Gradient biomaterials and their influences on cell migration

Abstract

Cell migration participates in a variety of physiological and pathological processes such as embryonic development, cancer metastasis, blood vessel formation and remoulding, tissue regeneration, immune surveillance and inflammation. The cells specifically migrate to destiny sites induced by the gradually varying concentration (gradient) of soluble signal factors and the ligands bound with the extracellular matrix in the body during a wound healing process. Therefore, regulation of the cell migration behaviours is of paramount importance in regenerative medicine. One important way is to create a microenvironment that mimics the in vivo cellular and tissue complexity by incorporating physical, chemical and biological signal gradients into engineered biomaterials. In this review, the gradients existing in vivo and their influences on cell migration are briefly described. Recent developments in the fabrication of gradient biomaterials for controlling cellular behaviours, especially the cell migration, are summarized, highlighting the importance of the intrinsic driving mechanism for tissue regeneration and the design principle of complicated and advanced tissue regenerative materials. The potential uses of the gradient biomaterials in regenerative medicine are introduced. The current and future trends in gradient biomaterials and programmed cell migration in terms of the long-term goals of tissue regeneration are prospected.

Keywords: biointerfaces; biomaterials; cell migration; gradient; regenerative medicine.

Figure 1.

A migrating cell seen from the top (left) and side (right). A migrating cell needs to perform a coordinated series of steps to move. Cdc42 regulates the direction of migration, Rac induces membrane protrusion at the front of the cell through stimulation of actin polymerization and integrin adhesion complexes and Rho promotes contraction in the cell body and at the rear [16].

Figure 2.

(a) Schematic (side view) of the glow-discharge reactor chamber, with the electrodes, sample cover and sample position [51]. (b) Schematic of the apparatus for preparation of a gradient on PE surfaces by corona discharge [52]. (c) Scheme of the preparation process of the gradient by a chemical degradation method. (d) Remote photocatalytic oxidation of a thiol self-assembled monolayer (SAM) under a gradient of UV illumination [53].

Figure 3.

(a) A molecular gradient of n-octyl trichlorosilane is formed on a surface by a vapour deposition technique. As the silane evaporates, it diffuses in the vapour phase and generates a concentration gradient along the silica substrate. A silane self-assembled monolayer (SAM) is formed when impinging on the substrate, followed by backfill of the unreacted regions with (11-(2-bromo-2-methyl)propionyloxy)undecyltrichlorosilane (BMPUS) [58]. (b) Thiol diffuses into the stamp from an ink pad. It leaves the stamp because of adsorption to the gold surface and creates a partially covered surface [80]. (c) Symmetrical lateral gradients are generated using hemicylindrical stamps. The contact area increases under increased compression. The darker areas indicate the more hydrophobic region where the contact time is longer. The sketch is not to scale [81]. (d) Schematic of a representative gradient-generating microfluidic network. Solutions containing different chemicals are introduced from the top inlets and allowed to flow through the network. When all the branches are recombined, a concentration gradient is established across the outlet channel (modified from Dertinger et al. [82]). (e) A solution of poly-d-lysine is used as the soaking solution. An appropriate voltage is applied between Ag/AgCl electrodes. After the system comes to equilibrium, the stamp is carefully immersed and soaked for 10 min. The stamp is then dried with nitrogen and contacts the surfaces under pressure in order to achieve pattern transfer (modified from Venkateswar et al. [83]).

Figure 4.

(a) Schematic of the successive binding of heparin and the growth factor onto the fibril surface of the PCL/F127 cylindrical scaffold and the formation of a three-dimensional growth factor gradient on the scaffold. (b) Gross appearance and fluorescence microscopy images showing the rhodamine-labelled VEGF165 gradient along the longitudinal direction of the PCL/F127 cylindrical scaffold. The VEGF165 immobilized on the cylindrical scaffold is expressed as a red colour. (c) Loading amount of growth factors (BMP-7, TGF-β2 and VEGF165) immobilized onto the PCL/F127/heparin scaffold sections. The scaffolds show the gradually decreasing concentration of growth factors along the longitudinal direction from the bottom position to the top position (growth factor concentration gradient scaffolds) [119].

Figure 5.

(a) Mask patterns used to control the intensity of UV light during photopolymerization of acrylamide. Patterns are printed on transparencies using a standard laser printer. (i) Greyscale intensity is shown to vary from 10 to 70%, in increments of 10%. (ii) A radial gradient pattern is used to generate substrata with a gradient in mechanical compliance. The centre of the circle is clear, gradually darkening in increasing greys to black on the outside. (iii) An inverse radial gradient pattern where the centre of the circle is dark, gradually decreasing to clear on the outside. (b) Young's modulus values for a radial-gradient gel using the microindentation method. (c) Cell speed on the polyacrylamide gel polymerized under a 30% grey filter (filled bar) and the polyacrylamide gel polymerized under no filter (open bar). The asterisk indicates that these two datasets are significantly different (p < 0.005). (d) Cell path on a radial-gradient gel. Cells start in the soft region of the gel and translocate towards the stiff region of the gel. (e) Cell path on a uniform-compliance gel polymerized under a 30% grey filter. The arrow length designates 50 mm. Open squares indicate the starting position of the cell [117].

Figure 6.

(a,b) Mc3t3 osteoblasts in contact with a biofunctionalized 80 nm pattern and exhibiting cell protrusions sensing the pattern. Scale bars, (a) 20 μm and (b) 200 nm. (c) Projected cell area (±s.e.m.) as a function of substrate position. Insets: Mc3t3 osteoblasts after 23 h adherence on a homogeneously nanopatterned area with 50 nm c(-RGDfK-) patch spacing (left) and along the spacing gradient (right), respectively. Cells are immunostained for vinculin (green), and actin is visualized using TRITC (6-tetramethylrhodamine isothiocyanate)-phalloidin (red). Scale bars, 20 mm [47].

Figure 7.

(a) Schematic of the process to produce a surface density gradient of a protein using an electrochemical approach. (b) Integrated area of the carbonyl band at 1735 cm^–1 measured from FTIR as a function of position for two C₁₅COOH/C₁₁OH gradients after EDC/N-hydroxysuccinimide (NHS) activation. (c) Bovine aortic endothelial cell displacements along gradients towards higher protein surface densities after 24 h cell culture. Data shown are for uniform (red bar), steep or sharp gradient (green bar), and smooth gradient (blue bar) surfaces of Fn, VEGF or both. Asterisks denote significant difference between two compared results (p < 0.05) [140].

Figure 8.

(a,b) Gradient steepness and stability of TG-L1Ig6-Alexa-488 in three-dimensional fibrin matrices. The gradient matrices with steepness of 0–10 mg and 0–25 mg ml^–1 TG-L1Ig6-Alexa-488 incorporated into 2 mg ml^–1 fibrin-based matrices are analysed (a) 1 and (b) 24 h after polymerization. (c) Normal dermal human fibroblasts (NDHF) adhered on the top and within three-dimensional fibrin matrices containing a gradient of 0–10 mg ml^–1 TG-L1Ig6. (d) The fibroblasts align with the direction of the gradient (increase between 5 and 40 mm from the beginning of the 0–10 mg ml^–1 gradient of TG-L1Ig6 [39].