# Biocompatible Dispersants for Carbon Nanomaterials
## 6 Biocompatible Dispersants
### Polymer Dispersants  
Unlike ACSs, polymer dispersants can be used to functionalise CNMs through a polymer wrapping mechanism, producing a static dispersion. This wrapping results in favourable CNM surface coverage, dispersion stability, and homogeneity [35]. Furthermore, by applying this thermodynamically stable CNM coating, free surfactant molecules can be removed from the final solution without the re-agglomeration of CNMs. This removal can be achieved through centrifugation [59], dialysis [60; 61], and filtration [62] methods. A range of synthetic and natural biocompatible polymers can be utilised as surfactants (Figure 6). Biocompatible block copolymers with hydrophobic and hydrophilic components such as Pluronic F127 (F127) offer excellent, tunable properties that can be investigated experimentally to develop dispersant systems for different CNMs. This was demonstrated by [PERSON] et al. in 2018, where SWCNTs were exfoliated and dispersed with F127 [25] (Table 5 (1)). The hydrophobicity of F127 comes from poly(propylene oxide) (PPO), the middle block of 30 units in Figure 6, whereas the poly(ethylene oxide) (PEO) component, the two edge blocks of 75 units in Figure 6, provides the polymer with hydrophilicity. The hydrophobic PPO units wrap around the CNMs surface via hydrophobic interactions. The hydrophilic PEO tails then interact with the surrounding solvent, causing them to orient away from the CNM surface, thereby providing steric repulsion between nanoparticles. A previous study found that F127 had excellent surface coverage of SWCNTs, with nearly full saturation achieved [63]. The polydispersity index and copolymer composition were critical factors in dispersant effectiveness and efficiency. However, these will likely have to be optimised for different nanomaterials. [PERSON] et al. improved upon the structure of F127, developing a PPO/PEO block copolymer with a starlike structure, a hydrophilic backbone, and hydrophilic arms (AP432) [64]. This interesting surfactant could exfoliate and disperse both SWCNTs and MWCNTs (Table 5 (2)). It was a better dispersant than F127 due to the increased steric repulsion between functionalised CNTs due to the hydrophilic arms repelling each other. The copolymer dispersant approach was also applied by [PERSON] et al. in 2019, who successfully developed a polydehydroalanine (PDha)-based copolymer capable of dispersing MWCNTs [65]. This polyzwitterion dispersant has exceptional charge density, and, by extension, great hydrophilicity. When combined with hydrophobic elements to anchor it to the CNMs surface, this material makes for an ideal dispersant. One downside of polyzwitterions is their sensitivity to pH and temperature, which may affect their hydrophilicity. Comprehensive biocompatibility data is not yet available on this material either (Table 5 (3)). Figure 6: Structures of two polymer surfactants: (**A**) Pluronic F127 (synthetic); and (**B**) a proposed structure of polydopamine (natural). The main drawback of polymer wrapping is that the final polymer coating is quite hard to remove from the CNMs as it is so strongly bound. Therefore, it makes sense for biomedical applications to develop a range of functional dispersants that also act as targeting/imaging/therapeutic agents whilst utilising the CNM as a stable scaffold. Our group has recently investigated this approach using CNOs wrapped in a hyaluronic acid-1,2-dimyristoyl-\(sn\)-glycero-3-phosphoethanolamine (HA-DMPE) conjugate polymer [61] (Table 5 (4)). The HA portion of the coating is hydrophilic and interacts with the solvent. It also acts as a targeting ligand for the CD44\({}^{+}\) receptor, which is overexpressed in a range of cancer cells [66]. The phospholipid (DMPE) component of the conjugate adsorbs onto the surface of the CNOs through hydrophobic interactions. Conjugate systems such as these have a range of potential applications; we integrated fluoresceinamine (a fluorophore) into the polymer conjugate to track in vitro and in vivo biodistribution.