a, Cylindrical particles prepared using PRINT method82; scale bar, 5 μm (© 2008 PNAS). b, UFO-shaped particles prepared by film-stretching18; scale bar, 5 μm (© 2007 PNAS). c, Rectangular discs of SU-8 (ref. 53); scale bar, 150 μm (© 2006 NPG). d, Polymer rings90 (© 2006 NPG). e, Phagocytosis of particles depends on shape17 (© 2006 PNAS).

Particles can pass through biological barriers by a number of different processes. These include passive (diffusive) and active processes ranging from extravasation to transdermal uptake. Most of these processes affect distribution and clearance of micro- and nanoparticles in the human body and they strongly depend on particle size.

a, Biphasic particles22 (© 2005 NPG). b, Composite liposome-nanoparticle carriers119; nanocells, scale bar 100 nm (© 2005 NPG). c, Vesosomes137 (© 2002 ACS). d, Core–shell microparticles138; scale bar 25 μm (© 2007 STM). e, Differential release of two drugs from two compartments of nanocell particles119 (© 2005 NPG).

a, Biphasic discs with smooth and rough surface; scale bar, 1 μm (image: Joerg Lahann). b, Pillar arrays for mapping force of epithelial cell migration134; scale bar, 5 μm (© 2005 PNAS). c, Asymmetrically coated particles136; scale bar, 5 μm (© 2003 RSC). d, Micropatterned particles114; scale bar, 100 μm (© 2007 PNAS). e, Control of spreading of an endothelial cell through a micropatterned substrate27; scale bar, 10 μm (© 1997 AAAS).

a, Human herpesvirus 3; scale bar, 100 nm (image: Frank Fenner). b, Ebola virus; scale bar, 500 nm (image: Frederick A. Murphy). c, Enterobacteria phage λ; scale bar, 50 nm (image: University of Wisconsin-Madison). d, Human erythrocytes; scale bar approximately 10 μm. e, Escherichia coli; image size approximately 7 μm × 6 μm. (Panels d and e © Dennis Kunkel Microscopy.) f, Surface texture in alveolar macrophages73; scale bar, 5 μm (© 2008 STM). g, Pollen; image size approximately 50 μm × 45 μm (Dartmouth Electron Microscope Facility). h, Intestinal villi; approximate magnification μ5,950 (© Dennis Kunkel Microscopy). i, The immunological synapse; T cell forming a synapse with a supported membrane containing GPI-linked pMHC and ICAM (ref. 134); © 1999 AAAS). j, A schematic of cellular compartmentalization showing several organelles surrounding the nucleus. The images clearly establish that nature uses physical parameters such as size, shape, texture and compartmentalization in designing life.