Wesley R. Burghardt

A Multimaterial Bioink Method for 3D Printing Tunable, Cell‐Compatible Hydrogels

A multimaterial bio-ink method using polyethylene glycol crosslinking is presented for expanding the biomaterial palette required for 3D bioprinting of more mimetic and customizable tissue and organ constructs. Lightly crosslinked, soft hydrogels are produced from precursor solutions of various materials and 3D printed. Rheological and biological characterizations are presented, and the promise of this new bio-ink synthesis strategy is discussed.

Molecular orientation and rheology in sheared lyotropic liquid crystalline polymers

This article summarizes recent experiments relating measurements of molecular orientation to bulk rheological behavior in liquid crystalline polymers (LCPs) under shear. The principal experimental techniques are flow birefringence and x-ray scattering. Since LCPs usually exhibit a “polydomain” texture, measurements of flow-induced orientation reflect both the local distribution of molecular orientation around the director and the heterogeneous distribution of director orientations in the sheared LCP. In model lyotropic solutions of poly(benzyl glutamate) (PBG) and hydroxypropylcellulose (HPC), there are clear structural signatures of a transition from director tumbling dynamics at low Deborah number to flow alignment at high Deborah number. Rheo-optical measurements of the full refractive index tensor in PBG allow the orientation predictions of microstructural theories for LCP rheology to be quantitatively tested. At low shear rates the two model materials differ: PBG solutions exhibit significant orientation, while HPC solutions show little orientation. This is correlated with the presence of so-called “Region I” shear thinning in HPC solutions. Conversely, in PBG solutions of high concentration, x-ray scattering measurements demonstrate that Region I arises from the presence of a hexagonal phase. The model systems are further differentiated in relaxation. Molecular orientation increases in PBG solutions, but decreases in HPC solutions upon flow cessation; these differences are manifested in the evolution of dynamic properties. Finally, structural investigations of a PBG solution and a nematic surfactant solution during step changes in shear conditions are used to interrogate tumbling dynamics at low shear rates and test microstructural tumbling models.

Transient shear flow of nematic liquid crystals: Manifestations of director tumbling

Calculations of a variety of transient flow phenomena based on the Leslie–Ericksen model for nematic liquid crystals are presented. Emphasis is placed on the behavior of nematics subject to director tumbling. A wide range of complicated transient phenomena such as oscillatory responses are predicted when the Ericksen number is sufficiently high to cause significant rotation of the director at steady state. Particular attention is given to relaxation processes such as constrained elastic recoil and stress relaxation that arise from the interaction of the relaxing director profile with the viscous response of the nematic. These relaxation processes are dramatically different in tumbling and flow aligning systems, due to the saturation of the director profile at high Ericksen number for the flow aligning case. A qualitative physical model of a textured polymer liquid crystal is presented in which the dynamics on a local scale governed by the distance between defects are assumed to be similar to the macroscop...

Oral and Oropharyngeal Perceptions of Fluid Viscosity Across the Age Span

Research demonstrates that varying sensory input, including the characteristics of a bolus, changes swallow physiology. Altering the consistency of fluids is a common compensatory technique used in dysphagia management to facilitate change. However, it is not known what variations in viscosity can be perceived in the oral cavity or oropharynx or if age affects oral and oropharyngeal perceptions of fluid viscosity. This study aims to establish the ability of normal adults to perceive fluid viscosity in the oral cavity and oropharynx and to determine if, within this population, there are age-related changes in oral and oropharyngeal perceptions. Sensitivity was established by deriving the exponent for the psychophysical law for fluid viscosity in both the oral cavity and the oropharynx, using modulus-free magnitude estimation with Newtonian fluids of corn syrup and water. Sixty normal volunteers, aged 21–84 years, participated. Results indicate that the exponent for oral perception of fluid viscosity was 0.3298, while for oropharyngeal perception it was 0.3148. Viscosity perception deteriorates with increasing age. Men exhibited a more marked deterioration in sensitivity than women. This study contributes to the literature on oral and oropharyngeal perceptions and on aging. The results provide a basis for work with individuals with dysphagia.

Flow birefringence and computational studies of a shear thinning polymer solution in axisymmetric stagnation flow

Abstract Flow of a shear thinning, moderately concentrated solution of monodisperse high molecular weight polystyrene has been studied in a nonhomogeneous axisymmetric stagnation flow. Shear and extensional rheometry have been used to characterize the rheology of the solution. Multimode Giesekus and Phan–Thien–Tanner (PTT) models, and a single mode PTT model with a White–Metzner rate dependent relaxation time (PTT–WM) were fit to the rheological data. Transient uniaxial elongational measurements show significant extension hardening, which could only be predicted by the PTT model. The Giesekus model provided the best representation of shear flow data, but substantially underpredicted the measured extensional viscosity. Flow birefringence was used to measure integrated axisymmetric shear and normal stress profiles as a function of position. These measurements were compared with the results of numerical simulations, performed using an adaptive finite element technique, with the three constitutive models. The computed results with the 3-mode Giesekus model accurately portrayed experimental shear and normal stresses in both forward and rear stagnation flow up to Weissenberg numbers (We) of O(1). At higher We, the Giesekus model underpredicts normal stresses in the rear stagnation flow, where elongational gradients dominate. Both PTT and PTT–WM models gave rise to unrealistically large extensional stresses in the rear stagnation flow even at moderate We which led to poor agreement with experimental data. On the basis of these comparisons, it appears that the elongational properties of the solution are intermediate between the Giesekus and PTT, but much closer to the Giesekus, predictions. This is in conflict with the experimentally measured uniaxial viscosity, which suggests that the PTT model should be superior in flows with significant uniaxial extension.

Oral sensory discrimination of fluid viscosity

Abstract. This study was designed to investigate the ability of normal young adult volunteers to sensorially identify Newtonian fluids of specified viscosities. Twenty subjects, 10 men and 10 women between the ages of 18 and 29 years participated. Seven stimuli, consisting of combinations of corn syrup and water, with viscosities ranging from 2 to 2,240 centipoise (cP) were prepared and characterized using a coaxial rotational viscometer. Subjects were presented with two anchor stimuli representing the extremes of the range of viscosities as a basis from which the experimental stimuli were judged. The seven experimental stimuli were randomly presented to each subject 10 times. The accuracy with which the subjects identified the viscosity of the fluid was significant at p < 0.01. The pattern of response was not significantly different across subjects nor gender. There were no differences in performance throughout the duration of the study. The repeat presentation of the anchor points did not significantly affect performance. Further research on oral perception of viscosity, and the processes that mediate changes in swallow physiology resulting from changes in viscosity is required.

Poly(N-isopropylacrylamide)-based semi-interpenetrating polymer networks for tissue engineering applications. Effects of linear poly(acrylic acid) chains on rheology

Semi-interpenetrating polymer networks (semi-IPNs), comprised of poly(N-isopropylacrylamide-co-acrylic acid) (p(NIPAAm-co-AAc)) hydrogels and linear p(AAc) chains, were synthesized, and the effects of the p(AAc) chains on semi-IPN rheology were examined. Oscillatory shear rheometry studies were performed and the rheological data were analyzed as a function of temperature, frequency, and p(AAc) chain amount (weight average molecular weight (M w ) 4.5 × 105 g/mol). At 22°C, the semi-IPNs, as well as control p(NIPAAm-co-AAc) hydrogels, demonstrated rheological data that were representative of soft, loosely cross-linked solids. Furthermore, only the highest p(AAc) chain amount tested affected the rigidity of the p(NIPAAm-co-AAc)-based semi-IPNs, as compared to the p(NIPAAm-co-AAc) hydrogels. At 37°C, the complex shear moduli (G ∗) demonstrated by the p(NIPAAm-co-AAc)-based semi-IPNs were significantly greater than G ∗ exhibited by the p(NIPAAm-co-AAc) hydrogels, and the semi-IPN G ∗ values significantly increased with increasing p(AAc) chain amount. These results can be used to develop p(NIPAAm)-based semi-IPNs with tailored mechanical properties that may function as scaffolds in tissue engineering initiatives.

Comparison of optical and mechanical measurements of second normal stress difference relaxation following step strain

Relaxation of the second normal stress difference (N2) following step strain of a concentrated monodisperse polystyrene solution has been studied using mechanical and optical rheometry. Measurements of normal thrust in a parallel plate geometry are corrected for strain inhomogeneity and combined with independent measurements of the first normal stress difference (N1) to determine N2. Optical experiments were performed using a novel configuration where flow birefringence data collected using multiple light paths within the shear plane are combined with the stress-optical law to determine all three independent stress components for shearing deformations. This technique eliminates end effects, and provides an opportunity to oversample the stress tensor and develop consistency checks of experimental data. N2 is found to be nonzero at all accessible times, and relaxes in roughly constant proportion to N1. This reflects nonaffine distribution of chain segments, even well within the regime of chain retraction at short times. Data collected with the two techniques are reasonably consistent with each other, and with results of previous studies, generally lying between the predictions of the Doi-Edwards model with and without the independent alignment approximation. The normal stress ratio −N2/N1 shows pronounced strain thinning in the nonlinear regime.

Molecular orientation of a liquid‐crystalline polymer solution in mixed shear‐extensional flows

A lyotropic solution of hydroxypropylcellulose in m‐cresol has been studied in a nonhomogeneous shear (plane Poiseuille) flow, and in a set of flows with mixed shear and extension (slit contractions). The average molecular orientation is measured using flow birefringence, while laser‐Doppler velocimetry is used to characterize extensional kinematics in the slit‐contraction flows. In slit flow, we observe very similar behavior to recently published observations in another model lyotropic, PBG in m‐cresol [B. D. Bedford and W. R. Burghardt, J. Rheol. 38, 1657 (1994)]. Steady flow at low rates gives way to an instability characterized by large scale structural heterogeneities and time‐dependent birefringence at higher rates. Throughout the entire flow rate range, however, average birefringence measured in slit flow may be quantitatively predicted from simple shear flow data, assuming that the locally averaged rheological and structural properties in the slit flow are equivalent to those occurring in simple s...

Transient molecular orientation and rheology in flow aligning thermotropic liquid crystalline polymers

Quantitative measurements of molecular orientation and rheology are reported for various transient shear flows of a nematic semiflexible copolyether. Unlike the case of lyotropic liquid crystalline polymers (LCPs), whose structure and rheology in shear are dominated by director tumbling, this material exhibits flow aligning behavior. The observed behavior is quite similar to that seen in a copolyester that we have recently studied [Ugaz and Burghardt (1998)], suggesting that flow aligning dynamics may predominate in main-chain thermotropes that incorporate significant chain flexibility. Since the flow aligning regime has received little attention in previous attempts to model the rheology of textured, polydomain LCPs, we attempt to determine whether available models are capable of predicting the orientation and stress response of this class of LCP. We first examine the predictions of the polydomain Ericksen model, an adaptation of Ericksen’s transversely isotropic fluid model which accounts for the polydo...