomes, and circulating tumor cells (CTCs). Circulating material can be collected in a peripheral blood draw, which is minimally invasive compared to conventional biopsy. The collection of information from ctDNA, exosomes, and/or CTCs is called a liquid biopsy.

Figure 2.

Conformations of surface-grafted polymers. (a) Linear polymers conjugated to a surface at low density can be employed to increase the accessibility and flexibility of a capture moiety, such as an antibody. However, nonspecific adsorption of non-targeted material can occur in the gaps between chains. (b) Linear, hydrophilic polymers achieve anti-fouling behavior when packed tightly enough to form a comb structure of extended chains. Conjugation of capture moieties to a non-fouling surface enhances the specificity of a liquid biopsy. (c) Branched bottlebrush polymers have superior non-fouling characteristics compared to linear polymers but at a much lower density.

Figure 3.

Chemical structures of select branched and hyperbranched polymers. a) Bottlebrush poly(L-lysine)-graft-poly(ethylene glycol). b) G3 PAMAM dendrimer. c) G3 PAMAM dendron with a nitrile functional group/ d) Hyperbranched polyglycerol.

Figure 4.

Mechanisms of increased affinity and avidity. a) Polymers separate the capture moiety from the surface, increasing accessibility. b) Polymers further offer conformational flexibility and avoid nonfunctional orientations. c) Hyperbranched polymers offer a high density of functional sites compared to rigid surfaces. d) Hyperbranched polymers facilitate multivalent recognition on the nanometer scale, which increase specificity. Non-targeted materials may have transient interactions with the capture moiety (left), while multivalent specific interactions (right) have high avidity even if each individual interaction has relatively weak binding affinity.

Figure 5.

Mechanisms of signal amplification. a) Fluorophores can be attached to functional groups on a dendrimer at exceptionally high density compared to antibodies. b) Electrochemical detection typically involves an enzyme-mediated redox reaction. The electron generated by the oxidation of NADH is transferred through a high density of ferrocenyl groups on dendrimers to a conductive surface below, generating a current proportional to the enzyme activity.