**5. Outlook**

as identical concentrations of the divalent ions Mg2+ and Ca2+ lead to different experimental outcome [65]. This ion-specific effect suggests that, in addition to electrostatic forces, also hydrophobic interactions are likely to contribute to the selective filtering properties of the basal

214 Composition and Function of the Extracellular Matrix in the Human Body

Systematic permeability studies with artificial particles were very helpful to unravel the physical mechanisms which are responsible for the trapping of solutes in the basal lamina. However, most compounds which encounter the basal lamina layer under physiological conditions are small molecules rather than microparticles. To investigate the selective prop‐ erties of the basal lamina toward small molecules, a microfluidic setup (**Figure 6**) was recently introduced [66]. Here, customized peptides with tailored amino acid sequences and thus different net charges were used as diffusion probes. To ensure optimal comparability, the molecular weight of those oligopeptides was kept constant. The penetration behavior of those peptides into an ECM gel was visualized by fluorescent microscopy, and similar to the SPT experiments discussed above also the behavior of those molecules critically depended on their charge. Positively charged peptides accumulated at the gel/buffer interface, whereas nega‐

**Figure 5.** Single-particle tracking experiments can locally map the permeability of ECM gels. *Upper panel*: Exemplary trajectories and the corresponding MSD curves of a particle showing free diffusion and a particle showing retarded diffusion. For freely diffusing particles, the dependence of the MSD on time is linear and a diffusion coefficient *D* can be calculated according to the formula shown in the graph. For the calculation of *D,* only the first 10% of the MSD data is used to avoid errors arising from statistical uncertainties. *Lower panel*: Trajectories of a particle which transiently switches between a diffusing and a bound state and of a particle which alternates between a strongly and a weakly bound configuration. The states of motion can be distinguished based on the fluctuation amplitude of the particle. Both trajectories were obtained at a salt concentration of 1 M KCl. The histogram shows that in ECM gels, the fraction of mobile particles depends on the concentration, valency, and detailed species of the ions used. Adapted with permis‐

sion from Arends et al. [65]. Copyright ©2013 American Chemical Society.

lamina.

Here, we have summarized selected aspects of our current understanding how the biochemical composition of the basal lamina is mirrored in the complex microarchitecture as well as the multi-facetted material properties of the biopolymer network. Deciphering the physicochem‐ ical principles which dictate the microstructure, viscoelastic properties, and selective perme‐ ability of the basal lamina layer are not only interesting for cell biology studies [67, 68], drug

**Figure 6.** Illustration of a microfluidic setup used to probe the diffusive penetration of peptides from a buffer compart‐ ment into a basal lamina gel. Typical intensity profile images for positively and negatively charged peptides are shown next to the microfluidic channel. For positively charged peptides, an intensity peak is observed at the buffer/gel inter‐ face, whereas such an accumulation does not occur for negatively charged peptides. Similar results are obtained when those peptides are injected into the connective tissue of mice. Tissue immunostaining confirmed that the positively charged peptides colocalize with collagen IV, a main component of the basal lamina. Adapted with permission from Arends et al. [66]. © 2015 Arends et al. Published under CC BY license.

delivery questions and tumor treatment [69–72], but might also have strong implications for tattoo removal applications: Here, ink nanoparticles trapped in the skin tissue have to be mobilized, for example, by soaking the tissue in salt solutions, so that they can be washed out from the skin rather than removed by painful and scar-inducing laser treatment. The lessons learned from systematically unraveling the physical and chemical mechanisms, which give rise to the complex properties of the basal lamina, may also help in the rational design of artificial hydrogel systems for tissue engineering approaches: The synthesis of complex macromolecules with well-defined chemical properties may allow for constructing hydrogels with both tailored mechanical properties and selective permeability behavior.
