Tissue-implant interactions

As better materials and more sophisticated devices are implanted in humans, new problems in the biocompatibility of these materials and devices will appear as a result of the improved functional longevity of the materials and devices. These so-called second generation problems will be recognized and solved by a better understanding of the tissue/implant interaction. 

Interactions at surfaces and interfaces also play an essential role in the design and function of clinical implants and biomedical devices. With a few recent exceptions, implants do not attach well to tissue, and the resulting mobility of the tissue-implant interface encourages chroitic inflammation. The result can be a gathering of platelets at the site, leading to a blood clot or to the formation of a fibrous capsule, or scar, around the implant (Figure 3.3). 

The porous surface may enhance tissue-polymer interactions, thus increasing the compatibility of implant prostheses. 

Using the above multimicroprocessed surfaces, which are manufactured by semiautomatic manipulations, early events of tissue-material interactions possibly predicting the tissue compatibility of implant materials have been... 

General Tissue-Fluid Interactions. The exact response of the body to any implant depends not only on the chemical composition of the implant but also on the form of the polymer (sheet, fiber, foam, etc.), the shape of the implant, whether the implant can move, and the location of the implant within the body. (Infection can also occur if proper sterilization techniques are not used.) The reaction of the body can vary from a relatively benign acceptance of the implant to an outright rejection of the material with an attempt, by the body, to extrude the implant and/or to destroy the implant by chemical means. Chemical destruction is usually... 

A wide number and great variety of clinically important cardiovascular implants and devises exist. Some (e.g., catheters) may only contact the blood once, and for a relatively short time others (e.g., kidney dialyzers and blood oxygenators) may be exposed to blood for hours, while tissue implants (e.g., heart valves and vascular grafts) will hopefully last for years, or the lifetime of the patient. All of these implants nd devices contain materials that are recognized by blood as foreign the result is a process of thrombosis often followed by formation of thromboemboli. This process generally involves a sequence of protein adsorption steps followed by blood cell interactions (especially involving platelets). 

The foreign body reaction occurring around soft tissue implants and thrombosis on surfaces in contact with blood are the major reactions encountered with implants. Both reactions involve the interaction of cells with the implant, especially in the later stages, and much previous study has therefore emphasized cellular events in the biocompatibility process. However, cells encounter foreign polymer implants under conditions that ensure the prior adsorption of a layer of protein to the polymer interface. The properties of the adsorbed layer are therefore important in mediating cellular response to the material. 

The placement procedure initiates a response to injury by the tissue, organ, or body and mechanisms are activated to maintain homeostasis. The degrees to which the homeostatic mechanisms are perturbed and the extent to which pathophysiologic conditions are created and tmdergo resolution are a measure of the host reactions to the biomaterial and may ultimately determine its biocompatibility. Although it is convenient to separate homeostatic mechanisms into blood-material or tissue-material interactions, it must be remembered that the various components or mechanisms involved in homeostasis are present in both blood and tissue and are a part of the physiologic continuum. Furthermore, it must be noted that host reactions may be tissue dependent, organ dependent, and species dependent. Obviously, the extent of injury varies with the implantation procedure. 

Wu and co-workers [13] incorporated copolymer poly(3HB-co-3HV) with calcium silicate(s) (CS) to increase the hydrophilicity of the copolymer in order to enhance cell adhesion on scaffolds used for cartilage tissue engineering. Interactions between poly(3HB-co-3HV)/CS composite scaffolds and chondrocytes in vitro and the formation of neocartilage were evaluated after the implantation of scaffolds into rabbits. It was found that the adhesion of chondrocytes onto the scaffolds and cell proliferation improved with the addition of CS. Enhanced penetration of chondrocytes into the scaffolds was observed with the increase in hydrophilicity of the poly(3HB-co-3HV)/CS composite scaffolds. A higher amount of collagen and glycosaminoglycan were detected in the composite scaffold compared with pure poly(3HB-co-3HV), indicating that poly(3HB-co-3HV)/CS composite scaffolds stimulated the extracellular matrix synthesis of chondrocytes. 

In the musculoskeletal system, bone is the primary tissue/organ interacting with prosthetic implants/biomaterials and their interface is a crucial region where the interactions pertinent to new tissue formation and implant efficacy occur. Bone is a complex biological system that comprises both hierarchical structures and living boneremodeling components. The architecture of bone is composed of nanoscale fibrous... 

On the other hand, retrieval studies in animals permit detailed monitoring of biocompatibility, biofunctionahty and biostabiUty of the implants at any desired and prescheduled interval of time. They also allow detailed studies of tissue-device interactions at the surface of the biotexthe since the surrounding tissue can be removed as appropriate. Animal models facilitate the study of certain complications such as caldfication in an accelerated time frame. Moreover, experimental conditions can be held constant in animal studies. However, animal models do not rephcate exact human physiological behavior, particularly for iU and injured patients with particular pathologies. 

An important further development of the retrieval study would be the simulation and modeling of in vivo performance of the biotextile implant. By simulating the various parameters such as blood flow and tissue-material interactions it may in the future be possible to predict such biological response variables as thrombosis, aneurysm formation and the rate of healing. 

Adsorption of proteins from solution onto synthetic materials is a key factor in the response of a living body to artificial implanted materials and devices. Adsorbed proteins mediate cell attachment and spreading through specific peptide sequence-integrin receptor interactions and may therefore favorably influence the mechanical stability of the subsequently developed tissue—implant interface. However, the uncontrolled nonspecific adsorption of proteins from the extracellular matrix results in interfaces with many types of proteins in different conformations—a situation that is believed to cause deleterious reactions of the body, such as foreign-body response and fibrous encapsulation. ... 

Considering these mechanisms of interaction between an electrochemical device and a tissue, it is necessary to investigate overall tissue and tissue-implant interfacial impedance responses for several reasons. Firstly, studying interfacial impedance response allows determining the... 

When human MSCs are cultured with human umbilical vein endothelial cells (HUVECs) in suspension, they can spontaneously form spheroids. In one particular approach, this phenomenon was used to fabricate a 3D prevascular network that could be maintained after implantation (Rouwkema et al. 2006). Direct interaction between endothehal cells and MSCs also resulted in an upregulation of osteogenic markers such as ALP and mineralization in vivo (Kaigler et al. 2005). For larger tissue implants, the initial stage of neovascularization by endothelial cells was implicated in greater bone regeneration in a critical-sized defect (Seebach et al. 2010). The complementary roles that endothelial cells and MSCs play may sustain viable bone formation. 

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