MSE-536

Wound Healing and the Presence of Biomaterials

Topics:

•Formation of Granulation Tissue

•Foreign Body Reaction

•Fibrous Encapsulation

•Chronic Inflammation

•Types of Implant Resolution

•Repair vs. Regeneration

•In Vivo Assays for Inflammatory Response

MSE-536

Responses following injury:

2. Acute inflammation:

• activation of neutrophils

• phagocytosis of foreign bodies

•release of hyaluronic acid and glycosaminoglycan (chemoattractants) into ECM.

3. Inflammatory response:

influx of fibroblasts into ECM

beginnings of granulation tissue formation

generation of new blood vessels

deposition of type III collagen fibers (thin and randomly oriented)

fibrin clot is dissolved, enzymes released and phagocytosis continues

1. Blood clotting and formation of fibrin network 4. Remodeling and scar

formation:

•Type III collagen replaced by type I collagen: collagen bundles are larger and oriented with principal lines of stress in tissue

•Increased amounts of chemicals such as chrondroitin and dermatan sulfate

•Scar tissue continues to form for several months

•Blood vessels that are unattached are resorbed

•Scar becomes pale and avascular

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Definition:

Granulation Tissue: characterized by a pebbly, granular appearance caused by the creation of many vascular buds sprouting from existing blood vessels. This process is called neovascularization or angiogenesis.

Fibroblasts: committed cell type found in many tissues. Fibroblasts synthesize and maintain

connective tissues by producing and extracellular matrix (ECM) rich in collagen and proteoglycans. Fibroblasts with features of smooth muscle cells

are called myofibroblasts and are responsible for wound contraction.

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Formation of Granulation Tissue

Wound-healing response of the body after injury or biomaterial

implantation

Granulation tissue formation at the tissue/material interface. (G)

zone of granulation tissue separates the spleen (S) from

the polymer implant (I)

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Foreign Body ReactionDefinition:

Foreign Body Giant Cells (FBGCs): multinucleated cells formed by fusion of monocytes/macrophages in an attempt to phagocytose biomaterials much larger than a single cell.

Large cells: FGGCs

Small Cells: macrophages

Bright particles: polypropylene

Factors affecting Foreign Body Composition:

1. Topography

2. Surface Chemistry

Factors affecting Foreign Body Reaction:

1. Shape

2. Surface/Volume Ratio

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(a) Foreign body reaction to embedded PMMA. Arrow points to macrophages in tissue

(b) Foreign body reaction to large particles of UHMWPE showing macrophages and FBGCs

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In vivo response to a biodegradable, polymeric biomaterial implanted in a rat for 12 weeks. (a) 4 days (b) 3 weeks (c) 12 weeks. P indicates polymer, or space left by polymer; N: neutrophils, FC: fibrous capsule, M: macrophages, PF: polymer fragments embedded in fibrous capsule. Infiltration of neutrophils into implantation area is seen within a few days, followed by slower development of fibrous capsule surrounding implant. Because material is biodegradable, polymer fragmentation is present at later times.

Fibrous Encapsulation

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Fibrous Encapsulation, cont.Final stage of healing for implants made of nondegradable

materials.Steps in granulation tissue maturation:

•Presence of larger blood vessels

•Alignment of collagen fibers in response to local mechanical forces

•Collapse of capsule surrounding implant and formation of a scar

Factors affecting capsule formation:

•Degree of original injury during implantation

•Amount of subsequent cell death

•Location of implant site

•Degradation time of implant

Factors affecting capsule thickness:

•Amount and composition of small particulates produced

•Mechanical factors at implant site

•Shape of implant

•Electrical currents

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Chronic InflammationCharacterized by the presence of mononuclear cells, including lymphocytes and plasma cells

Can include presence of granulomas – a layered structure comprised of a nonphagocytosable particle surrounded by a layer of FBGCs, a layer of modified macrophages called Epithelioid cells, and surrounded by a layer of lymphocytes

Subcutaneous model showing:

•Polymer hydrogel implant (h)

•Macrophages (right arrow)

•Lymphocytes (left arrow)

•C: beginning of fibrous capsule

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Four types of implant response resolution:

1. Extrusion: material forced out of the body (e.g. splinter)

2. Resorption: material biodegrades, no fibrous capsule forms

3. Integration: implant and host tissue grow together (e.g. porous titanium implant in bone)

4. Encapsulation: implant surrounded by fibrous tissue

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Repair vs. RegenerationWound healing in Skin

Repair involves healing of the internal dermal layer

Regeneration is regrowth of thin outer epidermal layer

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Skin Regeneration

In the epidermis, this process is called reepithelilization.

Cells at edge of wound flatten to cover more of the wound, releasing attachment to ECM to migrate across wound

Epithelial cells gradually cover the entire wound site

ECM attachments are reestablished, and cells recover original shape

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In vivo Assays for Inflammatory Response

Items in the table at right may cause biological

response through:

•Interactions of biomolecules (e.g. proteins and ions) or cells with implant

•Interactions of biomolecules or cells with soluble agents leached from implant

•Interactions of biomolecules or cells with insoluble particulates

•Alterations in load or strain in the area around the implant

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Biocompatibility: the ability of a medical device to perform with an appropriate host response in a specific application.

Biocompatible assessment: a measurement of the magnitude and duration of the adverse alterations in homeostatic mechanisms that determine the host response.

Evaluation of biocompatibility usually involves exposing a small animal to the selected biomaterial or its extract through injection or implantation.

Two primary reasons to carry out biocompatibility tests:

1. Screen novel materials to learn degree & type of inflammation response

2. Assess inflammation response to the material in a form very similar to that which will be implanted

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Choice of Animal:

•Select on similarity of physiology and healing response to that of humans in a given application

•Start with a small animal (e.g., rat, rabbit) and scale up as warranted

Choice of Implant Site:

•As close as possible to that used in final application

•Use accessible site (subcutaneous pouch) to check for inflammation response.

•Identify parameters that may affect degree of inflammation

Length of Study:

•Acute toxicity: up to 24 hours

•Subacute toxicity: 14-28 days

•Subchronic toxicity: up to first 90 days

•Chronic: > 90 days

Dose: should be same shape as final product.

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Factors that can affect dose in addition to shape in direct implants

1. Implant weight/bulk size

2. Implant surface area

3. Implant roughness

4. Number of implants per animal

Biomaterials may be introduced via:

1. Direct implantation

2. Injection of soluble products

3. Placing in a “cage” to isolate biomaterial – cage may affect inflammatory response. l

Stainless steel cage implant model. This allows investigators to examine inflammatory response without direct contact between biomaterial and

surrounding tissue.

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The End