Peptide Bolaamphiphile Research Articles

Electrochemical Suzuki–Miyaura cross-coupling using peptide bolaamphiphile hydrogel-supported Pd NPs as heterogeneous electrocatalyst

Electrochemical cross-coupling has arisen as one of the promising greener approaches for the construction of C–C bonds.

Design of short peptides and peptide amphiphiles as collagen mimics and an investigation of their interactions with collagen using molecular dynamics simulations and docking studies.

Short peptide sequences and bolaamphiphiles derived from natural proteins are gaining importance due to their ability to form unique nanoscale architectures for a variety of biological applications. In this work, we have designed six short peptides (triplet or monomeric forms) and two peptide bolaamphiphiles that either incorporate the bioactive collagen motif (Gly-X-Y) or sequences where Gly, Pro, or hydroxyproline (Hyp) are replaced by Ala or His. For the bolaamphiphiles, a malate moiety was used as the aliphatic linker for connecting His with Hyp to create collagen mimics. Stability of the assemblies was assessed through molecular dynamics simulations and results indicated that (Pro-Ala-His)3 and (Ala-His-Hyp)3 formed the most stable structures, while the amphiphiles and the monomers showed some disintegration over the course of the 200ns simulation, though most regained structural integrity and formed fibrillar structures, and micelles by the end of the simulation, likely due to the formation of more thermodynamically stable conformations. Multiple replica simulations (REMD) were also conducted where the sequences were simulated at different temperatures. Our results showed excellent convergence in most cases compared to constant temperature molecular dynamics simulation. Furthermore, molecular docking and MD simulations of the sequences bound to collagen triple helix structure revealed that several of the sequences had a high binding affinity and formed stable complexes, particularly (Pro-Ala-His)3 and (Ala-His-Hyp)3. Thus, we have designed new hybrid-peptide-based sequences which may be developed for potential applications as biomaterials for tissue engineering or drug delivery.

In Situ Self-Sorting Peptide Assemblies in Living Cells for Simultaneous Organelle Targeting.

Self-sorting is a common phenomenon in eukaryotic cells and represents one of the versatile strategies for the formation of advanced functional materials; however, developing artificial self-sorting assemblies within living cells remains challenging. Here, we report on the GSH-responsive in situ self-sorting peptide assemblies within cancer cells for simultaneous organelle targeting to promote combinatorial organelle dysfunction and thereby cell death. The self-sorting system was created via the design of two peptides E3C16E6 and EVMSeO derived from lipid-inspired peptide interdigitating amphiphiles and peptide bola-amphiphiles, respectively. The distinct organization patterns of the two peptides facilitate their GSH-induced self-sorting into isolated nanofibrils as a result of cleavage of disulfide-connected hydrophilic domains or reduction of selenoxide groups. The GSH-responsive in situ self-sorting in the peptide assemblies within HeLa cells was directly characterized by super-resolution structured illumination microscopy. Incorporation of the thiol and ER-targeting groups into the self-sorted assemblies endows their simultaneous targeting of endoplasmic reticulum and Golgi apparatus, thus leading to combinatorial organelle dysfunction and cell death. Our results demonstrate the establishment of the in situ self-sorting peptide assemblies within living cells, thus providing a unique platform for drug targeting delivery and an alternative strategy for modulating biological processes in the future.

Investigation of Self‐Assembly of Symmetric and Asymmetric Peptide Bolaamphiphiles by COSMO‐RS and Atomistic Simulations and Their Interactions with POPC Bilayers

AbstractIn this work, eight new peptide bolaamphiphiles (PbAs) are designed containing polar (serine or threonine) and nonpolar amino acid head groups (tryptophan or tyrosine) connected by varying aliphatic carbon chains of different lengths and saturation. The physicochemical properties of the bolaamphiphiles are examined by COSMO‐RS to determine sigma profiles, potential, and sigma surfaces. Self‐assembly is studied using atomistic molecular dynamic simulations under protonated and deprotonated conditions. To mimic physiological conditions, simulations are also carried out in the presence of sodium chloride and the impact of salt on the self‐assembly process is also studied. In general, hydrophobic PbAs are found to form higher‐order supramolecular structures while PbAs connected by shorter chains or hydrophilic head groups form globular, more compact structures. Self‐assembly is more favorable when the PbAs are protonated. Membrane interactions are then studied using MD simulations and the results indicate that the hydrophobic PbAs interact more favorably with the POPC membrane surfaces. These simulations provide insights about the intricate interactions and properties of the PbAs and shed light on the ability to form fine‐tuned structures at the nanoscale. This study may lead to the development of novel PbAs for potential applications in a wide variety of biological applications.

A platinum nanoparticle doped self-assembled peptide bolaamphiphile hydrogel as an efficient electrocatalyst for the hydrogen evolution reaction.

Noble metal-based nanomaterials have shown great potential for catalytic application with higher selectivity and activity. Owing to their self-assembly properties with various molecular interactions, peptides play an essential role in the controlled synthesis of noble metal-based catalysts with high surface area. In this work, a phenylalanine (F) and tyrosine (Y) based peptide bolaamphiphile is prepared by solution-phase peptide synthesis. The peptide bolaamphiphile readily self-assembles into a hydrogel with a cross-linked nanofibrillar network. The platinum nanoparticles (Pt NPs) are in situ generated within the cross-linked nanofibrillar network of the hydrogel matrix of the peptide bolaamphiphile. Benefiting from the synergistic properties of the Pt nanoparticles doped on three-dimensional fibrous networks, Pt6@hydrogel shows efficient catalytic activity for the electrochemical hydrogen evolution reaction (HER) in 0.5 M H2SO4 solution. The Pt6@hydrogel requires an overpotential of 45 mV at −10 mA cm−2 with a Tafel slope of 52 mV dec−1. The Pt6@hydrogel also shows electrocatalytic activity in basic and neutral pH solutions. The excellent activity and stability of Pt6@hydrogel for the HER shows great potential for energy conversion applications.

Monolayer wall nanotubes self-assembled from short peptide bolaamphiphiles

In spite of extensive research, it remains a formidable challenge to control the dimension of the nanostructures self-assembled from short designed peptides. In this work, we show that peptide bolaamphiphiles form monolayer wall nanotubes, facilitated by the interplay between the side chain structure and hydrophobicity of the central residues. The peptide KI4K self-assembles into nanotubes with a width of ~ 100 nm, but changes in the molecular structure of amino acid side chains could hugely impact the nanostructures formed. The three variants of KI4K, via the substitution of aromatic amino acids (F, Y, and Dopa) for the I residue closest to the C-terminus, could substantially reduce nanotube diameters, indicating a significant steric hindrance of the benzene rings on the lateral packing of β-sheets. However, the introduction of hydroxyl groups on the benzene rings alleviates the steric effect, with nanotube diameter increasing in the order of KI3FK, KI3YK, and KI3DopaK, suggesting the formation of side chain H-bonds between β-sheets in addition to hydrophobic contacts. Because the self-assembly process of KI3DopaK nanotubes is slow, key intermediates and their structural details are well characterized. With increasing incubation time, monolayered twisted ribbons and helical ribbons grow into mature KI3DopaK nanotubes via the pitch closing route.

Development of Nanoscale Hybrids from Ionic Liquid-Peptide Amphiphile Assemblies as New Functional Materials.

Over the years, ionic liquids (ILs) have gained tremendous importance because of their unique properties and plethora of applications. In this work, we have developed a new nanoscale hybrid gel consisting of 1-ethyl-3-methylimidazolium dimethyl phosphate, [C2mim][dmp], and self-assembled peptide nanoassemblies. The peptide nanoassemblies were formed by self-assembly of a newly synthesized peptide bolaamphiphile bis(N-α-amido-threonine) 1,7 heptane dicarboxylate (ThrC7). Upon mild heating and sonication of the IL and ThrC7 nanoassemblies, ThrC7-IL nanocomposites were formed. We explored the formation of nanohybrids by varying the ratio of IL to ThrC7 assemblies. While at lower IL ratios, a gelatinous matrix was formed, at higher IL ratios, highly ordered multilayered structures were observed by atomic force microscopy (AFM) imaging. The interactions between the ThrC7 nanofibers and [C2mim][dmp] IL were probed by Fourier transform infrared spectroscopy, transmission electron microscopy, and AFM imaging. Differential scanning calorimetry and thermogravimetric analysis showed that the nanohybrids illustrated distinct thermal phase changes due to changes in hydrogen bonding interactions and unfolding of the nanoassemblies. The viscoelastic behavior of the nanohybrids indicated that the materials displayed higher storage modulus upon incorporation of the ThrC7 nanoassemblies when compared to the IL. Furthermore, the nanohybrids were found to adhere to and promote proliferation of human dermal fibroblasts, while cytotoxicity was observed toward MCF-7 breast cancer cells. Thus, for the first time, we have developed peptide-based nanohybrids with an imidazolium-based IL with unique structural properties that may open new avenues for exploring potential biological applications.

Varying fish scale derived hydroxyapatite bound hybrid peptide nanofiber scaffolds for potential applications in periodontal tissue regeneration.

New peptide based hybrid scaffolds were prepared by blending two different fish scale derived hydroxyapatite with functionalized peptide nanofibers for potential applications in periodontal tissue regeneration. The nanofibers were prepared by self-assembly of the newly designed peptide bolaamphiphile Bis (N-α-amido-glutamic acid) 1,7 heptane tetracarboxylate and functionalized with a segment of the tyrosine rich amylogenin peptide sequence MPLPPHPGHPGYINF followed by polygalacturnonic acid and hydroxyapatite derived from salmon or red-snapper fish scales. The binding interactions of the components of the scaffold was confirmed by FTIR spectroscopy as well as SEM imaging. Hybrids scaffolds with salmon scale derived HaP showed higher mechanical strength and Young's Modulus compared to snapper scale derived scaffolds. Our results indicated that while both the scaffolds supported cell proliferation and efficiently formed cell-scaffold matrices with gingival fibroblasts, we observed greater alignment of the cells in the case of scaffolds that contained snapper scale derived hydroxyapatite. Furthermore, higher differentiation ability into osteoblast like cells was seen in the case of the snapper scale derived HaP based scaffolds. Our studies indicate that the hybrid peptide nanofiber scaffold matrices, particularly those prepared using snapper scales may have significant utility in the development of biomaterials for periodontal tissue regeneration.

Self-Assembly, Antimicrobial Activity, and Membrane Interactions of Arginine-Capped Peptide Bola-Amphiphiles.

The self-assembly and antimicrobial activity of two novel arginine-capped bola-amphiphile peptides, namely RA6R and RA9R (R, arginine; A, alanine) are investigated. RA6R does not self-assemble in water due to its high solubility, but RA9R self-assembles above a critical aggregation concentration into ordered nanofibers due to the high hydrophobicity of the A9block. The structure of the RA9R nanofibers is studied by cryogenic transmission electron microscopy (cryo-TEM) and small-angle X-ray scattering (SAXS). Circular dichroism spectroscopy shows that both RA6R and RA9R adopt coil conformations in water at low concentration, but only RA9R adopts a β-sheet conformation at high concentration. SAXS and differential scanning calorimetry are used to study RA6R and RA9R interactions with a mixed lipid membrane that models a bacterial cell wall, consisting of multilamellar 1,2-dipalmitoyl-sn-glycero-3-phosphoglycerol/1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine vesicles. Cytotoxicity studies show that RA6R is more cytocompatible than RA9R. RA6R has enhanced activity against the Gram-negative pathogen P. aeruginosa at a concentration where viability of mammalian cells is retained. RA9R has little antimicrobial activity, independently of concentration. Our results highlight the influence of the interplay between relative charge and hydrophobicity on the self-assembly, cytocompatibility, and bioactivity of peptide bola-amphiphiles.

Tuning the Handedness: Role of Chiral Component in Peptide-Appended Bolaamphiphile-Based Coassembled Hydrogels.

Chirality is the intrinsic property of a molecule that can be tuned by the change in chirality of a molecule or by the addition of a chiral component as an external stimulus. An l-leucine-based dipeptide-appended succinic acid-based bolaamphiphile coassembled with d-tartaric acid to form supramolecular right-handed nanostructured hydrogel, whereas l-tartaric acid coassembled to form supramolecular left-handed nanostructured hydrogel. Scanning electron microscopy and transmission electron microscopy experiments revealed the right- and left-handed helical nanofibers that are responsible for the formation of supramolecular nanostructured hydrogels. The synergistic chiral effect of l-leucine in peptide bolaamphiphile and d/l-tartaric acid plays a significant role in bicomponent gelation with helical nanofibers. The first two amino acids attached to both sides of succinic acid moiety act as a tuning button for supramolecular chirality of amino acids/peptides attached with succinic acid-based bolaamphiphiles. The second amino acid plays the role of modulating supramolecular chirality if the first two amino acids act neutrally to the chirality of bolaamphiphiles, which was confirmed by circular dichroism spectroscopy.

Restructuring of Lipid Membranes by an Arginine-Capped Peptide Bolaamphiphile.

We study the self-assembly of arginine-capped bolaamphiphile peptide RA3R (A: alanine, R: arginine) together with its binding to model membranes and its cytotoxicity and antimicrobial activity. Anionic 2-oleoyl-1-palmitoyl- sn-glycero-3-phospho-rac-(1-glycerol) sodium salt/2-oleoyl-1-palmitoyl- sn-glycero-3-phosphoethanolamine (POPG/POPE) vesicles and zwitterionic 1,2-dioleoyl- sn-glycero-3-phosphocholine/2-oleoyl-1-palmitoyl- sn-glycero-3-phosphocholine (POPC/DOPC) vesicles are used as model membranes to mimic bacterial and mammalian cell membranes, respectively. We show that RA3R adopts a polyproline-II collagen-like conformation in water. Binding of RA3R to POPG/POPE vesicles induces a strong correlation between the lipid bilayers, driven by RA3R/POPG attractive electrostatic interaction together with a shift of the intramolecular POPE zwitterionic interaction toward an attractive electrostatic interaction with the RA3R. Populations of RA3R/POPG/POPE vesicles comprise different bilayer spacings, dA and dB, controlled by the conformation of the lipid chains corresponding to the Lβ (gel-like) and Lα (liquid-crystal) phases, respectively. Cryo-TEM images reveal the presence of vesicles with no internal structure, compartmentalized thin-wall vesicles, or multilayer vesicles with uncorrelated layers and compartmentalization depending on the RA3R/POPG/POPE composition. In contrast, the interaction of RA3R with multilamellar POPC/DOPC vesicles leads to the decorrelation of the lipid bilayers. RA3R was tolerated by skin fibroblast cells for a concentration up to 0.01 wt %, while 0.25 wt % RA3R proved to be an efficient antibacterial agent against Gram-positive bacteria L. monocytogenes. Our results highlight the ability of RA3R to distinguish between bacterial and mammalian cells and establish this peptide as a candidate to reduce the proliferation of L. monocytogenes bacteria.

Dendritic peptide bolaamphiphiles for siRNA delivery to primary adipocytes

Obesity is a major risk factor for diabetes, heart disease and other health problems. Adipose tissue plays a central role in the development of obesity and obesity-associated diseases. Gene therapy targeting adipose tissue may provide a promising strategy for obesity treatment. However, nucleic acid delivery to adipose tissue or even cultured adipocytes is challenging due to low delivery efficacy and high toxicity of the current cationic lipid based delivery systems, or monoamphiphiles. Herein, we report using dendritic peptide bolaamphiphiles (bolas) to deliver siRNA to primary adipocytes and hepatocytes. The bola consists of two l-Lysine dendrons connected to a fluorocarbon core through disulfide linkages. The Lysine dendrons are functionalized with l-histidine and l-tryptophan to promote endosomal escape and cellular uptake. The bola exhibited over 70% knockdown of GAPDH gene in both primary adipocytes and hepatocytes. Importantly, different from Lipofectamine that significantly reduced genes involved in lipolysis, lipogenesis, fatty acid oxidation and ketogenesis, the bolas had little to no effect on these genes. These results demonstrate the bola as a promising new vector for clinical and experimental applications for delivery of siRNA to metabolic organs.

Tuning Supramolecular Structure and Functions of Peptide bola-Amphiphile by Solvent Evaporation-Dissolution.

Solvent molecules significantly affect the supramolecular self-assembly, for example, in forming solvent-bridged hydrogen bonding networks. Even small changes in solvent composition can have dramatic impact on supramolecular assembly. Herein, we demonstrate the use of trace solvents (as low as 0.04%) to tune the morphology and consequent functions of supramolecular nanostructures based on an aromatic peptide bola-amphiphile. Specifically, perylene bisimide-(di)glycine-tyrosine (PBI-[GY]2) bola-amphiphile was shown to give rise to red-emitting nanofibers when assembled in water, while exposure to trace organic solvents such as tetrahydrofuran (THF) and others via solvent-evaporation followed by aqueous assembly gave rise to white-light-emitting nanospheres. Differential hydrogen bonding between water (donor and acceptor) and THF (acceptor only) impacts supramolecular organization, which was verified using a density functional theory (DFT) simulation. The tunable consequent surface hydrophobicity was utilized in staining the cytoplasm and membrane of cells, respectively. The trace-solvent effect achieved through evaporation-dissolution provides a methodology to mediate the morphologies and consequent functions for supramolecular biomaterials controlled by the self-assembly pathway.

Self-Assembled Arginine-Capped Peptide Bolaamphiphile Nanosheets for Cell Culture and Controlled Wettability Surfaces.

The spontaneous assembly of a peptide bolaamphiphile in water, namely, RFL4FR (R, arginine; F, phenylalanine; L, leucine) is investigated, along with its novel properties in surface modification and usage as substrates for cell culture. RFL4FR self-assembles into nanosheets through lateral association of the peptide backbone. The L4 sequence is located within the core of the nanosheets, whereas the R moieties are exposed to the water at the surface of the nanosheets. Kinetic assays indicate that the self-assembly is driven by a remarkable two-step process, where a nucleation phase is followed by fast growth of nanosheets with an autocatalysis process. The internal structure of the nanosheets is formed from ultrathin bolaamphiphile monolayers with a crystalline orthorhombic symmetry with cross-β organization. We show that human corneal stromal fibroblast (hCSF) cells can grow on polystyrene films coated with films dried from RFL4FR solutions. For the first time, this type of amphiphilic peptide is used as a substrate to modulate the wettability of solid surfaces for cell culture applications.

Structure-Based Design of Dendritic Peptide Bolaamphiphiles for siRNA Delivery

Development of safe and effective delivery vectors is a critical challenge for the application of RNA interference (RNAi)-based biotechnologies. In this study we show the rational design of a series of novel dendritic peptide bolaamphiphile vectors that demonstrate high efficiency for the delivery of small interfering RNA (siRNA) while exhibiting low cytotoxicity and hemolytic activity. Systematic investigation into structure–property relationships revealed an important correlation between molecular design, self-assembled nanostructure, and biological activity. The unique bolaamphiphile architecture proved a key factor for improved complex stability and transfection efficiency. The optimal vector contains a fluorocarbon core and exhibited enhanced delivery efficiency to a variety of cell lines and improved serum resistance when compared to hydrocarbon analogues and lipofectamine RNAiMAX. In addition to introducing a promising new vector system for siRNA delivery, the structure–property relationships and “fluorocarbon effect” revealed herein offer critical insight for further development of novel materials for nucleic acid delivery and other biomaterial applications.

Lipase-catalyzed dissipative self-assembly of a thixotropic Peptide bolaamphiphile hydrogel for human umbilical cord stem-cell proliferation.

We report lipase-catalyzed inclusion of p-hydroxy benzylalcohol to peptide bolaamphiphiles. The lipase-catalyzed reactions of peptide bolaamphiphiles with p-hydroxy benzylalcohol generate dynamic combinatorial libraries (DCL) in aqueous medium that mimic the natural dissipative system. The peptide bolaamphiphile 1 (HO-WY-Suc-YW-OH) reacts with p-hydroxy benzylalcohol in the presence of lipase forming an activated diester building block. The activated diester building block self-assembles to produce nanofibrillar thixotropic hydrogel. The subsequent hydrolysis results in the dissipation of energy to form nonassembling bolaamphiphile 1 with collapsed nanofibers. The thixotropic DCL hydrogel matrix is used for 3D cell culture experiments for different periods of time, significantly supporting the cell survival and proliferation of human umbilical cord mesenchymal stem cells.

Peptide-Nanofiber-Supported Palladium Nanoparticles as an Efficient Catalyst for the Removal of N-Terminus Protecting Groups.

Sonication-induced tryptophan- and tyrosine-based peptide bolaamphiphile nanofibers have been used to synthesize and stabilize Pd nanoparticles under physiological conditions. The peptide bolaamphiphile self-assembly process has been thoroughly studied by using several spectroscopic and microscopic techniques. The stiffness of the soft hydrogel matrix was measured by an oscillatory rheological experiment. FTIR and circular dichroism (CD) experiments revealed a hydrogen-bonded β-sheet conformation of peptide bolaamphiphile molecules in a gel-phase medium. The π-π stacking interactions also played a crucial role in the self-assembly process, which was confirmed by fluorescence spectroscopy. Electron (SEM and TEM) and atomic force microscopy (AFM) studies showed that the peptide bolaamphiphile molecules self-assemble into nanofibrillar structures. Pd nanoparticles were synthesized in the hydrogel matrix in which redox-active tryptophan and tyrosine residues reduce the metal ions to metal nanoparticles. The size of the Pd nanoparticles are in the range of 3-9 nm, and are stabilized by peptide nanofibers. The peptide-nanofiber-supported Pd nanoparticles have shown effective catalytic activity for the removal of N-terminus protecting groups of amino acids and peptides.

Photophysical study of a π-stacked β-sheet nanofibril forming peptide bolaamphiphile hydrogel

We describe the state of molecular self-assembly of a peptide based bolaamphiphile molecule using spectroscopic and microscopic techniques. The tryptophan and phenylalanine containing peptide bolaamphiphile forms a hydrogel upon sonication under physiological conditions. Sonication helps to reorient the peptide molecules by providing the required energy for the self-assembly process. The disassembly and self-assembly processes are influenced by various stimuli, including heating–cooling and shaking–rest methods. The extensive hydrogen bonding and π–π stacking interactions are responsible for the self-assembly process, which is confirmed by FT-IR, temperature dependent NMR and fluorescence spectroscopy studies. FT-IR and powder X-ray diffraction studies reveal that the gelator molecules self-assemble into an antiparallel β-sheet type structure. The TEM image of the hydrogel shows a well-defined amyloid-like nanofibrillar structure. The amyloid-like behaviour of the fibril forming peptide bolaamphiphile hydrogel is confirmed by ThT and Congo red binding studies. The effect of concentration, time and temperature on the self-assembly mechanism of the peptide bolaamphiphile hydrogel is investigated by time resolved fluorescence spectroscopy.

Self-programmed nanovesicle to nanofiber transformation of a dipeptide appended bolaamphiphile and its dose dependent cytotoxic behaviour.

Nanostructural transition of a small peptide bolaamphiphile via molecular self-assembly is a challenging task. Here, we report the self-programmed morphological transformation from nanovesicles to nanofibers of a smart peptide bolaamphiphile in its self-assembling hydrogel state. The nanostructural transition occurs based on the structural continuity of the β-sheet structures. Spectroscopic studies confirmed the different molecular arrangements of the two different nanostructures. Furthermore, the smart bolaamphiphile shows a dose-dependent cytotoxicity and cell-proliferation behaviour.

Fabrication of Collagen–Elastin-Bound Peptide Microtubes for Mammalian Cell Attachment

In this work we have designed self-assembled peptide-based microconstructs and examined their interactions with elastin and collagen for potential application as scaffolds for chondrocyte cell attachment. Being biological in nature, peptide-based nano- and microstructures have intrinsic molecular recognition properties which allow extensive chemical, conformational and functional diversity. We have synthesized a new peptide bolaamphiphile, bis(N-α-amido-val)-1,5-pentane dicarboxylate, and examined its self-assembly at varying pH values. The formation of high-density networks of nano- and microtubular structures was found to be in the range of pH 4–6. The formed microtubes were then covalently bound to varying concentrations of the extracellular matrix protein elastin, a versatile protein that allows for an extensive array of physical and chemical modifications to attune properties towards diverse necessities of biomedical applications. We found that binding to microtubes was concentration dependent. The morphological and chemical changes complementing the processes of self-assembly and binding to elastin were examined by electron microscopic and spectroscopic methods. Furthermore, we also incorporated the extracellular matrix protein type-I collagen, a critical constituent for designing biocompatible scaffolds, into the elastin functionalized micro-tubes. Since the main goal is to develop highly biocompatible protein functionalized microstructures that support cellular interactions, we examined the interactions of the microcomposites with chondrocyte cell line, in order to assess the biocompatibility and interaction between the microconstructs and the cells. The designed elastin and collagen-bound peptide microtubes may potentially serve as a new class of biomaterials by promoting cell growth and proliferation.