Simulation of extracellular matrix remodeling by fibroblast cells in soft three-dimensional bioresorbable scaffolds

To culture functional soft tissues and organs in three-dimensional scaffolds, it is essential to elucidate the optimal scaffold mechanical properties. However, mechanoregulated soft tissue remodeling is not well understood. In this study, we hypothesized that individual cells are capable of remodeling extracellular matrix within a short proximity of themselves in order to match the stiffness of the broader surrounding matrix. This theory was implemented in a three-dimensional finite element model to simulate soft tissue remodeling of human fibroblast cells in two collagen–chitosan scaffolds with different mechanical properties. Simulation results closely matched with previously reported experimental data, showing that soft tissue growth in compliant (Scaf-A, 4.30 kPa) and stiff (Scaf-B, 17.03 kPa) scaffolds led to an almost eightfold difference in the resulting stiffnesses after 10 days (8.40 kPa for Scaf-A, 59.25 kPa for Scaf-B). Furthermore, varying the simulated rate for tissue remodeling by \(\pm \)50 % caused unequal changes in the resulting stiffness (+3.6 and \(-\)23 % for Scaf-A, +5 and \(-\)17 % for Scaf-B), and \(\pm \)50 % changes in the assumed upper limit on tissue stiffness only had larger effects on the stiff scaffold (+42 and \(-\)44 % for Scaf-B). These results reinforce the notion that soft tissue remodeling is governed by the stiffness of the surrounding matrix, until meeting an upper limit on tissue stiffness.     

Enzyme design by chemical modification of protein scaffolds

Covalent modification methods allow an almost unlimited range of functionality to be introduced into proteins. In concert with genetic techniques, chemical strategies have had significant impact in the field of enzyme design. Major recent developments include introducing catalytic activity into inactive proteins, modifying the selectivity and/or reactivity of existing enzymes and designing novel enzyme-based biosensors. New chemical methods promise to further increase the range of functionality that can be incorporated into proteins. These results suggest that semi-synthetic methods will play a key role in the development of future biocatalysts.     

Current status of clinically available bioresorbable scaffolds in percutaneous coronary interventions

Drug-eluting stents (DES) are widely used as first choice devices in percutaneous coronary interventions. However, certain concerns are associated with the use of DES, i.e. delayed arterial healing with a subsequent risk of neo-atherosclerosis, late stent thrombosis and hypersensitivity reactions to the DES polymer. Bioresorbable vascular scaffolds are the next step in percutaneous coronary interventions introducing the concept of supporting the natural healing process following initial intervention without leaving any foreign body materials resulting in late adverse events. The first-generation devices have shown encouraging results in multiple studies of selected patients up to the point of full bioresorption, supporting the introduction in regular patient care. During its introduction in daily clinical practice outside the previously selected patient groups, a careful approach should be followed in which outcome is continuously monitored.     

Calvarial bone distraction with a contractile bioresorbable polymer

The aim of the present study was to evaluate the possibility of mobilizing calvarial bone with a fully implantable and bioresorbable device. The animal model used was the New Zealand white rabbit (n = 12). An island bone flap attached to the dura mater was created in the parietal region and amalgam markers were placed in this bone flap and in the ipsilateral frontal bone. In one group of six rabbits (group 1), a specially processed contractile 70L/30D,L polylactic acid plate, 15 x 6 x 0.6 mm, was attached to the island flap by one extremity, and to the fixed ipsilateral frontal bone by the other. In group 2 (control), no plate was added. Bone marker movement was followed with serial radiography. In group 1, there was a progressive reduction in mean marker distance over the first 48 hours, and stability thereafter. In group 2 (control), mean marker distance remained stable until the second postoperative week, after which time there was a slight increase until the end of the experimental period. At 4 weeks, the mean marker separation differed significantly between group 1 (mean, -3.62 mm; SD, 0.79 mm) and group 2 (mean, 0.34 mm; SD, 0.14 mm; p <0.001). In conclusion, a totally implantable and bioresorbable device was successfully used to mobilize calvarial bone. Polymer contractility will likely constitute the basis of a new generation of bioresorbable distractors for use in craniofacial surgery.     

Recommendations for the use of bioresorbable vascular scaffolds in percutaneous coronary interventions

Background

To eliminate some of the potential late limitations of permanent metallic stents, the bioresorbable coronary stents or ‘bioresorbable vascular scaffolds’ (BVS) have been developed.

Methods

We reviewed all currently available clinical data on BVS implantation.

Results

Since the 2015 position statement on the appropriateness of BVS in percutaneous coronary interventions, several large randomised trials have been presented. These have demonstrated that achieving adequate 1 and 2 year outcomes with these first-generation BVS is not straightforward. These first adequately powered studies in non-complex lesions showed worse results if standard implantation techniques were used for these relatively thick scaffolds. Post-hoc analyses hypothesise that outcomes similar to current drug-eluting stents are still possible if aggressive lesion preparation, adequate sizing and high-pressure postdilatation are implemented rigorously. As long as this has not been confirmed in prospective studies the usage should be restricted to experienced centres with continuous outcome monitoring. For more complex lesions, results are even more disappointing and usage should be discouraged. When developed, newer generation scaffolds with thinner struts or faster resorption rates are expected to improve outcomes. In the meantime prolonged dual antiplatelet therapy (DAPT, beyond one year) is recommended in an individualised approach for patients treated with current generation BVS.

Conclusion

The new 2017 recommendations downgrade and limit the use of the current BVS to experienced centres within dedicated registries using the updated implantation protocol and advise the prolonged usage of DAPT. In line with these recommendations the manufacturer does not supply devices to the hospitals without such registries in place.

     

Spatio-temporal modification of collagen scaffolds mediated by triple helical propensity

Functionalized collagen that incorporates exogenous compounds may offer new and improved biomaterials applications, especially in drug-delivery, multifunctional implants, and tissue engineering. To that end, we developed a specific and reversible collagen modification technique utilizing associative chain interactions between synthetic collagen mimetic peptide (CMP) [(ProHypGly) chi; Hyp = hydroxyproline] and type I collagen. Here we show temperature-dependent collagen binding and subsequent release of a series of CMPs with varying chain lengths indicating a triple helical propensity driven binding mechanism. The binding took place when melted, single-strand CMPs were allowed to fold while in contact with reconstituted type I collagens. The binding affinity is highly specific to collagen as labeled CMP bound to nanometer scale periodic positions on type I collagen fibers and could be used to selectively image collagens in ex vivo human liver tissue. When heated to physiological temperature, bound CMPs discharged from the collagen at a sustained rate that correlated with CMP's triple helical propensity, suggesting that sustainability is mediated by dynamic collagen-CMP interactions. We also report on the spatially defined modification of collagen film with linear and multi-arm poly(ethylene glycol)-CMP conjugates; at 37 degrees C, these PEG-CMP conjugates exhibited temporary cell repelling activity lasting up to 9 days. These results demonstrate new opportunities for targeting pathologic collagens for diagnostic or therapeutic applications and for fabricating multifunctional collagen coatings and scaffolds that can temporally and spatially control the behavior of cells associated with the collagen matrices.     

Prevention of biofilm formation by polymer modification

Bacterial biofilm formation on synthetic polymers plays an important role in industry and in modern medicine, leading, for example, to difficult-to-treat infections caused by colonized foreign bodies. Prevention of biofilm formation is a necessary step in the successful prophylaxis of such infections. One approach is to inhibit bacterial adherence by polymer surface modification. We have investigated polymer modification by glow discharge treatment in order to study the influence of the modified surface on bacterial adherence. Surface roughness, surface charge density and contact angles of the modified polymers were determined and related to the adherence ofStaphylococcus epidermidis KH6. Although no influence of surface roughness and charge density on bacterial adherence was noticed, a correlation between the free enthalpy of adhesion (estimated from contact angle measurements) and adherence was observed. There seems to exist a certain minimum bacterial adherence, independent of the nature of the polymer surface. Modified polymers with negative surface charge allow for bacterial adherence close to the adherence minimum. These polymers could be improved further by the ionic bonding of silver ions to the surface. Such antimicrobial polymers are able to prevent bacterial colonization, which is a prerequisite for biofilm formation. It is suggested that modification of polymers and subsequent surface coupling of antimicrobials might be an effective approach for the prevention of bacterial biofilm formation.     

Appropriate use of bioresorbable vascular scaffolds in percutaneous coronary interventions: a recommendation from experienced users: A position statement on the use of bioresorbable vascular scaffolds in the Netherlands

Percutaneous coronary interventions (PCI) have become a reliable revascularisation option to treat ischaemic coronary artery disease. Drug-eluting stents (DES) are widely used as first choice devices in many procedures due to their established good medium to long term outcomes. These permanent implants, however, do not have any residual function after vascular healing following the PCI. Beyond this initial healing period, metallic stents may induce new problems, resulting in an average rate of 2 % reinterventions per year. To eliminate this potential late limitation of permanent metallic DES, bioresorbable coronary stents or ‘vascular scaffolds’ (BVS) have been developed. In a parallel publication in this journal, an overview of the current clinical performance of these scaffolds is presented. As these scaffolds are currently CE marked and commercially available in many countries and as clinical evidence is still limited, recommendations for their general usage are needed to allow successful clinical introduction.     

Renal tissue reconstitution by the implantation of renal segments on biodegradable polymer scaffolds

Renal units were created in vivo by transplanting isolated renal segments on three-dimensional, biodegradable polymer scaffolds. Renal segments, freshly isolated from rat kidneys, were seeded on polymer scaffolds and subcutaneously implanted in athymic mice for two and four weeks. Three-dimensional renal reconstructs were formed with glomeruli and tubules, showing a possibility of reconstituting renal structures by transplanting renal segments.     

Functionalized synthetic biodegradable polymer scaffolds for tissue engineering

Scaffolds (artificial ECMs) play a pivotal role in the process of regenerating tissues in 3D. Biodegradable synthetic polymers are the most widely used scaffolding materials. However, synthetic polymers usually lack the biological cues found in the natural extracellular matrix. Significant efforts have been made to synthesize biodegradable polymers with functional groups that are used to couple bioactive agents. Presenting bioactive agents on scaffolding surfaces is the most efficient way to elicit desired cell/material interactions. This paper reviews recent advancements in the development of functionalized biodegradable polymer scaffolds for tissue engineering, emphasizing the syntheses of functional biodegradable polymers, and surface modification of polymeric scaffolds.     

Site-specific polymer modification of therapeutic proteins

Recent advances in chemoselective ligation technology have made possible the modification of proteins with polymers in a site-specific and controlled manner. These approaches rely on the incorporation of chemoselective anchors into the protein backbone by either chemical or recombinant means, and subsequent modification with a polymer carrying a complementary linker. As a result, the assembly process and the covalent structure of the resulting protein-polymer conjugate are completely controlled, enabling the rational optimization of drug properties, in particular efficacy and pharmacokinetic properties. Application of chemoselective ligation technologies to cytokines and chemokines has led to the generation of new lead proteins for use as erythropoietic agents and HIV fusion inhibitors.     

Synthesis of versatile thiol-reactive polymer scaffolds via RAFT polymerization

Well-defined polymer scaffolds convertible to (multi)functional polymer structures via selective and efficient modifications potentially provide an easy, versatile, and useful approach for a wide variety of applications. Considering this, a homopolymer scaffold, poly(pyridyldisulfide ethylmethacrylate) (poly(PDSM)), having pendant groups selectively reactive with thiols, was synthesized by reversible addition fragmentation chain transfer (RAFT) polymerization. Soluble polymers with controlled molecular weights and narrow PDIs were generated efficiently. The versatility of the scaffold to generate random co- and ter-polymers combining multiple functionalities with controlled-composition was shown by separate and simultaneous conjugation of different mercapto-compounds, including a tripeptide in one-step. Conversion of water-insoluble scaffold to peptide-containing water-soluble copolymers was observed to yield nanometer-size particles with narrow polydispersity. The overall results suggest that the well-defined PDSM homopolymer scaffold generated via RAFT polymerization can be a versatile building block for generation of new structures having potential for drug delivery applications via a straightforward synthetic approach.     

Phosphodiester modification by zinc metalated adenine polymer with carboxyl pendants

This report describes a novel carboxyl pendant containing adenylated polymeric template, its metalation with Zn (II), and manifestation of catalytic activity for the hydrolysis of model phosphodiester, bis(p-nitrophenyl) phosphate (bNPP), and plasmid cleavage. Observation of a bell-shaped pH-K(obs) profile suggested influence of pH variation over hydrolysis rate. This metalated polymer also afforded facile relaxation of pBR322 supercoiled DNA, with an interesting reusability feature intricately associated with heterogeneous catalysis.     

Signaling protein inhibitors via the combinatorial modification of peptide scaffolds

Compounds that selectively interfere with protein-protein interactions are not only invaluable as biological reagents, but may ultimately serve as therapeutically useful drugs for the treatment of a wide variety of disease states. However, unlike active site directed inhibitors that bind to a relatively small, well-defined, hydrophobic pocket, reagents that disrupt protein-protein interactions must contend with a protein surface that is comparatively large, ill defined, and solvent exposed. We have developed a straightforward method for the acquisition of protein-protein interaction inhibitors. The library-based strategy starts with low affinity consensus sequence peptides, which are then transformed in a stepwise fashion into high affinity inhibitors. The approach has been used to create potent ligands for SH2 and SH3 domains, as well as powerful and highly selective inhibitors for protein kinases and phosphatases. The protocol is easily automated and therefore has the potential to be routinely applied, in a high throughput fashion.     

Comparison of early vascular morphological changes between bioresorbable poly-L-lactic acid scaffolds and metallic stents in porcine iliac arteries

Bioresorbable scaffolds have the potential to overcome several problems associated with metallic stents. Bioresorbable poly-L-lactic acid (PLLA) scaffold implantation for the treatment of peripheral artery disease has already been reported in animal models and clinical trials; however, no studies comparing PLLA scaffolds and bare metal stents (BMSs) with regard to early vascular morphological changes, identified using intravascular ultrasound (IVUS) analysis, have been reported. In this study, PLLA scaffolds and BMSs were implanted bilaterally in iliac arteries of five miniature pigs. Digital subtraction angiography and IVUS were performed before and immediately after stent implantation and at 6-week follow-up. All PLLA scaffolds and BMSs were patent at 6-week follow-up. Per IVUS analysis, the percent area stenosis did not significantly differ between PLLA scaffolds and BMSs (65.7% vs. 67.2%, P = .761). Furthermore, percent vessel lumen change also did not differ significantly. Neointima formation (the neointimal area plus medial area) was significantly less with PLLA scaffolds than with BMSs (15.65 mm² vs. 25.69 mm², P < .001). In conclusion, based on IVUS results, short-term results after stent implantation in porcine iliac arteries were comparable between PLLA scaffolds and BMSs. Therefore, PLLA scaffolds are safe and feasible for implantation in peripheral arteries.     

A numerical model of heterogeneous surface strains in polymer scaffolds

In vitro bone tissue growth inside porous scaffolds can be enhanced by macroscopic cyclic compression of the construct, but the heterogeneous strain generated inside the construct must be investigated to determine appropriate levels of compression. For this purpose a linear micro-finite element (muFE) technique based on micro-computed tomography (muCT) was verified for the calculation of local displacements inside polymer scaffolds, from which local strains may be estimated. Local displacements in the axial direction at the surface of microstructures inside the scaffold in 60 locations were calculated with the muFE model, based on compression simulation of a muCT reconstruction of the scaffold. These displacements were compared with accurately measured displacements in the axial direction in the same polymer scaffold at the same 60 locations, using a micro-compression chamber and muCT reconstructions of the scaffold under two fixed levels of compression (5% and 0%). The correlation between the calculated and the measured displacements, after correction for the dependence of the axial displacement on the axial position, was r=0.786 (r2=0.617). From this we conclude that the linear muFE model is suitable to estimate local surface strains inside polymer scaffolds for tissue engineering applications. This technique can not only be used to determine appropriate parameters such as the level of macroscopic compression in experimental design, but also to investigate the cellular response to local surface strains generated inside three-dimensional scaffolds.