On the biomechanical function of scaffolds for engineering load-bearing soft tissues

Abstract

Replacement or regeneration of load-bearing soft tissues has long been the impetus for the development of bioactive materials. While maturing, current efforts continue to be confounded by our lack of understanding of the intricate multi-scale hierarchical arrangements and interactions typically found in native tissues. The current state of the art in biomaterial processing enables a degree of controllable microstructure that can be used for the development of model systems to deduce fundamental biological implications of matrix morphologies on cell function. Furthermore, the development of computational frameworks which allow for the simulation of experimentally derived observations represents a positive departure from what has mostly been an empirically driven field, enabling a deeper understanding of the highly complex biological mechanisms we wish to ultimately emulate. Ongoing research is actively pursuing new materials and processing methods to control material structure down to the micro-scale to sustain or improve cell viability, guide tissue growth, and provide mechanical integrity, all while exhibiting the capacity to degrade in a controlled manner. The purpose of this review is not to focus solely on material processing but to assess the ability of these techniques to produce mechanically sound tissue surrogates, highlight the unique structural characteristics produced in these materials, and discuss how this translates to distinct macroscopic biomechanical behaviors.

Figure 1

Ability to produce scaffolds which mimic native tissue constituent scales. Viable tissue replacements are confounded by complex multi-scale architectures, hierarchical interactions, and modes of deformation characteristically observed in native tissues. Overcoming the limitations of current medical therapies necessitates new production methods or adaptations to current techniques to produce scaffolds in a controlled manner with characteristic lengths comparable to those observed in nature.

Figure 2

Cross-sectional scanning EM micrographs of non-fibrous methods to manipulate micro-morphology. (a, d) Porous poly (L-lactic acid) prepared via TIPS at 30 °C with metastable state residence times of (a) 5 min, (b) 60 min so as to control pore size. (b, e) Polycaprolactone scaffold created by particle leaching with a poly(ethyl methacrylate) bead (200 µm diameter) porogen. Increased porosity was attained by compressing the beads in a mould prior to injecting melted polycaprolactone. (c, f) A porous poly(ester amine) sample prepared via gas foaming at 105 °C. By increasing the gas saturation pressure, pore density is increased and average pore diameter decreases. Saturation pressure equals 20 bar and 40 bar respectively. Figure adapted with permission from [–177].

Figure 3

Attaining structural anisotropy through material processing. (a, d) Polyurethane scaffolds produced by thermally induced phase separation with oriented pores produced by imposing a heat transfer gradient during cooling. (b) Supercritical gas foaming used to create open cell composite foams for bone tissue engineering which exhibit morphologic and mechanical anisotropy with pores oriented in the foaming direction. (c, e) Electrospinning allows controlled fiber deposition by the use of a rotating collection surface and has the potential to produce scaffolds which exhibit gross, anisotropic soft tissue-like mechanical behaviors. Figure adapted with permission from [35, 178].

Figure 4

Scanning electron micrographs of methods commonly employed to create 3D scaffolds exhibiting fibrous structures with diameters on the order of native ECM. (a) Nano-scale self-assembled alkylated peptide amphiphiles forming tristed ribbon morphologies [179]. (b) Biopolymer gels like those made of fibrin readily support cellular viability and can be used to investigate cellular behavior in a reasonably well controlled environment. (b) Electrospinning scaffolds for tissue engineered applications have seen widespread use owing to the inherent ability to produce a synthetic matrix with tunable fiber architectures (diameter, orientation, etc.). (c) Needled nonwoven fabrics made of biocompatible polymers have shown promise in tissue engineered efforts to produce de-novo ECM in a well defined microstructure. Figure adapted with permission from [180, 181]

Figure 5

Synthetic scaffold production to recapitulate native tissue mechanical behavior. (a) Homogenation model prediction of material modulus for native inner and outer annulus fibrosus lamella (IAF and OAF respectively) and annulus fibrosus cell seeded electrospun scaffolds. (b) Representative stress versus strain curves from compression testing of porous poly (lactide-co-glycolide) scaffolds and articular cartilage sourced from porcine and ovine models. (c) Planar biaxial response comparison of native porcine pulmonary valve and highly aligned electrospun poly (ester urethane) urea construct. Figure adapted with permission from [79, 82, 182].

Figure 6

Expediting natural processes for the production of engineered tissue technologies. The time span required to produce practical options for tissue engineering is constrained significantly compared to native tissue development. As such, it is necessary to tailor culture parameters including mechanical and chemical cues in an effort to improve construct mass, composition of ECM constituents, and cell proliferation prior to implantation.

Figure 7

Native aortic valve cell-matrix interaction and cell deformation response to increasing transvalvular pressure. Cell deformation in native porcine aortic valve leaflets, as quantified by changes in nuclear aspect ratio, was highly dependent on local collagen fiber kinematics. Generally speaking, increasing transvalvular pressure resulted in increased cell nuclear aspect ratios but unique layer dependent responses were observed. Furthermore, a bimodal trend was observed between cell deformation and increased diastolic loads. Figure adapted with permission from [144].

Figure 8

Strain induced changes in electrospun polyester micro-architecture and resulting nuclei deformation. When exposed to biaxial modes of deformation, electrospun fibers were observed to transition from a tortuous configuration in the unstrained state to an interconnected web-like architecture at high strains. A composite of all NAR measurements (mean ± s.e.m) demonstrated a rapid increase to ~60% strain, after which a plateau was observed with further strain increases, indicating that nuclei deformations are dominated by local fiber straightening. A composite cell-scaffold deformation response (bottom) is provided for native porcine aortic valve leaflet, cell integrated electrospun PEUU, and a theoretical purely affine cell deformation response to macroscopic strain. Figure reproduced with permission from [88].

Figure 9

Application of traditional engineering metrics of material behavior are often inadequate descriptors of the complex behaviors observed in biological materials. (a) Load bearing matrix rich biological materials, like native aortic valve leaflets, typically exhibit highly non-linear and highly anisotropic tensile behaviors. (b) The transition from low to high stiffness is attributed to a coupled fiber recruitment process capable of exhibiting lateral contraction at high stress levels (circumferential shortening). As a consequence, the determination of traditional engineering indices of material behavior, such as tensile modulus has little meaning (i.e. negative modulus). Adequate characterization of biological materials can necessitate more sophisticated methods to characterize their true mechanical behaviors.

Figure 10

Cell integrated electrospun scaffold hierarchical structure and function. Despite exhibiting a tissue-like mechanical response at the macro scale, the scaffold exhibits vastly different micro and meso mechanical behaviors. For instance, at the micro-scale a heterogeneous deformation response is observed. In addition, fibers in the unstrained configuration exhibit an undulated or tortuous morphology which transitions to a highly interconnected web-like architecture at finite strains. At the macro scale, we observe a complex 3D scaffold with tunable tissue-level mechanical behavior that can be remarkably similar to the biaxial mechanical response of the native porcine pulmonary leaflet. Figure adapted with permission from [181].