Abstract
As
key components of antifouling material surfaces, the design and
screening of polymer molecules grafted on the substrate are
critical. However, current
experimental and computational models still retain an empirical flavor
due to the complex structure of polymers.
Here, we report a simple and general
strategy that enables multi-scale design and screening of easily
synthesized functional polymer molecules to address this challenge.
Specifically, the required functions
of the antifouling material are decomposed and assigned to different
modules of the polymer molecules. By
designing different modules, a novel bio-inspired polymer with three
zwitterionic poly (sulfobetaine methacrylate) (PSBMA) chains, three
catechol (DOPA) anchors (tri-DOPA-PSBMA), and a tris(2-aminoethyl) amine
(TREN) scaffold were screened out.
Moreover, it was successfully synthesized via an atom transfer radical
polymerization (ATRP). The
excellent performance of tri-DOPA-PSBMA with a versatile and convenient
grafting strategy makes it a promising material for marine devices,
biomedical devices, and industrial applications.
Keywords: rational design, zwitterionic polymer, antifouling
material surfaces, rapid adhesive, long-term stability
Introduction
Antifouling
surfaces resisting nonspecific protein adsorption and biofilm adhesion
improve function, efficiency, and safety in products such as
water purification membranes,
marine
vehicle coatings, medical implants, and other industrial
applications1, 2.
The strong hydrophilicity of
zwitterionic polymer gives them good biocompatibility and anti-protein
adhesion properties. Zwitterionic polymer grafted surfaces have been
investigated to offer high resistance toward nonspecific protein
adsorption and cell adhesion under physiological conditions.
Due
to their highly hydrophilic nature and flexibility, zwitterionic polymer
grafted surface have met with varying success in vitro and in vivo
antifouling tests3. Commonly,
zwitterionic
polymers were immobilized to the surfaces by two strategies, i.e.,
adsorption of polymer from solution (so-called “graft-to” approach)
and surface-initiated polymerization (SIP) of monomers from
surface-bound initiators (“graft-from” approach)4,
5. In contrast to the “graft-from” approach, the “graft-to” method
is simpler and more convenient, especially suitable for practical
applications on a large scale. However, for the “graft-to” approach,
especially for binding in the presence of water, specific surface
functional groups with high binding energies and versatile feasibility
to various substrate surfaces are required. Inspired by the adhesion
mechanism of mussels, 3,4-dihydroxyphenylalanine (DOPA) is believed to
be a versatile residue for wet adhesion. Various zwitterionic polymers
with catechol adhesive groups have been designed to achieve surface
adhesion in aqueous solutions6-8. Although these
zwitterionic polymers, such as one zwitterionic chain with one catechol
anchor, two zwitterionic chains with two catechol anchors, and
zwitterionic copolymers with multiple anchors, have been proven to be
effective for one-step anchoring and enhanced antifouling
properties.
Little
attention has been paid to the adsorption kinetics and thermodynamics of
rapid adhesion and long-term stability of polymer grafts, especially
from the sophisticated design of different polymer structures with the
construction scaffolds. Recently, Waite et al. reported that a
siderophore with a robust tris(2-aminoethyl) amine (TREN) scaffold and
three amino acid 3,4-dihydroxy-L-phenylalanine (DOPA) residues exhibited
strong adsorption and adhesion behavior and retained adhesive integrity
over a wide range of pH9, 10. However, the effects of
multiple conjugations, different hydrophilic groups, and different
scaffolds on the properties of the materials were still unrevealing.
Therefore,
there is an ongoing need for rapid and precise polymer design strategies
capable of robustly anchoring zwitterionic polymers onto various
material surfaces and having excellent antifouling functions.
Due
to the uncertainty of polymer properties and complicated experimental
procedures, it is relatively difficult and time-consuming to screen
polymers with excellent antifouling properties by synthesizing a number
of polymers with different structures and determining their surface
properties.
Related
reports on machine learning have enabled rapid polymer molecular design
and high-throughput screening of optimal materials by drawing on
concepts and ideas from
combinatorial chemistry and
materials informatics11, 12, namely by combining
”building blocks” of different structures or components in parallel,
systematically, and repeatedly13, 14. High-throughput
screening gives us the idea of screening out advantageous ”building
blocks” according to the required functions. Moreover, due to the
competition between the polymer chains dissolved in water and adsorbed
on the solid surface, the hydrophilic-hydrophobic balance of the polymer
and extensively optimized polymer grafting conformations should be
considered and adjusted comprehensively to achieve effective underwater
adhesion and good antifouling property. Especially, the complex system
with multiple components and multiple interactions will result in
differences in the underwater adhesion mechanism, solvation free energy
(G solv), and electrostatic potential of polymer
molecules which need to be considered from the molecular microstructure
and the adsorption configuration information15, 16.
Molecular simulation and density functional theory (DFT) calculation can
be used to explain the adsorption kinetics and thermodynamics of polymer
molecules from the molecular information of ”block”
properties17. Hence, it is necessary to propose a
strategy that combines multi-scale molecular design and molecular
simulation based on the actual function of the polymer rather than the
blind screening of large throughput, to design and screen the specific
polymer molecules which are easy to synthesize.
Herein, we propose a design and screen strategy for a novel bio-inspired
zwitterionic polymer for rapid underwater adhesion and long-term
antifouling stability. The target molecule is divided into three parts
according to the desired function: tail chain, head group, and scaffold.
In molecular design, different colors represent different functions
performed in polymer molecules( Figure
1a ).
Head groups conjugate to tail chains
to fabricate the adhesive antifouling polymers which will be grafted to
various substrates using catechol-mediated adhesion to resist protein
adsorption. Twelve polymer molecules were designed by a preliminary
screening of different head groups, tail chains, and scaffolds. The
surface properties, G solv, and adsorption energy
(E ads) with hydroxylated silicon surface of
design molecules were calculated by
DFT
and molecular mechanics (MM) calculation, and the most stable adsorption
configuration of molecules on the silica surface was analyzed by
molecular dynamics (MD) annealing simulation. (Figure 1b ). By
comparing these results, we selected a zwitterionic polymer with a TREN
scaffold and three DOPA residues for surface anchoring, and three PSBMA
polymer chains for antifouling to synthesize and investigate its
adhesion behavior and antifouling property.
From the calculation and the
experimental results, we demonstrated that this material has a fast
surface adhesion and antifouling surfaces with long-term stability in an
aqueous solution. On this basis, it
is
interesting to note that tri-DOPA-PSBMA tethered on hydroxylated silicon
wafers in 10 minutes and remained stable for more than 30 days without
compromising performance, which is of great significance to industrial
application. (Figure 1c )