Biomimetic hierarchical structures

Choi SJ, Suh KY, Lee HH. Direct UV-replica molding of biomimetic hierarchical structure for selective wetting. J Am Chem Soc 2008 130(20) 6312-3. 

Techniques to produce multiscale biomaterial scaffolds with designer geometries are the need of the hour to provide improved biomimetic properties for functional tissue replacements. While micrometer fibers generate an open pore stnicture, nanofibers support cell adhesion and facilitate cell-cell interactions. This was further proven by cell penetration studies, which showed superior ingrowth of cells into hierarchical structures. Mixed bimodal scaffolds of two different polymers are another promising approach, because they exhibit hierarchical pore/ surface systems and combine the beneficial properties of both polymers at two different scales. Vaiious 3D micro- and nanoscale multiscale scaffolds have been fabricated through various techniques and were found to have the potential to essentially recreate natural bone, cardiac, neural, and vascular tissues. 

Inspired by the hierarchical structures that enable bone function, Deng et al. recently developed a mechanically competent 3D scaffold mimicking the bone marrow cavity and the lamellar structure of bone by orienting electrospun polyphosphazene-polyester blend nanofibers in a concentric manner with an open central cavity (Figure 11.9b and c) [66]. The 3D biomimetic scaffold exhibited mechanical characteristic similar to native bone. Compressive modulus of the scaffold was found to be within the range of human trabecular bone. When tuned to have desired properties, the concentric open macrostructures of nanofibers that structurally and mechanically mimic the native bone can be a potential scaffold design for accelerated bone healing. 

The enormous scale of the challenge to build a truly biomimetic muscle can be appreciated by considering what is known about natural muscle structure. The contractions of these motors involve a highly complex and coordinated sequence of electrical, chemical, and physical phenomena within a composite, gel-like polymeric material that is known to possess a detailed hierarchical structure, stretching from the nano-scale assembly of proteins through the cellular fibrillar textures to the macroscopic tissue. The exact function of many of these features is still the subject of on-going research. In general, the skeletal muscles consist of tendons (non-active) and muscle belly (active). While tendons mainly provide muscle connectivity to hard bones,... 

B. N. Sahoo and B. Kandasubramanian, Recent progress in fabrication and characterization of hierarchical biomimetic superhydrophobic structures, RSCAdv., 4, 22053-22093 (2014). 

Tokarev, I. and Minko, S. (2009a) Multiresponsive, hierarchically structured membranes New, challenging, biomimetic materials for biosensors, controlled release, biochemical gates, and nanoreactors . Advanced Materials, 21,241-247. 

The knowledge about bone tissue structure and morphology described in detail in [2, 13, 20-25] proves to be very complex because bone is arranged in several hierarchical structures. Rational design of artificial implant materials should take into account the tissue s characteristics - the more similarities possessed by the fabricated implant to the bone tissue (biomimetism), the greater the chance of acceptance of the alien system by the human organism. 

Abstract Life as we know it is impossible without formation of hierarchical structures. First and foremost, proteins, that is, sequence-specific polypeptides, are nature s vanguard in this respect. Peptoids and polypeptoids are structural isomers and analogs to peptides and polypeptides. Here, we review the advancements over the last few years on biomimetic hierarchical stmctures obtained using polypeptoids. Although the inherently more flexible amide bond in peptoids make the stabilization of secondary stmcture challenging, it also gives us a tool to direct the conformation of the amide bond by design. As will be seen, this is a particularly important feature of peptoids. 

Here, we aim to give our perspective on the advances, remaining problems, and potential of biomimetic hierarchical stmcture that can be (potentially) created from polypeptoids. In our opinion, of the different synthetic polymers, polypeptoids form a very interesting platform for the study of complex hierarchical structure. 

Abstract The aggregation behaviour of biomimetic polypeptide hybrid copolymers and copolypeptides is here reviewed with a particular eye on the occurrence of secondary structure effects. Structure elements like a-helix or / -sheet can induce a deviation from the classical phase behaviour and promote the formation of vesicles or hierarchical superstructures with ordering in the length-scale of microns. Polypeptide copolymers are therefore considered as models to study self-assembly processes in biological systems. In addition, they offer a great potential for a production of novel advanced materials and colloids. 

The phase behaviour of biomimetic polypeptide-based copolymers in solution was described and discussed with respect to the occurrence of secondary structure effects. Evidently, incorporation of crystallisable polypeptide segments inside the core of an aggregate has impact on the curvature of the corecorona interface and promotes the formation of fibrils or vesicles or other flat superstructures. Spherical micelles are usually not observed. Copolymers with soluble polypeptide segments, on the other hand, seem to behave like conventional block copolymers. A pH-induced change of the conformation of coronal polypeptide chains only affects the size of aggregates but not their shape. The lyotropic phases of polypeptide copolymers indicate the existence of hierarchical superstructures with ordering in the length-scale of microns. 

It is important to emphasize that many natural tissues are essentially composed of nanoscale biopolymers or biocomposites with hierarchical architectures. Therefore, by mimicking the structure and property of their natural counterparts, synthetic nanopoiymers and nanocomposites are very likely to enhance/regulate the functions of specific cells or tissues. This principle has been demonstrated by the success of bioinspired polymers and composites in both clinical practice and in laboratory research. In particular, bone is the hierarchical tissue that has inspired a myriad of biomimetic materials, devices, and systems for decades. This chapter focuses on this well-developed area of biomimetic or bioinspired nanopoiymers and nanocomposites for bone substitution and regeneration, especially those with high potentials for clinical applications in the near future. 

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