Spatial regulation of controlled bioactive factor delivery for bone tissue engineering

Abstract

Limitations of current treatment options for critical size bone defects create a significant clinical need for tissue engineered bone strategies. This review describes how control over the spatiotemporal delivery of growth factors, nucleic acids, and drugs and small molecules may aid in recapitulating signals present in bone development and healing, regenerating interfaces of bone with other connective tissues, and enhancing vascularization of tissue engineered bone. State-of-the-art technologies used to create spatially controlled patterns of bioactive factors on the surfaces of materials, to build up 3D materials with patterns of signal presentation within their bulk, and to pattern bioactive factor delivery after scaffold fabrication are presented, highlighting their applications in bone tissue engineering. As these techniques improve in areas such as spatial resolution and speed of patterning, they will continue to grow in value as model systems for understanding cell responses to spatially regulated bioactive factor signal presentation in vitro, and as strategies to investigate the capacity of the defined spatial arrangement of these signals to drive bone regeneration in vivo.

Keywords: Biomaterials patterning; Bone regeneration; Drug delivery; Spatial regulation; Temporal regulation.

Figure 1

Schematic illustrating technologies to spatially control bioactive factor presentation.

Figure 2

ALP staining (blue) of C2C12 cells on 5 mm decellularized skin discs printed with bioactive factors resulting from (A-C) varying amounts of BMP-2 printed on the right halves of the scaffolds, (D-F) BMP-2 printed uniformly on the scaffolds with inhibitors printed on the left halves, (G) GDF-5 printed on the left halves, BMP-2 on the right halves, as well as (H) on a square piece with increasing number of BMP-2 overprints (OP). Adapted, with permission, from Cooper, et al. [179]. Copyright Mary Ann Liebert, Inc. 2010.

Figure 3

Examples of bioactive factor gradient formation. (A) Schematic of a commercially available gradient maker design. (B) Dual syringe pump system used for gradient fabrication in alginate/heparin hydrogels, where (C) the flow rate from each syringe is controlled over time to create (D) measurable BMP-2 and TGF-β1 linear gradients in opposite directions. Encapsulated hMSCs expressed increased (E) osteogenic and (F) chondrogenic differentiation markers on the side of the gradient with increased BMP-2 and TGF-β1, respectively. (A) Adapted, with permission, from Chatterjee, et al. [205]. Copyright Bentham Science Publishers 2011. (B-F) Adapted, with permission, from Jeon, et al. [206]. Copyright John Wiley and Sons 2013.

Figure 4

Example of modular assembly of microgels. Schematics representing assembly of (A) hexagonal microgels and (D) lock and key shaped microgels, with (B, E) phase contrast and (C, F) fluorescence photomicrographs of centimeter-scale hydrogel constructs with fluorescently labeled encapsulated fibroblasts. Scale bars: 100 μm. Adapted, with permission, from Zamanian et al. [248]. Copyright John Wiley and Sons 2010.

Figure 5

FITC-labeled siRNA retained (A) uniformly in PEG hydrogels, as well as (B-D) in specific regions controlled by UV exposure through a photomask. Scale bar = 100 μm. Alsberg laboratory unpublished data.

Figure 6

Multiphoton patterning in 3D agarose hydrogels showing (A) oblique and (B) side views of a 4 × 4 × 4 array of squares (60 μm per side) of maleimide-conjugated green fluorescent dye (AF488-Mal), and a 4 × 4 × 3 array of circles (50 μ diameter) of maleimide-conjugated red fluorescent dye (AF546-Mal) created in a second multiphoton irradiation step. Adapted, with permission, from Wosnick et al. [285]. Copyright American Chemical Society 2008.