Benjamin D. Almquist

Fusion of biomimetic stealth probes into lipid bilayer cores

Many biomaterials are designed to regulate the interactions between artificial and natural surfaces. However, when materials are inserted through the cell membrane itself the interface formed between the interior edge of the membrane and the material surface is not well understood and poorly controlled. Here we demonstrate that by replicating the nanometer-scale hydrophilic-hydrophobic-hydrophilic architecture of transmembrane proteins, artificial “stealth” probes spontaneously insert and anchor within the lipid bilayer core, forming a high-strength interface. These nanometer-scale hydrophobic bands are readily fabricated on metallic probes by functionalizing the exposed sidewall of an ultrathin evaporated Au metal layer rather than by lithography. Penetration and adhesion forces for butanethiol and dodecanethiol functionalized probes were directly measured using atomic force microscopy (AFM) on thick stacks of lipid bilayers to eliminate substrate effects. The penetration dynamics were starkly different for hydrophobic versus hydrophilic probes. Both 5- and 10 nm thick hydrophobically functionalized probes naturally resided within the lipid core, while hydrophilic probes remained in the aqueous region. Surprisingly, the barrier to probe penetration with short butanethiol chains (Eo,5 nm = 21.8kbT, Eo,10 nm = 15.3kbT) was dramatically higher than longer dodecanethiol chains (Eo,5 nm = 14.0kbT, Eo,10 nm = 10.9kbT), indicating that molecular mobility and orientation also play a role in addition to hydrophobicity in determining interface stability. These results highlight a new strategy for designing artificial cell interfaces that can nondestructively penetrate the lipid bilayer.

Formation and characterization of fluid lipid bilayers on alumina.

Fluid lipid bilayers were deposited on alumina substrates with the use of bubble collapse deposition (BCD). Previous studies using vesicle rupture have required the use of charged lipids or surface functionalization to induce bilayer formation on alumina, but these modifications are not necessary with BCD. Photobleaching experiments reveal that the diffusion coefficient of POPC on alumina is 0.6 microm (2)/s, which is much lower than the 1.4-2.0 microm (2)/s reported on silica. Systematically accounting for roughness, immobile regions and membrane viscosity shows that pinning sites account for about half of this drop in diffusivity. The remainder of the difference is attributed to a more tightly bound water state on the alumina surface, which induces a larger drag on the bilayer.

Dynamic actuation using nano-bio interfaces

The nanoscale dimensions, sensitive electronic control, and flexible architecture of new generations of nanomaterials and nanofabrication techniques hold immense promise not only for electronic devices, but also biological interfaces. As the size scales of these materials approach biological species, interfaces with characteristics designed to emulate their nanoscale biological counterparts are becoming possible. These new systems have higher biocompatibility, functionality, and lower cell toxicity than their microscale predecessors. While stellar examples have been demonstrated for biomolecular detection and imaging, exciting new possibilities for long-term integration and dynamic stimulation are now emerging, including protein activation, membrane integration and intracellular delivery. These tailored interfaces may lead to improved regenerative medicine, gene therapy and neural prosthetics.

Molecular Structure Influences the Stability of Membrane Penetrating Biointerfaces.

Nanoscale patterning of hydrophobic bands on otherwise hydrophilic surfaces allows integration of inorganic structures through biological membranes, reminiscent of transmembrane proteins. Here we show that a set of innate molecular properties of the self-assembling hydrophobic band determine the resulting interface stability. Surprisingly, hydrophobicity is found to be a secondary factor with monolayer crystallinity the major determinate of interface strength. These results begin to establish guidelines for seamless bioinorganic integration of nanoscale probes with lipid membranes.

Combination Growth Factor Therapy via Electrostatically Assembled Wound Dressings Improves Diabetic Ulcer Healing In Vivo.

Chronic skin ulcerations are a common complication of diabetes mellitus, affecting up to one in four diabetic individuals. Despite the prevalence of these wounds, current pharmacologic options for treating them remain limited. Growth factor-based therapies have displayed a mixed ability to drive successful healing, which may be due to nonoptimal delivery strategies. Here, a method for coating commercially available nylon dressings using the layer-by-layer process is described to enable both sustained release and independent control over the release kinetics of vascular endothelial growth factor 165 and platelet-derived growth factor BB. It is shown that the use of strategically spaced diffusion barriers formed spontaneously by disulfide bonds enables independent control over the release rates of incorporated growth factors, and that in vivo these dressings improve several aspects of wound healing in db/db mice.

Biomaterials: a potential pathway to healing chronic wounds?

Chronic dermal wounds are a devastating problem, which disproportionally affect individuals with conditions such as diabetes, paralysis, or simply old age. These wounds are extremely challenging to treat due to a heterogeneous combination of causative factors, creating a substantial burden on healthcare systems worldwide. Despite their large impact, there is currently a startling lack of options for effectively treating the underlying biological changes that occur within the wounds. Biomaterials possess an enticing ability to provide new comprehensive approaches to healing these devastating wounds; advanced wound dressings are now being developed that enable the ability to coordinate temporal delivery of multiple therapeutics, protect sensitive biologics from degradation, and provide supportive matrices that encourage the growth of tissue. This positions biomaterials as a potential “conductor” of wound repair, allowing them to simultaneously address numerous barriers to healing, and in turn providing a promising pathway to innovative new technologies for driving successful healing.

Programmable biomaterials for dynamic and responsive drug delivery

Biomaterials are continually being designed that enable new methods for interacting dynamically with cell and tissues, in turn unlocking new capabilities in areas ranging from drug delivery to regenerative medicine. In this review, we explore some of the recent advances being made in regards to programming biomaterials for improved drug delivery, with a focus on cancer and infection. We begin by explaining several of the underlying concepts that are being used to design this new wave of drug delivery vehicles, followed by examining recent materials systems that are able to coordinate the temporal delivery of multiple therapeutics, dynamically respond to changing tissue environments, and reprogram their bioactivity over time.

The Hair Follicle: An Underutilized Source of Cells and Materials for Regenerative Medicine

The hair follicle is one of only two structures within the adult body that selectively degenerates and regenerates, making it an intriguing organ to study and use for regenerative medicine. Hair follicles have been shown to influence wound healing, angiogenesis, neurogenesis, and harbor distinct populations of stem cells; this has led to cells from the follicle being used in clinical trials for tendinosis and chronic ulcers. In addition, keratin produced by the follicle in the form of a hair fiber provides an abundant source of biomaterials for regenerative medicine. In this review, we provide an overview of the structure of a hair follicle, explain the role of the follicle in regulating the microenvironment of skin and the impact on wound healing, explore individual cell types of interest for regenerative medicine, and cover several applications of keratin-based biomaterials.