Construction Of Gradient Materials Based On Surface Initiated ATRP And The Manipulation Of Cell Migration

Cell migration plays an essential role in many physiological processes, such as embryonic development, wound healing, immune reaction and metastasis. Directional migration in vivo is induced by gradient signals (biological, chemical or mechanical). Creating gradient cues in vitro can avoid the complicate environment and mimic the corresponding biological events in vivo, and thereby provides a way to disclose the cascade responses in tissue regeneration processes and to develop novel criteria for design of tissue-inductive biomaterials.In this work, molecular weight gradients of poly(2-hydroxyethyl methacrylate) (PHEMA) brushes with slopes of 0.8-3.2 nm/mm were fabricated using surface-initiated atom transfer radical polymerization (ATRP) and a micro-injection method. The PHEMA gradients were characterized by X-ray photoelectron spectrometry (XPS) and ellipsometry. The adhesion number, spreading area, adhesion force, and expression of focal adhesion and actin fibers of vascular smooth muscle cells (VSMCs) decreased along with the increase of the PHEMA brushes length. The VSMCs exhibited preferential orientation and enhanced directional migration toward the direction of reduced PHEMA thickness, whose extent was dependent on the gradient slope and polymer thickness. Most of the cells were oriented along the gradients, and 87% of the cells moved directionally at the optimal condition.Tissue regeneration in vivo involves the migration of various types of cells, some of which are beneficial for tissue regeneration, while others are not. Considering the case of vascular atherosclerosis and "restenosis", to selectively promote the migration of vascular endotheliocytes (ECs) over smooth muscle cells (SMCs) is necessary. Herein a complementary density gradient of poly(2-hydroxyethyl methacrylate) (PHEMA) brushes and YIGSR peptide, a sequence specifically improving the mobility of ECs, was fabricated using a microinjection-backfill method combining with SI-ATRP and click chemistry. The gradients were visualized by fluorescent labeling and further quantified by X-ray photoelectron spectrometry (XPS) and quartz crystal microbalance with dissipation (QCM-d). PHEMA density decreased linearly along the gradient, while YIGSR density increased linearly, with a slope of-48.9 ng/cm2·mm and 80.4 ng/cm2·mm, respectively. At the location where PHEMA density was 193 ng/cm2 (YIGSR density was 308 ng/cm2) on the complementary gradient,82% of ECs exhibited preferential orientation and enhanced directional migration behavior on the gradient surface toward the region of lower PHEMA density and higher YIGSR density. The migration rate of the ECs was significantly enhanced to 18.2μm/h, 5-fold as on TCPS, whereas the mobility of SMCs was not significantly influenced, leading to faster migration of ECs than SMCs. Therefore, the success of the complementary gradient relies on the appropriate interplay between the PHEMA brushes and the cell-specific ligand 67kD laminin receptor (67 LR), enabling the selective guidance of EC migration.Selective enhancement of directional migration of Schwann cells (SCs) over fibroblasts (FIBs) plays a significant role in peripheral nerve regeneration, since it can improve neuron repair and suppress fibrosis. Herein a complementary density gradient of poly(3-dimethyl-(methacryloyloxyethyl) ammonium propane sulfonate) (PDMAPS, a zwitterionic polymer with superhydrophilic and antifouling property) and KHIFSDDSSE peptide (derived from neural cell adhesion molecule NCAM, which mediates cell-cell adhesion and cell aggregation) was fabricated and characterized by using the methods mentioned above. The SCs exhibited preferential orientation and enhanced directional migration behavior on the gradient surface toward the region of lower PDMAPS density and higher KHIFSDDSSE density. Meantime, the migration rate of the SCs was significantly enhanced to at least 2-fold, all the net migration distance increased to 3-fold; whereas the mobility of FIBs was reduced to 60% of its natural state, leading to faster migration of SCs than FIBs. Therefore, the success of the complementary gradient relies on the appropriate interplay between the PDMAPS brushes and the cell-specific ligands, enabling the selective guidance of SCs migration.In this study, controllable 2D gradient materials were successfully prepared by SI-ATRP. We developed strategies to achieve desired tissue regeneration by using a complementary gradient with a synergic effect of two signals to dominate the selective cell migration, highlighting a new perspective on designing complex biomaterials.