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An ancestral lower-jaw bone contributing to the evolutionary origin of the mammalian ectotympanic.

A bone forming the ancestral lower component of the articular–quadrate jaw joint, homologous to the mammalian malleus.

An ankle bone also called the talus. The detailed double-pulley construction in early whales helps test their artiodactyl ancestry.

A bioactive glass is a glass designed to elicit a useful biological response at its interface, notably bonding with bone for some compositions and uses. It is a class of biomaterials, not a claim that every glass has the same response. Related glass-ceramics contain crystalline phases and should be distinguished from wholly glassy materials.

A motivating early experiment compared glass-ceramic solubility and interfacial hydroxyapatite crystallization with observations around implants in rat femurs. Larry L. Hench and colleagues examined histology and ultrastructure as well as laboratory interfacial changes. The question was how bonding developed at the material–bone interface, beyond the bulk material's mechanical strength.^[ref]

Contact with fluid can change the material's surface and release dissolved constituents. Geometry therefore matters alongside composition. For an original monodisperse spherical-powder model, radius $r$ and density $\rho$ give surface area per massAt stipulated $\rho =2500\mathrm{kg}{\mathrm{m}}^{-3}$ and $r=10\mu \mathrm{m}$, this is $120{\mathrm{m}}^2\mathrm{k}{\mathrm{g}}^{-1}$, or $0.12{\mathrm{m}}^2{\mathrm{g}}^{-1}$. Increasing radius to $100\mu \mathrm{m}$ reduces that area per mass tenfold. The example neglects pores and aggregation and is not a particular product's specification.

If initial release flux per unit area were equal in two such powders, their initial release rates per mass would differ by the same factor. Actual flux can change with solution composition and surface evolution; a biological response need not scale linearly with geometric area. Comparisons should therefore specify particle dimensions, exposed area, fluid conditions and time. Surface-layer formation, cytotoxicity, mechanical behavior and tissue bonding remain distinct observations rather than interchangeable proof of suitability.

Biocompatibility is the ability of a biomaterial to function with an appropriate response from the living system in a specified application. It relates the material, its intended function and its biological environment. It is therefore more informative to ask whether a particular finished component is compatible with a particular contact site and duration than to call a substance universally “biocompatible.”^[ref]

Consider the same nominal polymer used as a short-lived skin-contacting film or as part of a long-lived component contacting blood. Its chemistry alone does not specify the biological exposure. Surface treatment, processing residues, final preparation and the length and type of contact can change the relevant response. The FDA’s primary resource emphasizes the final finished device and explicitly identifies contact type, duration and component interactions as assessment variables.^[ref]

A cytotoxicity experiment addresses one part of this question by examining adverse effects on biological cells. In a simple illustrative cell culture comparison, suppose a sample-exposed culture has 900 viable cells and a matched control has 1000 at the same observation time. The relative viable-cell count is $900/1000=0.90$. That ratio is an observation under the chosen conditions, not a complete compatibility verdict: growth rate, exposure preparation, measurement method and repeatability affect its interpretation. A control can distinguish a material-associated effect from an effect of the culture medium or preparation procedure.

Functional compatibility also need not mean that nothing happens at the interface. Some biomaterials are designed to bond to bone, so the desired response differs from that of a material intended merely to remain separated from surrounding tissue. Evidence about cell viability, local tissue organization and the component’s mechanical function answers different questions. A useful experimental account states which response was measured, the biological setting and exposure history, and the uncertainty in the comparison.

In biomedical engineering, a biomaterial is a material selected or designed to interact with a biological system for a medical purpose. Its defining feature is this intended interaction, rather than whether it originated in an organism. A synthetic polymer, a metal or a ceramic can therefore be a biomaterial; natural collagen can also be processed for such a use. The biomedical meaning should be distinguished from the broader use of “bio” to mean material obtained from biomass.^[ref]

A biomaterial has both a material function and a biological environment. A load-bearing component must transmit forces without unacceptable deformation, but its surface also contacts fluid, proteins and biological cells. Biocompatibility consequently concerns an appropriate host response for a particular application; it is not a universal label that guarantees identical behavior at every site or for every exposure duration.^[ref]

An illuminating historical example is the 1971 work of L. L. Hench and colleagues on a bone-bonding glass-ceramic. They compared laboratory measurements of glass-ceramic solubility and interfacial hydroxyapatite crystallization with histology and ultrastructure observations around implants in rat femurs. The important question was how the material–bone interface developed, rather than merely whether the bulk ceramic was strong.^[ref]

To separate bulk behavior from an interface effect in a model experiment, one could compare specimens with identical dimensions and bulk Young's modulus but different surface treatments, while holding the cell culture conditions fixed. Measure both the mechanical response and cell adhesion, instead of substituting one for the other. Conversely, changing specimen thickness while retaining the same surface can change bending resistance without changing the nominal surface chemistry. These controlled comparisons show why evaluating a biomaterial requires specifying the material, its geometry, its interface and the biological setting together.

Biomechanics applies mechanics to living organisms, from the forces exerted by a muscle to the deformation of a biological cell. It connects geometry, motion, material response and biological activity. A bone supporting a load, blood moving through a vessel, and a muscle producing work are all mechanical systems, but their living components can remodel, consume chemical energy and change their properties.

A simple torque balance explains why an internal muscle force can greatly exceed an external load. In an idealized stationary forearm, suppose a downward load $W=20\mathrm{N}$ acts with perpendicular moment arm $r_W=0.30\mathrm{m}$ about the elbow and the balancing tendon force has muscle moment arm $r_m=0.030\mathrm{m}$. Neglect the forearm’s own gravitational weight and other muscles. Zero net torque requiresThese are perpendicular moment arms, not necessarily distances measured along the bone. Force balance also requires a joint reaction; the torque equation alone does not determine every internal force.

Geometry must also be separated from material behavior. A tendon with sectional area $20{\mathrm{mm}}^2$ carrying that illustrative $200\mathrm{N}$ has mean axial stress $10\mathrm{MPa}$. Its extension depends on its length, constitutive equation and loading history, not just the applied force. Measurements of stress against strain therefore answer a different question from measurements of force against extension. For a living specimen, hydration, temperature and the rate of deformation can all be controlled experimental variables.

An influential motivating experiment is Archibald Vivian Hill’s measurement of heat and work during muscle contraction. In his 1938 primary paper, a thermopile detected tiny temperature changes; an optically amplified galvanometer made the response fast enough to study shortening. Comparing contractions under different loads connected shortening speed, mechanical work and liberated heat. Such measurements constrain an active muscle model in a way that simply stretching a passive spring cannot.^[ref] The example captures a central aim of biomechanics: use measurable forces, motion and energy transfers to test how a biological mechanism works.

A body fossil preserves evidence of an organism's body or a part of it, such as a tooth, bone, shell or leaf. Preservation can retain original material, a mineralized structure, an impression or a cast. A body fossil therefore need not be an intact body or contain the original organic substances.

A trace fossil, such as a footprint or burrow, records activity rather than the body itself. Both are biological evidence, but they answer different questions: a tooth can constrain anatomy, whereas a trackway can constrain movement. Decay experiments show why preserved anatomy requires care: some characters disappear before others, biasing the apparent similarities used in phylogenetics.^[ref]

Bone is living mineralized connective tissue forming major parts of the vertebrate endoskeleton. Its extracellular matrix combines organic material with mineral, supporting both toughness and stiffness. Cells continually remodel it, so a bone is a regulated tissue rather than an inert stone.

The mineral-rich residue left after combustion of bone organic material. It contains calcium phosphates and should not automatically be treated as pure tricalcium phosphate.

Bone marrow is tissue within the internal spaces of bones. Hematopoietic regions produce blood cells, while adipose-rich regions have a different composition. Marrow is therefore not simply an empty bone cavity or the mineralized bone matrix itself.

A bursa is a fluid-containing sac positioned where tissues move against one another, such as a tendon crossing bone. Its sliding surfaces reduce friction. A bursa can lie near a synovial joint without necessarily communicating with the joint cavity.

cartilage with mineral deposited in its matrix. Mineralization does not convert its histological identity automatically into bone.

The cartilaginous-fish clade, including sharks, ray fish and chimaeras. Extant lack of extensive bone does not demonstrate that their ancestors never possessed it.

The clavicle is the curved bone connecting the sternum with the scapula. It helps hold the shoulder laterally and transmits forces between the upper limb and axial skeleton. Its strut-like position supports a mobile shoulder while maintaining separation from the thorax.

Connective tissue supports, connects or separates other tissues through cells embedded in extracellular matrix. Its forms include loose supporting tissue, tendons, cartilage and bone. Variation in matrix composition and organization explains their very different mechanical behavior.

David H. Kohn is a biomedical engineer whose research connects biomineralization, biomaterials and biomechanics. His University of Michigan profile describes two related questions: how the organization of mineralized tissue produces its mechanical behavior, and how understanding natural material formation can inform engineered biomaterials. This is an example of biomimetics: learning design principles from biological structures rather than merely copying their chemical ingredients.^[ref]

His joint work with M. D. Morris, W. F. Finney and Rupak M. Rajachar used Raman spectroscopy to investigate bone during elastic deformation. Their custom mechanical tester fitted the stage of a Raman microprobe and accepted hydrated specimens. The experiment paired loading of murine femora in tension with measurements of mineral phosphate-band shifts, while keeping loading below yield.^[ref] The combination asks how a local molecular response accompanies the deformation of an entire composite material.

An original idealized calculation explains why the two measurements are complementary. For a harmonic oscillator with effective bond force constant $k$ and reduced mass $\mu$, the vibrational wavenumber iswhere $c$ is the speed of light. Holding $\mu$ fixed gives the first-order relation $\Delta \tilde{\nu }/\tilde{\nu }\simeq \frac{1}{2}\Delta k/k$. A hypothetical band moving from $1000$ to $998\mathrm{c}{\mathrm{m}}^{-1}$ would correspond to approximately a $0.4\%$ decrease in this model's effective bond force constant. This Raman shift as a probe of bond stiffness is a local vibration model; the chosen numbers are not measurements from Kohn's paper.

A specimen's Young's modulus instead relates its macroscopic stress to its strain in a specified loading direction and regime. It cannot be read directly from one vibrational wavenumber. Composite material geometry, phase interactions and orientation mediate the connection between a local bond force constant and a bulk elastic modulus. Even a large local spectral response need not imply an equally large change in bulk stiffness.

This distinction gives the experiment its interest: a tensile testing apparatus and a spectroscopic probe observe different levels of the same loaded material. Their agreement or disagreement can discriminate structural explanations that a single bulk measurement would leave unresolved. Repeating both measurements during unloading would additionally test whether the changes reverse. Kohn's institutional description places this multiscale mechanics question alongside the development of new biomaterials.^[ref]

The principal tooth-bearing lower-jaw bone of mammals; enlargement and changes in articulation are informative in early mammalian evolution.

bone forming within dermal or related connective tissues rather than replacing a cartilaginous template. Large skull-roof bones and small surface odontodes are distinct anatomical organizations.

bone deposited during replacement of a cartilaginous skeletal template. It should be distinguished from calcified cartilage and a superficial perichondral bone layer.