One directional error bars in ggplot2

I am plotting some data from a test that was repeated pre and post an intervention. As such I would like to plot the results on the same figure with error bars. As the error bars of the two trials cross, making it difficult to decipher, I would like to plot the error bars upwards only for the post test and downwards only for the pre test. Is this possible in ggplot2?

Example code:

library("ggplot2")
set.seed(1)
dat <- data.frame(Trial = c(rep("Pre",9),rep("Post",9)), 
                  Time = rep.int(seq(0,120,15),2), 
                  Insulin = c(rnorm(9,15,2),rnorm(9,22,2)),
                  Insulin_sd = c(rnorm(18,3,1)))


p3 <- ggplot(data = dat, aes(x = Time, y = Insulin, group = Trial))+
  geom_errorbar(aes(ymin = Insulin - Insulin_sd, ymax = Insulin + Insulin_sd,linetype = Trial), width = 4) +
  geom_line(aes(linetype = Trial)) +
  geom_point(aes(shape= Trial, fill = Trial), size = 2) +
  scale_shape_manual(values=c(21,24),guide = guide_legend(reverse = TRUE)) +
  scale_fill_manual(values=c("black","white"),guide = guide_legend(reverse = TRUE)) +
  scale_linetype_manual(values = c("solid","dashed"),guide = guide_legend(reverse = TRUE)) +
  scale_y_continuous(limits= c(0,35))

(sorry- not able to upload an image)

Any help greatly appreciated.

One way to get close to what you want is to manually set error bar maxima and minima using Boolean logic based on your existing data.

dat$min <- dat$Insulin - (dat$Trial=='Pre')*dat$Insulin_sd

dat$max <- dat$Insulin + (dat$Trial=='Post')*dat$Insulin_sd

p3 <- ggplot(data = dat, aes(x = Time, y = Insulin, group = Trial)) +
       geom_errorbar(aes(ymin=min, ymax=max, linetype = Trial), width = 4) +
       geom_line(aes(linetype = Trial)) +
       geom_point(aes(shape= Trial, fill = Trial), size=2) +
       scale_shape_manual(values=c(21,24), guide=guide_legend(reverse = TRUE)) +
       scale_fill_manual(values=c("black","white"), guide=
             guide_legend(reverse = TRUE)) +
       scale_linetype_manual(values = c("solid","dashed"), 
             guide=guide_legend(reverse = TRUE)) +
       scale_y_continuous(limits= c(0,35))