BME 320: Lecture #1-2 Review (Key Terminology, Historical Trajectory of Biomaterials)

biomaterial

A material intended to interface with biological systems to evaluate, treat, augment, or replace any tissue, organ, or function of the body

biomaterials science

The physical and biological study of materials and their interaction with the biological environment

The goal of biomaterials is to . . . as . . . and . . .

ensure a good/high quality of life; people age; the body naturally begins to break down

biocompatibility

The ability of a biomaterial to be tolerated by the body and coexist with cells, tissues, and organs without causing any adverse reaction or toxicity

Despite general understanding, no formal definition exists for . . .

biocompatibility

In the case of non-biocompatibility, toxicity may be . . . or . . ., . . . or . . ., and may be . . . or . . .

local; systemic; negligible; harsh; acute; manifest after a long time

bioinert

A material that serves a purely physical and/or mechanical function but otherwise remains biologically inert inside the body (i.e. does not influence cell or tissue behavior)

bioactivity

The ability of a material to actively stimulate or influence cell and tissue behavior (e.g. by inducing upregulation or downregulation of genes leading to changes in cell function)

Bioactivity can be achieved through the release of . . ., . . ., . . ., etc. and/or through . . . and . . .

ions; bioactive signals; cytokines; drugs; biomaterial structure; chemical characteristics

Examples of bioactive effects: (3)

(1) cell migration; (2) cell proliferation; (3) differentiation (cell type change)

bioresorbable

The ability of a natural material to resorb (i.e. dissolve/degrade/erode) over time after implantation in the body

Discuss the use of biomaterials in the pre-industrial era.

From ancient civilizations In the pre-industrial era, spanning ancient civilizations through the onset of the Industrial Revolution, biomaterial use was largely constrained to naturally available materials such as wood, bone, ivory, shells, linen, and precious metals. Although scientific understanding of human anatomy, physiology, and immune function was limited, early practitioners demonstrated substantial empirical knowledge by observing clinical outcomes and adapting material choices accordingly. Through repeated experience, they recognized phenomena that would now be described as implant rejection, toxicity, allergic responses, and material failure.

Many of the clinical challenges faced during this period parallel those encountered in modern biomaterials science, including fracture repair, tissue loss, infection, and the long-term compatibility of implanted materials. Ancient surgeons and healers sought to restore function and replace damaged tissues using accessible materials, often evaluating success through observation of healing, pain, and survival outcomes. These early efforts established the foundational principle of biomaterials science: selecting materials that can interact with the human body while minimizing adverse biological responses and maximizing therapeutic benefit.

Examples of pre-industrial biomaterial use: (5)

(1) wooden toe prosthesis (c. 1065-740 B.C.); (2) animal sinew as suture; (3) nacre as a natural ceramic; (4) glass eyes; (5) dentures using gold setting (inert, malleable)

Explain why a pre-industrial toe prosthesis (c. 1065-740 B.C.) was selectively fabricated with wood as opposed to other materials (e.g. metals).

The selection of wood for a pre-industrial toe prosthesis was likely driven by a combination of biomechanical, practical, and safety considerations. From a functional perspective, wood possesses a lower density than most metals and may more closely approximate the mass and mechanical behavior of the missing digit. This reduced weight would minimize disruption to gait mechanics and locomotion, thereby improving the prosthesis's ability to restore normal foot function. In modern biomaterials terms, wood would have offered more favorable biomechanical compatibility for a load-bearing appendage than a substantially heavier metallic alternative.

Wood also provides thermal advantages. As a relatively poor conductor of heat, it is less susceptible to rapid temperature fluctuations during daily use. By contrast, metals readily conduct thermal energy and could become uncomfortably hot under prolonged sunlight exposure, creating the potential for tissue irritation or even thermal injury. Although ancient users would not have understood thermal conductivity at a materials-science level, they could readily observe and avoid materials associated with discomfort or harm.

Finally, wood was generally more accessible, easier to shape into anatomically relevant geometries, and simpler to repair or replace than metal. Fabrication of metallic prostheses would have required specialized resources and metallurgical expertise, whereas wood could be worked using comparatively simple tools. Consequently, wood represented a practical biomaterial choice that balanced functionality, user comfort, manufacturability, and availability, all of which remain important design considerations in contemporary biomaterials engineering.

Biomaterial idea-to-clinic trajectory, steps and facilitators: (10)

(1) Step: Identify a need, Facilitator(s): Physician/dentist, researcher, inventor, entrepreneur; (2) Step: Device design, Facilitator: Physician (consults), engineer (implements); (3) Step: Materials synthesis, Facilitator: Ceramicist, metallurgist, polymer chemist; (4) Step: Materials testing, Facilitator: Bioengineer, mechanical engineer, biochemist, cell biologist, veterinary surgeon; (5) Step: Fabrication, Facilitator: Engineer, machinist; (6) Step: Sterilization and packaging, Facilitator: Bioengineer, industrial designer; (7) Step: Device testing, Facilitator: Bioengineer, veterinary surgeon, physician/dentist; (8) Step: Regulatory, Facilitator: Regulatory specialist, regulatory agency, legislators; (9) Step: Clinical use, Facilitator: Physician, dentist, optometrist; (10) Step: Explant analysis, Facilitator: Pathologist, bioengineer

Distinguish between materials in medicine and biomaterials.

The distinction between materials in medicine and biomaterials lies in the intended nature of their interaction with biological systems. Biomaterials are specifically engineered to interface with living tissues or physiological processes in order to evaluate, treat, augment, or replace a tissue, organ, or bodily function. Consequently, their design must account for factors such as biocompatibility, host response, and long-term functional performance within the body. In contrast, materials in medicine are used in healthcare applications but are not intended to establish a sustained functional interface with biological tissues.

This distinction is reflected in common medical devices. Surgical adhesives, intraocular lenses, craniomaxillofacial implants, orthopedic fixation hardware, cardiac stents, and hip implants are considered biomaterials because they directly interact with tissues and contribute to physiological function. Conversely, syringes, IV bags and tubing, removable balloon catheters, and strictly extracorporeal prostheses are materials used in medicine, as their primary purpose is treatment delivery or functional assistance rather than integration with or replacement of biological structures.

Discuss the case of the "Kennewick Man" as historical evidence of biocompatibility.

The case of the Kennewick Man, an approximately 8,500-year-old skeleton discovered in the Pacific Northwest, provides a notable historical example of biocompatibility. Examination of the remains revealed a stone spear point embedded within the pelvic bone. Importantly, the surrounding bone exhibited substantial remodeling, indicating that the individual survived the injury for an extended period after the spear became lodged.

From a biomaterials perspective, the retained spear point functioned as a foreign material that remained in intimate contact with living tissue without eliciting a destructive biological response. Rather than showing evidence of extensive bone resorption, chronic degradation, or local toxicity, the affected region demonstrated new bone formation around the object. This tissue adaptation suggests a degree of biological tolerance and integration, making the Kennewick Man a compelling early example of biocompatibility and illustrating that successful long-term tissue-material interactions have occurred throughout human history.

Expound on the discovery of the intraocular lens as an example of biocompatibility.

During World War II, physicians treating injured pilots noted that conventional glass fragments lodged in the eye often provoked inflammation and adverse tissue responses. In contrast, fragments of poly(methyl methacrylate) (PMMA) from aircraft canopies frequently remained embedded in ocular tissues without eliciting a significant inflammatory reaction.

This phenomenon was observed by the English ophthalmologist Sir Harold Ridley, who recognized that PMMA appeared highly biocompatible within the ocular environment. Inspired by these clinical observations, Ridley proposed using PMMA to fabricate an artificial lens capable of replacing the natural lens removed during cataract surgery. The first successful implantation of a PMMA intraocular lens was performed on November 29, 1949, demonstrating that the material could function long-term within the eye while minimizing adverse host responses. PMMA-based IOLs ultimately transformed cataract treatment, becoming widely adopted by the 1980s and serving as a landmark example of the clinical importance of biocompatibility.

Historical trajectory of biomaterials: (4)

(1) 1st Generation -> (2) 2nd Generation -> (3) 3rd Generation -> (4) 4th Generation

Historical trajectory of biomaterials: (1) 1st Generation

Biomaterials were used to replace a body part with an artificial one having similar physical and/or mechanical properties; crucially, these materials were known to be bioinert (e.g. Titanium-based alloy

What were some common bioinert substances used for 1st Generation biomaterial synthesis? (3)

(1) metals (e.g. titanium-based alloy); (2) ceramics (e.g. Alumina); (3) polymers (e.g. PMMA)

Examples of 1st Generation biomaterials/implementations: (3)

Hip and knee prostheses made from (1) titanium-based alloys, (2) alumina, (3) PMMA (poly methyl methacrylate)

Discuss why 1st Generation biomaterials, though they could replicate general functionality, failed to last a lifetime like natural organs or tissues could.

First-generation biomaterials were designed primarily to be bioinert and mechanically functional, enabling them to replace or support damaged tissues while minimizing adverse biological responses. Although they could often replicate the gross function of a tissue or organ, they lacked the ability to self-repair, remodel, or adapt to changing physiological conditions. Consequently, their lifespan was limited by processes such as wear, fatigue, corrosion, and material degradation.

In contrast, many native tissues maintain themselves through continuous cellular activity and remodeling, allowing them to repair damage and adapt to their environment. While not all tissues possess strong regenerative capacity, biological systems generally exhibit greater adaptability than synthetic implants. In addition, biomaterial devices may introduce challenges absent in native tissues, including wear-particle generation, fibrous encapsulation, and infection risks associated with implantation interfaces (e.g., LVAD driveline connections). As a result, first-generation biomaterials could restore function but generally could not match the long-term durability and biological integration of living tissues.

Historical trajectory of biomaterials: (2) 2nd Generation

Biomaterials were bioactive OR bioresorbable (but not both)

Examples of 2nd Generation biomaterials: (3)

(1) sutures made from PLGA (poly lactic-co-glycolic acid) dissolve within weeks; (2) ceramic implants made from hydroxyapatite; (3) implants with textured surfaces (e.g. sandblasted or engraved, allowing better tissue integration compared to smooth ones

Explain why ceramic implants made from hydroxyapatite satisfy the conditions of a 2nd Generation biomaterial.

Hydroxyapatite-based ceramic implants are classified as second-generation biomaterials because they are bioactive, rather than merely bioinert. Unlike first-generation materials that were designed primarily to avoid adverse biological responses, hydroxyapatite actively interacts with surrounding tissue and promotes a favorable cellular response, being readily recognized by native osteoblasts to facilitate osseointegration. Importantly, hydroxyapatite is not bioresorbable--implants characteristically integrate with local bone tissue--fulfilling the "or" requirement of 2nd generation classification.

Explain how texture may constitute a dimension of bioactivity.

Texture can constitute a dimension of bioactivity because the physical surface characteristics of a biomaterial influence how cells interact with it. Cellular behaviors such as adhesion, migration, proliferation, differentiation, and extracellular matrix deposition are affected not only by a material's chemistry but also by its micro- and nanoscale topography. Consequently, surface texture can actively modulate the biological response to an implant rather than simply serving as a passive structural feature.

For instance, textured or roughened bone implants generally promote superior tissue integration compared to smooth implants because the increased surface area and topographical cues enhance osteoblast attachment and bone ingrowth, resulting in stronger osseointegration. Thus, by directly influencing cell behavior and tissue integration, surface texture serves as an important determinant of bioactivity.

Historical trajectory of biomaterials: (3) 3rd Generation

Biomaterials were bioactive AND bioresorbable, designed to stimulate healing

Examples of 3rd Generation biomaterials: (2)

(1) Decellularized tissue extracellular matrices retaining bioactive factors and proteins; (2) chirality of peptides leading to regenerative immune response

Elaborate as to why injectable porcine tissue-derived ECM repair after MI constitutes an example of a 3rd Generation biomaterial application.

Injectable porcine-derived decellularized extracellular matrix (ECM) for myocardial infarction (MI) repair is classified as a third-generation biomaterial because it is both bioactive and bioresorbable, with the explicit purpose of stimulating endogenous tissue healing. The decellularized ECM retains matrix proteins and biological cues that influence host cell behavior, promoting cell recruitment, infiltration, and tissue remodeling within the damaged myocardium. At the same time, the material is gradually degraded and replaced by host tissue, allowing it to serve as a temporary regenerative scaffold rather than a permanent implant.

This application is not first-generation because first-generation biomaterials are designed to be largely bioinert, functioning primarily as passive structural replacements. In contrast, ECM-derived materials actively interact with native cells and are intended to induce a healing response.

It is also not second-generation because those biomaterials are typically either bioactive or bioresorbable. Decellularized ECM fulfills both criteria simultaneously, exceeding the defining characteristics of the second generation.

Finally, it is not fourth-generation because its classification does not depend on advanced technologies such as nanotechnology, genetic engineering, or other hybrid approaches aimed at replicating complex native tissue functions. Rather, it fits cleanly within the established third-generation paradigm of a biomaterial that is simultaneously bioactive, bioresorbable, and designed to promote regeneration and healing.

Historical trajectory of biomaterials: (4) 4th Generation

Biomaterials can mimic native tissue functionality

Examples of 4th Generation biomaterials: (4)

(1) Biofabricated tissues and organoids (miniature organs in a dish); (2) Micro/nano-scale particles (e.g. liposomes) that can perform targeted delivery of a bioactive molecule into specific cells; (3) Smart, stimuli-responsive biomaterials that can release a drug (e.g. insulin) in response to elevated glucose levels; (4) Cancer vaccines to reprogram the host immune system against tumor cells

Distinguish between 1st, 2nd, 3rd, and 4th Generation biomaterials.

Biomaterial development has progressed from passive material replacement toward increasingly sophisticated materials capable of directing biological processes and emulating native tissue function.

First-generation biomaterials were designed primarily to replace damaged tissues or body parts with materials possessing similar physical or mechanical properties. The central design philosophy was bioinertness, meaning the material should minimize interaction with surrounding tissues and avoid provoking a significant host response. Examples include traditional metallic implants such as titanium alloys used in orthopedic applications.

Second-generation biomaterials represented a shift toward materials that actively participate in the biological environment. These materials are either bioactive or bioresorbable, but not both. Bioactive materials promote beneficial interactions with surrounding tissues, whereas bioresorbable materials are designed to degrade over time as healing occurs.

Third-generation biomaterials combine both features of the second generation, being simultaneously bioactive and bioresorbable. Rather than simply replacing tissue, they are specifically designed to stimulate and guide endogenous healing and tissue regeneration. Decellularized extracellular matrix scaffolds and other regenerative biomaterials exemplify this approach.

Fourth-generation biomaterials extend beyond the regenerative paradigm by seeking to mimic native tissue functionality itself. These materials are often engineered to reproduce aspects of the dynamic, adaptive behavior of biological tissues, frequently drawing on advances in fields such as nanotechnology, bioengineering, materials chemistry, and molecular biology.