Antibacterial approaches in tissue engineering using metal ions and nanoparticles: From mechanisms to applications

Bacterial infection of implanted scaffolds may have fatal consequences and, in combination with the emergence of multidrug bacterial resistance, the development of advanced antibacterial biomaterials and constructs is of great interest. Since decades ago, metals and their ions had been used to minimize bacterial infection risk and, more recently, metal-based nanomaterials, with improved antimicrobial properties, have been advocated as a novel and tunable alternative. A comprehensive review is provided on how metal ions and ion nanoparticles have the potential to decrease or eliminate unwanted bacteria. Antibacterial mechanisms such as oxidative stress induction, ion release and disruption of biomolecules are currently well accepted. However, the exact antimicrobial mechanisms of the discussed metal compounds remain poorly understood. The combination of different metal ions and surface decorations of nanoparticles will lead to synergistic effects and improved microbial killing, and allow to mitigate potential side effects to the host. Starting with a general overview of antibacterial mechanisms, we subsequently focus on specific metal ions such as silver, zinc, copper, iron and gold, and outline their distinct modes of action. Finally, we discuss the use of these metal ions and nanoparticles in tissue engineering to prevent implant failure.

Biological Roles and Delivery Strategies for Ions to Promote Osteogenic Induction

Bone is the most studied tissue in the field of tissue regeneration. Even though it has intrinsic capability to regenerate upon injury, several pathologies and injuries could hamper the highly orchestrated bone formation and resorption process. Bone tissue engineering seeks to mimic the extracellular matrix of the tissue and the different biochemical pathways that lead to successful regeneration. For many years, the use of extrinsic factors (i.e., growth factors and drugs) to modulate these biological processes have been the preferred choice in the field. Even though it has been successful in some instances, this approach presents several drawbacks, such as safety-concerns, short release profile and half-time life of the compounds. On the other hand, the use of inorganic ions has attracted significant attention due to their therapeutic effects, stability and lower biological risks. Biomaterials play a key role in such strategies where they serve as a substrate for the incorporation and release of the ions. In this review, the methodologies used to incorporate ions in biomaterials is presented, highlighting the osteogenic properties of such ions and the roles of biomaterials in controlling their release.

A Dual Molecular Biointerface Combining RGD and KRSR Sequences Improves Osteoblastic Functions by Synergizing Integrin and Cell-Membrane Proteoglycan Binding

Synergizing integrin and cell-membrane heparan sulfate proteoglycan signaling on biomaterials through peptidic sequences is known to have beneficial effects in the attachment and behavior of osteoblasts; however, controlling the exact amount and ratio of peptides tethered on a surface is challenging. Here, we present a dual molecular-based biointerface combining integrin (RGD) and heparin (KRSR)-binding peptides in a chemically controlled fashion. To this end, a tailor-made synthetic platform (PLATF) was designed and synthesized by solid-phase methodologies. The PLATF and the control linear peptides (RGD or KRSR) were covalently bound to titanium via silanization. Physicochemical characterization by means of contact angle, Raman spectroscopy and XPS proved the successful and stable grafting of the molecules. The biological potential of the biointerfaces was measured with osteoblastic (Saos-2) cells both at short and long incubation periods. Biomolecule grafting (either the PLATF, RGD or KRSR) statistically improved (p < 0.05) cell attachment, spreading, proliferation and mineralization, compared to control titanium. Moreover, the molecular PLATF biointerface synergistically enhanced mineralization (p < 0.05) of Saos-2 cells compared to RGD or KRSR alone. These results indicate that dual-function coatings may serve to improve the bioactivity of medical implants by mimicking synergistic receptor binding.

Antimicrobial Peptides: Powerful Biorecognition Elements to Detect Bacteria in Biosensing Technologies

Bacterial infections represent a serious threat in modern medicine. In particular, biofilm treatment in clinical settings is challenging, as biofilms are very resistant to conventional antibiotic therapy and may spread infecting other tissues. To address this problem, biosensing technologies are emerging as a powerful solution to detect and identify bacterial pathogens at the very early stages of the infection, thus allowing rapid and effective treatments before biofilms are formed. Biosensors typically consist of two main parts, a biorecognition moiety that interacts with the target (i.e., bacteria) and a platform that transduces such interaction into a measurable signal. This review will focus on the development of impedimetric biosensors using antimicrobial peptides (AMPs) as biorecognition elements. AMPs belong to the innate immune system of living organisms and are very effective in interacting with bacterial membranes. They offer unique advantages compared to other classical bioreceptor molecules such as enzymes or antibodies. Moreover, impedance-based sensors allow the development of label-free, rapid, sensitive, specific and cost-effective sensing platforms. In summary, AMPs and impedimetric transducers combine excellent properties to produce robust biosensors for the early detection of bacterial infections.

Regenerating Bone via Multifunctional Coatings: The Blending of Cell Integration and Bacterial Inhibition Properties on the Surface of Biomaterials

In dentistry and orthopedics, it is well accepted that implant fixation is a major goal. However, an emerging concern is bacterial infection. Infection of metallic implants can be catastrophic and significantly reduce patient quality of life. Accordingly, in this work, we focus on multifunctional coatings to simultaneously address and mitigate both these problems. We have developed a tailor-made peptide-based chemical platform that integrates the well-known RGD cell adhesive sequence and the lactoferrin-derived LF1-11 antimicrobial peptide. The platform was covalently grafted on titanium via silanization and the functionalization process characterized by contact angle, XPS, and QCM-D. The presence of the platform statistically improved the adhesion, proliferation and mineralization of osteoblast-like cells compared to control surfaces. At the same time, colonization by representative bacterial strains was significantly reduced on the surfaces. Furthermore, the biological potency of the multifunctional platform was verified in a co-culture in vitro model. Our findings demonstrate that this multifunctional approach can be useful to functionalize biomaterials to both improve cell integration and reduce the risk of bacterial infection.