Many functional units in biology exist as stabilized, discrete entities within the water-based milieu of living systems. Materials can also be designed as nano- to microscale water-dispersed colloids that can engage, interact with, or mimic biology. Disperse biomaterials include nanomaterials, such as micelles and nanoparticles, as well as structures ranging from a few microns to submillimeter in size, including liposomes, microparticles and microgels. This broad range of materials addresses applications ranging from drug delivery and imaging to in-situ tools that enhance the understanding of cellular behavior. As new synthetic routes and processes are developed to generate organic, inorganic, and hybrid composite particles with a range of shapes, sizes, and compositions, the possibilities for combined function and cell- or tissue-responsive behavior in these materials abound.
Nanoparticles have been of extreme interest for in vivo applications addressing cancer and certain other disease conditions because the biodistribution of nanoscale objects is highly influenced by size. Thus, nanomaterials are driven to enhance their concentration in different regions based on tissue vascularity, the presence of leaky or damaged vasculature, macrophage uptake, and the nature of the clearing process within the body. One of the challenges for the in vivo design of nano-biomaterials is the ability to enhance uptake of materials that are delivered through the bloodstream to a desired area, while minimizing their uptake in the filtration organs of the liver and spleen. For example, nanomaterials within a given size range (sub-100-150 nm) can passively target solid tumors, but usually are found in even higher concentrations in the liver, which can lead to significant side effects. The use of inorganic semiconductors and a range of metallic and inorganic nanomaterials enables exploitation of the unique properties achieved in nanomaterials, such as the highly stable photoluminescence of quantum dots, which can be applied to imaging, and light-activated rupture of gold nanocarriers that can be disrupted remotely within the desired region of the body with lasers. In general, it is of great interest to understand the extracellular and intracellular tracking of nanomaterials for imaging and delivery, and to better understand how composition and surface functionality can impact circulation time and toxicity for systemic release.
Micron and submicron scale materials systems can be tuned to enable optimized incorporation of hydrophilic or hydrophobic molecules, and can be designed as drug carriers that target different cell types based on molecular targeting and size. On the other hand, larger micron-scale materials can be designed to approach the size of cells. In this case, there is particular interest in the ability to functionalize microparticle surfaces with receptor proteins or ligands that enable particle-cell interactions and communication. These cell mimicking systems may be used to address several important biological questions, and the mechanical modulus as well as chemical composition of the surface may be modified to control cell behavior in 3D systems. Micron-scale materials systems have been used to encapsulate individual cells and create artificial membranes that may also regulate cell interactions. For both the micron and nanoscale disperse materials, new opportunities exist in the exploration of particle shape and aspect ratio with regard to cell interactions, as well as mechanical stiffness and released or surface-bound chemical factors or cues. Challenges and opportunities in this area include the ability to control polydispersity and shape of particles to achieve reversible assembly and disassembly in response to biological or external cues, and the potential to regulate molecular permeability through these materials systems for release, sensing, or imaging.