Biofoam

Figure 2: Depiction of various honeycomb patterns[14]

Honeycomb refers to bioinspired patterns that provide a lightweight design for energy absorbing structures. Honeycomb design can be found in different structural biological components such as spongy bone and plant vasculature. Biologically inspired honeycomb structures include Kelvin, Weaire and Floret honeycomb (see Figure 2); each with a slightly different structure in comparison to the natural hexagonal honeycomb. These variations on the biological design have yielded significantly improved energy absorption results in comparison to traditional hexagonal honeycomb biofoam.[14]

Due to these increased energy absorption performances, honeycomb inspired structures are being researched for use inside vehicle crumple zones. By using honeycomb structures as the inner core and surrounding the structure with a more rigid structural shell, these components can absorb impact energy during a crash and reduce the amount of energy the driver experiences.[15]

Figure 3: Bioinspired Macroscopic Carbon Tube Aerogel (CTA) compared with polar bear fur at high magnification[16]

Aerogels are able to fill large volumes with minimal material yielding special properties such as low density and low thermal conductivity. These aerogels tend to have internal structures categorized as open or closed cell structures,[17] the same cell structure that is used to define many 3-dimensional honeycomb biofoams. Aerogels are also being engineered to mirror the internal foam structures of animal hairs (see Figure 3). These biomimetic aerogels are being actively researched for their promising elastic and insulative properties.[16]

A foam is considered open-celled if at least two of its facets are holes rather than walls.[18] In this case the entirety of the load on the foam is on the cross-beams that make up the edges of the cell.[6] If no more than one of the walls of the cell are holes, the foam is considered closed-celled in nature.[18] For most synthetic foams, a mixture of closed cell and open cell character is observed due to cells rupturing during the foaming process and then the matrix solidifying.[18][5]

The mechanical properties of the foam then depend on the closed cell character of the foam as derived by Gibson and Ashby:[19]

${\displaystyle \left({\frac {E}{E_{s}}}\right)\propto \left({\frac {\rho }{\rho _{s}}}\right)^{2}\left({\frac {1}{(1+\phi )^{2}}}\right)\left[1+{\frac {\rho }{\rho _{s}}}({\frac {\phi ^{3}}{1+\phi }})\right]}$

Where E is the elastic modulus, ρ is the density of the material, φ is the ratio of the volume of the face to the volume of the edge of the material, and the subscript s denotes the bulk property of the material rather than that of the foam sample.

For many polymeric foams, a solidified foam is formed by polymerizing and foaming a liquid polymer mixture and then allowing that foam to solidify.[5][12][18][3] Thus, liquid foam aging effects do occur before solidification. In the liquid foam, gravitational forces and internal pressures cause a flow of the liquid toward the bottom of the foam.[12][20] This causes some of the foam cells to form into irregular polyhedra as liquid drains, which are less stable structures than the spherical structures of a traditional foam.[12] These structures can however be stabilized by the presence of a surfactant.[20]

The foam structure before solidification is an inherently unstable one, as the voids present greatly increase the surface free energy of the structure.[12][20] In some synthetic biofoams, a surfactant can be used in order to lower the surface free energy of the foam and therefore stabilize the foam. In some natural biofoams, proteins can act as the surfactants for the foams to form and stabilize.[12]

During the solidification of synthetic biofoams, fibers may be added as a reinforcement agent for the matrix.[18][4] This additionally will create a heterogeneous nucleation site for the air pockets of the foam itself during the foaming process.[18] However, as fiber content increases, it can begin to inhibit formation of the cellular structure of the matrix.[4]

Figure 4. PLA being produced through ring-opening polymerization of lactide.[21]

In relation to packaging, starches and biopolyesters make up these biofoams as they are adequate replacements to expanded polystyrene.[22] Polylactic acids (PLAs) are a common form of the basis of these biofoams since they offer a substitute for polyolefin-based foams that are commonly used in automotive parts, pharmaceutical products, and short life-time disposable packaging industries due to their bio-based and biodegradable properties.[23] PLA comes from the formation of lactide produced from lactic acid due to bacterial fermentation through ring-opening polymerization, in which the process is shown through Figure 4.[21]

PLA does not have the most desirable traits for biodegradability in the packaging industry as it contains a low heat distortion temperature and has unfavorable water barrier characteristics.[22] On the other hand, PLA has been shown to have desirable packaging properties including high ultraviolet light barrier properties, and low melting and glass transition temperatures.[24] As of recently, PGA has been introduced in the packaging industry as it is a good solvent and comparable to PLA. Table 1 shows the characteristics of both biofoams and how they compare.[25] As shown, PGA contains a strong stereochemistry structure which in turn causes it to have high barrier and mechanical properties making it desirable for the packaging industry.[25] The study of mixing both PGA and PLA has been explored by using copolymerization in order for PGA to help enhance the barrier properties of PLA when used in packaging.[25]

Table 1: The properties of PLA in comparison to PGA.[25]

Figure 5: The FFF-based 3D printed PLA undergoing compression testing and its shape after heating the sample greater than its glass transition temperature (Tg).[21]

The most popular biofoam in the use of biomedical devices is PLA as well. PLA's properties are also desirable in biomedical applications, especially in combination with other polymers.[26] Specifically, its biocompatibility and biodegradability make it favorable in tissue engineering through the use of FDM-3D printing.[26] PLA does well in these printing environments as its glass transition temperature as well as shape memory is small.[21] In recent studies, PLA has been specifically combined with hydroxyapatite (HA) in order to make the modulus of the sample more favorable for its application in repairing bone failure.[26] Specifically in tissue engineering, HA has also been shown to generate osteogenesis by triggering osteoblasts and pre-osteoblastic cells.[27] HA is a strong material, which makes it ideal to add to PLA, due to the fact that PLA has weak toughness with a 10% elongation before failure.[26] FFF-based 3D printing was used as well as compression tests demonstrated in Figure 5.[21] The results showed that there was a self-healing capability of the sample, which could be used in certain biomedical practices.[21]