A peptide-based hydrogelation strategy has been developed that allows homogenous encapsulation and subsequent delivery of C3H10t1/2 mesenchymal stem cells. Structure-based peptide design afforded MAX8, a 20-residue peptide that folds and selfassembles in response to DMEM resulting in mechanically rigid hydrogels. The folding and self-assembly kinetics of MAX8 have been tuned so that when hydrogelation is triggered in the presence of cells, the cells become homogeneously impregnated within the gel. A unique characteristic of these gel-cell constructs is that when an appropriate shear stress is applied, the hydrogel will shear-thin resulting in a low-viscosity gel. However, after the application of shear has stopped, the gel quickly resets and recovers its initial mechanical rigidity in a near quantitative fashion. This property allows gel/cell constructs to be delivered via syringe with precision to target sites. Homogenous cellular distribution and cell viability are unaffected by the shear thinning process and gel/cell constructs stay fixed at the point of introduction, suggesting that these gels may be useful for the delivery of cells to target biological sites in tissue regeneration efforts.hydrogel ͉ self-assembly ͉ stem cell

A growing number of medications must be administered through parenteral delivery, i.e., intravenous, intramuscular, or subcutaneous injection, to ensure effectiveness of the therapeutic. For some therapeutics, the use of delivery vehicles in conjunction with this delivery mechanism can improve drug efficacy and patient compliance. Macromolecular self-assembly has been exploited recently to engineer materials for the encapsulation and controlled delivery of therapeutics. Selfassembled materials offer the advantages of conventional crosslinked materials normally used for release, but also provide the ability to tailor specific bulk material properties, such as release profiles, at the molecular level via monomer design. As a result, the design of materials from the "bottom up" approach has generated a variety of supramolecular devices for biomedical applications. This review provides an overview of self-assembling molecules, their resultant structures, and their use in therapeutic delivery. It highlights the current progress in the design of polymer-and peptide-based self-assembled materials.

Self-assembling peptide hydrogels are used to directly encapsulate and controllably release model FITC–dextran macromolecules of varying size and hydrodynamic diameters. MAX1 and MAX8 are two peptide sequences with different charge states that have been designed to intramolecularly fold and self assemble into hydrogels at physiological buffer conditions (pH 7.4, 150 mM NaCl). When self-assembly is initiated in the presence of dextran or protein probes, these macromolecules are directly encapsulated in the gel. Self-diffusion studies using fluorescence recovery after photobleaching (FRAP) and bulk release studies indicate that macromolecule mobility within, and release out of, these gels can be modulated by varying the hydrogel mesh size. The average mesh size can be modulated by simply varying the concentration of a given peptide used to construct the gel or by altering the peptide sequence. In addition, results suggest that electrostatic interactions between the macromolecules and the peptide network influence mobility and release. Depending on probe size, release half-lives can be varied from 8 h to over a month.

Hydrogel materials are promising vehicles for the delivery of protein therapeutics. Proteins can impart physical interactions, both steric and electrostatic in nature, that influence their release from a given gel network. Here, model proteins of varying hydrodynamic diameter and charge are directly encapsulated and their release studied from electropositive fibrillar hydrogels prepared from the self assembling peptide, MAX8. Hydrogelation of MAX8 can be triggered in the presence of proteins for their direct encapsulation with no effect on protein structure nor the hydrogel's mechanical properties. Bulk release of the encapsulated proteins from the hydrogels was assessed for a month time period at 37°C before and after syringe delivery of the loaded gels to determine the influence of protein structure on release. Release of positively charged and neutral proteins was largely governed by the sterics imposed by the network. Conversely, negatively charged proteins interacted strongly with the positively charged fibrillar network, greatly restricting their release to <10% of the initial protein load. Partition and retention studies indicated that electrostatic interactions dictate the amount of protein available for release. Importantly, when protein encapsulated gels were delivered via syringe, the release profiles of the macromolecules showed similar trends as those observed for non-sheared gels. This study demonstrates that proteins can be directly encapsulated in self assembled MAX8 hydrogels, which can then be syringe delivered to a site where subsequent release is controlled by protein structure.

A de novo designed beta-hairpin peptide (MAX8), capable of undergoing intramolecular folding and consequent intermolecular self-assembly into a cytocompatible hydrogel, has been studied. A combination of small angle neutron scattering (SANS) and cryogenic-transmission electron microscopy (cryo-TEM) have been used to quantitatively investigate the MAX8 nanofibrillar hydrogel network morphology. A change in the peptide concentration from 0.5 to 2 wt% resulted in a denser fibrillar network as revealed via SANS by a change in the high q (q = (4 pi/lambda) x sin (theta/2), where lambda = wavelength of incident neutrons and theta = scattering angle) mass fractal exponent from 2.5 to 3 and by a decrease in the measured correlation length from 23 to 16 A. A slope of -4 in the USANS regime indicates well-defined gel microporosity, an important characteristic for cellular substrate applications. These changes, both at the network as well as the individual fibril lengthscales, can be directly visualized in situ by cryo-TEM. Fibrillar nanostructures and network properties are directly related to bulk hydrogel stiffness via oscillatory rheology. Preliminary cell viability and anchorage studies at varying hydrogel stiffness confirm cell adhesion at early stages of cell culture within the window of stiffness investigated. Knowledge of the precise structure spanning length scales from the nanoscale up to the microscale can help in the formation of future, specific structure-bioproperty relationships when studying in vitro and in vivo behavior of these new peptide scaffolds.