Figure 1. (A) Basic concept of surface engineering of biomaterials to control the interaction between living matter and biomaterials. (B) Surface engineering includes modification on original surface and additional layer coating to control over surface properties. By altering surface characteristics, various purposes can be fulfilled including enhanced biocompatibility, antibacterial ability, cells regulation and delivery of bioactive agents for specific applications.
When extracorporeal devices such as orthopedic implants, drug-eluting stents, tissue engineering scaffolds and microfluidics first contact with the living matter of human body, the body would elicit a foreign body response involving inflammation, blood coagulation, fibrous encapsulation, and rejection in extreme cases[1].
Protein adsorption is the first major event in the interaction between the living matter and implanted devices. Subsequent events such as cellular activities and signaling pathways initiation are largely dependent on their interactions with the deposited protein layer. For example, the complement system can be activated by protein adsorption. Blood-coagulation process will be initiated for wound healing. Neutrophils are responsible for the acute inflammatory response; they will migrate to the interface and degrade the foreign objective. Monocytes will be recruited to the biomaterials-tissue interface and differentiated into macrophages attempting to eliminate foreign objects, which marks the chronic inflammatory response. Macrophages uptake the debris as well as injured tissue and clear them through phagocytosis. However, with a large mass of foreign objective, a “frustrated phagocytosis” occurs resulting in aggregation of macrophages to form multinucleated foreign body giant cells. Fibroblasts will be activated and secrets collagen fibers aligned parallel to the surface of biomaterials that forms a fibrous capsule. Fibrous encapsulation is always formed around the implant to screen it from the body. Those undesirable reactions result in destruction of local tissue as well as implants failure. Such non-specific protein adsorption is governed by protein properties such as protein structure, polarity and charge distribution, and features of biomaterial surface including chemistry and topography as well as environmental conditions including pH and temperature[2]. Engineering biomaterials with an anti-fouling surface will create a protein-resistance layer to improve their performance. Herein, we first described surface engineering methods to construct anti-fouling surface by underscoring the use of PEG in both macroscopic surface and nanoparticle system.
The adsorption of plasma protein will benefit the bacterial adhesion as well. Upon binding to the surface, bacteria will proliferate rapidly and secrete extracellular matrix leading to biofilm formation (Figure 2). The biofilm is a colony of immobilized bacteria on the surface of biomaterial that exhibits a robust structure. Bacterial biofilms are much harder to eradicate by antibiotics than circulating bacteria[3]. The biofilm formation results in device-related infections limiting the success of implant and medical interventions. Anti-biofouling surface formation is known as the “passive” strategy to address the problems of bacterial adhesion. “Active” strategy by construction of anti-bacterial surface is discussed as well. Various bactericidal substances are incorporated in surface modifications including silver ions, antimicrobial peptides, antibiotics and antibacterial polymers.